python_doc_md/Beginner/whatsnew/2.7.md

What's New in Python 2.7

Author : A.M. Kuchling (amk at amk.ca)

This article explains the new features in Python 2.7. Python 2.7 was released on July 3, 2010.

Numeric handling has been improved in many ways, for both floating-point numbers and for the ~decimal.Decimal{.interpreted-text role="class"} class. There are some useful additions to the standard library, such as a greatly enhanced unittest{.interpreted-text role="mod"} module, the argparse{.interpreted-text role="mod"} module for parsing command-line options, convenient ~collections.OrderedDict{.interpreted-text role="class"} and ~collections.Counter{.interpreted-text role="class"} classes in the collections{.interpreted-text role="mod"} module, and many other improvements.

Python 2.7 is planned to be the last of the 2.x releases, so we worked on making it a good release for the long term. To help with porting to Python 3, several new features from the Python 3.x series have been included in 2.7.

This article doesn't attempt to provide a complete specification of the new features, but instead provides a convenient overview. For full details, you should refer to the documentation for Python 2.7 at https://docs.python.org. If you want to understand the rationale for the design and implementation, refer to the PEP for a particular new feature or the issue on https://bugs.python.org in which a change was discussed. Whenever possible, "What's New in Python" links to the bug/patch item for each change.

The Future for Python 2.x {#whatsnew27-python31}

Python 2.7 is the last major release in the 2.x series, as the Python maintainers have shifted the focus of their new feature development efforts to the Python 3.x series. This means that while Python 2 continues to receive bug fixes, and to be updated to build correctly on new hardware and versions of supported operated systems, there will be no new full feature releases for the language or standard library.

However, while there is a large common subset between Python 2.7 and Python 3, and many of the changes involved in migrating to that common subset, or directly to Python 3, can be safely automated, some other changes (notably those associated with Unicode handling) may require careful consideration, and preferably robust automated regression test suites, to migrate effectively.

This means that Python 2.7 will remain in place for a long time, providing a stable and supported base platform for production systems that have not yet been ported to Python 3. The full expected lifecycle of the Python 2.7 series is detailed in 373{.interpreted-text role="pep"}.

Some key consequences of the long-term significance of 2.7 are:

• As noted above, the 2.7 release has a much longer period of maintenance when compared to earlier 2.x versions. Python 2.7 is currently expected to remain supported by the core development team (receiving security updates and other bug fixes) until at least 2020 (10 years after its initial release, compared to the more typical support period of 18--24 months).
• As the Python 2.7 standard library ages, making effective use of the Python Package Index (either directly or via a redistributor) becomes more important for Python 2 users. In addition to a wide variety of third party packages for various tasks, the available packages include backports of new modules and features from the Python 3 standard library that are compatible with Python 2, as well as various tools and libraries that can make it easier to migrate to Python 3. The Python Packaging User Guide provides guidance on downloading and installing software from the Python Package Index.
• While the preferred approach to enhancing Python 2 is now the publication of new packages on the Python Package Index, this approach doesn't necessarily work in all cases, especially those related to network security. In exceptional cases that cannot be handled adequately by publishing new or updated packages on PyPI, the Python Enhancement Proposal process may be used to make the case for adding new features directly to the Python 2 standard library. Any such additions, and the maintenance releases where they were added, will be noted in the py27-maintenance-enhancements{.interpreted-text role="ref"} section below.

For projects wishing to migrate from Python 2 to Python 3, or for library and framework developers wishing to support users on both Python 2 and Python 3, there are a variety of tools and guides available to help decide on a suitable approach and manage some of the technical details involved. The recommended starting point is the pyporting-howto{.interpreted-text role="ref"} HOWTO guide.

Changes to the Handling of Deprecation Warnings

For Python 2.7, a policy decision was made to silence warnings only of interest to developers by default. DeprecationWarning{.interpreted-text role="exc"} and its descendants are now ignored unless otherwise requested, preventing users from seeing warnings triggered by an application. This change was also made in the branch that became Python 3.2. (Discussed on stdlib-sig and carried out in 7319{.interpreted-text role="issue"}.)

In previous releases, DeprecationWarning{.interpreted-text role="exc"} messages were enabled by default, providing Python developers with a clear indication of where their code may break in a future major version of Python.

However, there are increasingly many users of Python-based applications who are not directly involved in the development of those applications. DeprecationWarning{.interpreted-text role="exc"} messages are irrelevant to such users, making them worry about an application that's actually working correctly and burdening application developers with responding to these concerns.

You can re-enable display of DeprecationWarning{.interpreted-text role="exc"} messages by running Python with the -Wdefault <-W>{.interpreted-text role="option"} (short form: -Wd <-W>{.interpreted-text role="option"}) switch, or by setting the PYTHONWARNINGS{.interpreted-text role="envvar"} environment variable to "default" (or "d") before running Python. Python code can also re-enable them by calling warnings.simplefilter('default').

The unittest module also automatically reenables deprecation warnings when running tests.

Python 3.1 Features

Much as Python 2.6 incorporated features from Python 3.0, version 2.7 incorporates some of the new features in Python 3.1. The 2.x series continues to provide tools for migrating to the 3.x series.

A partial list of 3.1 features that were backported to 2.7:

• The syntax for set literals ({1,2,3} is a mutable set).
• Dictionary and set comprehensions ({i: i*2 for i in range(3)}).
• Multiple context managers in a single with{.interpreted-text role="keyword"} statement.
• A new version of the io{.interpreted-text role="mod"} library, rewritten in C for performance.
• The ordered-dictionary type described in pep-0372{.interpreted-text role="ref"}.
• The new "," format specifier described in pep-0378{.interpreted-text role="ref"}.
• The memoryview{.interpreted-text role="class"} object.
• A small subset of the importlib{.interpreted-text role="mod"} module, described below.
• The repr{.interpreted-text role="func"} of a float x is shorter in many cases: it's now based on the shortest decimal string that's guaranteed to round back to x. As in previous versions of Python, it's guaranteed that float(repr(x)) recovers x.
• Float-to-string and string-to-float conversions are correctly rounded. The round{.interpreted-text role="func"} function is also now correctly rounded.
• The PyCapsule{.interpreted-text role="c:type"} type, used to provide a C API for extension modules.
• The PyLong_AsLongAndOverflow{.interpreted-text role="c:func"} C API function.

Other new Python3-mode warnings include:

• !operator.isCallable{.interpreted-text role="func"} and !operator.sequenceIncludes{.interpreted-text role="func"}, which are not supported in 3.x, now trigger warnings.
• The !-3{.interpreted-text role="option"} switch now automatically enables the !-Qwarn{.interpreted-text role="option"} switch that causes warnings about using classic division with integers and long integers.

PEP 372: Adding an Ordered Dictionary to collections {#pep-0372}

Regular Python dictionaries iterate over key/value pairs in arbitrary order. Over the years, a number of authors have written alternative implementations that remember the order that the keys were originally inserted. Based on the experiences from those implementations, 2.7 introduces a new ~collections.OrderedDict{.interpreted-text role="class"} class in the collections{.interpreted-text role="mod"} module.

The ~collections.OrderedDict{.interpreted-text role="class"} API provides the same interface as regular dictionaries but iterates over keys and values in a guaranteed order depending on when a key was first inserted:

>>> from collections import OrderedDict
>>> d = OrderedDict([('first', 1),
...                  ('second', 2),
...                  ('third', 3)])
>>> d.items()
[('first', 1), ('second', 2), ('third', 3)]

If a new entry overwrites an existing entry, the original insertion position is left unchanged:

>>> d['second'] = 4
>>> d.items()
[('first', 1), ('second', 4), ('third', 3)]

Deleting an entry and reinserting it will move it to the end:

>>> del d['second']
>>> d['second'] = 5
>>> d.items()
[('first', 1), ('third', 3), ('second', 5)]

The ~collections.OrderedDict.popitem{.interpreted-text role="meth"} method has an optional last argument that defaults to True. If last is true, the most recently added key is returned and removed; if it's false, the oldest key is selected:

>>> od = OrderedDict([(x,0) for x in range(20)])
>>> od.popitem()
(19, 0)
>>> od.popitem()
(18, 0)
>>> od.popitem(last=False)
(0, 0)
>>> od.popitem(last=False)
(1, 0)

Comparing two ordered dictionaries checks both the keys and values, and requires that the insertion order was the same:

>>> od1 = OrderedDict([('first', 1),
...                    ('second', 2),
...                    ('third', 3)])
>>> od2 = OrderedDict([('third', 3),
...                    ('first', 1),
...                    ('second', 2)])
>>> od1 == od2
False
>>> # Move 'third' key to the end
>>> del od2['third']; od2['third'] = 3
>>> od1 == od2
True

Comparing an ~collections.OrderedDict{.interpreted-text role="class"} with a regular dictionary ignores the insertion order and just compares the keys and values.

How does the ~collections.OrderedDict{.interpreted-text role="class"} work? It maintains a doubly linked list of keys, appending new keys to the list as they're inserted. A secondary dictionary maps keys to their corresponding list node, so deletion doesn't have to traverse the entire linked list and therefore remains O(1).

The standard library now supports use of ordered dictionaries in several modules.

• The ConfigParser <configparser>{.interpreted-text role="mod"} module uses them by default, meaning that configuration files can now be read, modified, and then written back in their original order.
• The ~collections.somenamedtuple._asdict{.interpreted-text role="meth"} method for collections.namedtuple{.interpreted-text role="func"} now returns an ordered dictionary with the values appearing in the same order as the underlying tuple indices.
• The json{.interpreted-text role="mod"} module's ~json.JSONDecoder{.interpreted-text role="class"} class constructor was extended with an object_pairs_hook parameter to allow OrderedDict{.interpreted-text role="class"} instances to be built by the decoder. Support was also added for third-party tools like PyYAML.

::: seealso

372{.interpreted-text role="pep"} - Adding an ordered dictionary to collections

: PEP written by Armin Ronacher and Raymond Hettinger; implemented by Raymond Hettinger. :::

PEP 378: Format Specifier for Thousands Separator {#pep-0378}

To make program output more readable, it can be useful to add separators to large numbers, rendering them as 18,446,744,073,709,551,616 instead of 18446744073709551616.

The fully general solution for doing this is the locale{.interpreted-text role="mod"} module, which can use different separators ("," in North America, "." in Europe) and different grouping sizes, but locale{.interpreted-text role="mod"} is complicated to use and unsuitable for multi-threaded applications where different threads are producing output for different locales.

Therefore, a simple comma-grouping mechanism has been added to the mini-language used by the str.format{.interpreted-text role="meth"} method. When formatting a floating-point number, simply include a comma between the width and the precision:

>>> '{:20,.2f}'.format(18446744073709551616.0)
'18,446,744,073,709,551,616.00'

When formatting an integer, include the comma after the width:

>>> '{:20,d}'.format(18446744073709551616) '18,446,744,073,709,551,616'

This mechanism is not adaptable at all; commas are always used as the separator and the grouping is always into three-digit groups. The comma-formatting mechanism isn't as general as the locale{.interpreted-text role="mod"} module, but it's easier to use.

::: seealso

378{.interpreted-text role="pep"} - Format Specifier for Thousands Separator

: PEP written by Raymond Hettinger; implemented by Eric Smith. :::

PEP 389: The argparse Module for Parsing Command Lines

The argparse{.interpreted-text role="mod"} module for parsing command-line arguments was added as a more powerful replacement for the optparse{.interpreted-text role="mod"} module.

This means Python now supports three different modules for parsing command-line arguments: getopt{.interpreted-text role="mod"}, optparse{.interpreted-text role="mod"}, and argparse{.interpreted-text role="mod"}. The getopt{.interpreted-text role="mod"} module closely resembles the C library's !getopt{.interpreted-text role="c:func"} function, so it remains useful if you're writing a Python prototype that will eventually be rewritten in C. optparse{.interpreted-text role="mod"} becomes redundant, but there are no plans to remove it because there are many scripts still using it, and there's no automated way to update these scripts. (Making the argparse{.interpreted-text role="mod"} API consistent with optparse{.interpreted-text role="mod"}'s interface was discussed but rejected as too messy and difficult.)

In short, if you're writing a new script and don't need to worry about compatibility with earlier versions of Python, use argparse{.interpreted-text role="mod"} instead of optparse{.interpreted-text role="mod"}.

Here's an example:

import argparse

parser = argparse.ArgumentParser(description='Command-line example.')

# Add optional switches
parser.add_argument('-v', action='store_true', dest='is_verbose',
                    help='produce verbose output')
parser.add_argument('-o', action='store', dest='output',
                    metavar='FILE',
                    help='direct output to FILE instead of stdout')
parser.add_argument('-C', action='store', type=int, dest='context',
                    metavar='NUM', default=0,
                    help='display NUM lines of added context')

# Allow any number of additional arguments.
parser.add_argument(nargs='*', action='store', dest='inputs',
                    help='input filenames (default is stdin)')

args = parser.parse_args()
print args.__dict__

Unless you override it, !-h{.interpreted-text role="option"} and !--help{.interpreted-text role="option"} switches are automatically added, and produce neatly formatted output:

-> ./python.exe argparse-example.py --help
usage: argparse-example.py [-h] [-v] [-o FILE] [-C NUM] [inputs [inputs ...]]

Command-line example.

positional arguments:
  inputs      input filenames (default is stdin)

optional arguments:
  -h, --help  show this help message and exit
  -v          produce verbose output
  -o FILE     direct output to FILE instead of stdout
  -C NUM      display NUM lines of added context

As with optparse{.interpreted-text role="mod"}, the command-line switches and arguments are returned as an object with attributes named by the dest parameters:

-> ./python.exe argparse-example.py -v
{'output': None,
 'is_verbose': True,
 'context': 0,
 'inputs': []}

-> ./python.exe argparse-example.py -v -o /tmp/output -C 4 file1 file2
{'output': '/tmp/output',
 'is_verbose': True,
 'context': 4,
 'inputs': ['file1', 'file2']}

argparse{.interpreted-text role="mod"} has much fancier validation than optparse{.interpreted-text role="mod"}; you can specify an exact number of arguments as an integer, 0 or more arguments by passing '*', 1 or more by passing '+', or an optional argument with '?'. A top-level parser can contain sub-parsers to define subcommands that have different sets of switches, as in svn commit, svn checkout, etc. You can specify an argument's type as ~argparse.FileType{.interpreted-text role="class"}, which will automatically open files for you and understands that '-' means standard input or output.

::: seealso

argparse{.interpreted-text role="mod"} documentation

: The documentation page of the argparse module.

upgrading-optparse-code{.interpreted-text role="ref"}

: Part of the Python documentation, describing how to convert code that uses optparse{.interpreted-text role="mod"}.

389{.interpreted-text role="pep"} - argparse - New Command Line Parsing Module

: PEP written and implemented by Steven Bethard. :::

PEP 391: Dictionary-Based Configuration For Logging

The logging{.interpreted-text role="mod"} module is very flexible; applications can define a tree of logging subsystems, and each logger in this tree can filter out certain messages, format them differently, and direct messages to a varying number of handlers.

All this flexibility can require a lot of configuration. You can write Python statements to create objects and set their properties, but a complex set-up requires verbose but boring code. logging{.interpreted-text role="mod"} also supports a ~logging.config.fileConfig{.interpreted-text role="func"} function that parses a file, but the file format doesn't support configuring filters, and it's messier to generate programmatically.

Python 2.7 adds a ~logging.config.dictConfig{.interpreted-text role="func"} function that uses a dictionary to configure logging. There are many ways to produce a dictionary from different sources: construct one with code; parse a file containing JSON; or use a YAML parsing library if one is installed. For more information see logging-config-api{.interpreted-text role="ref"}.

The following example configures two loggers, the root logger and a logger named "network". Messages sent to the root logger will be sent to the system log using the syslog protocol, and messages to the "network" logger will be written to a network.log{.interpreted-text role="file"} file that will be rotated once the log reaches 1MB.

import logging
import logging.config

configdict = {
 'version': 1,    # Configuration schema in use; must be 1 for now
 'formatters': {
     'standard': {
         'format': ('%(asctime)s %(name)-15s '
                    '%(levelname)-8s %(message)s')}},

 'handlers': {'netlog': {'backupCount': 10,
                     'class': 'logging.handlers.RotatingFileHandler',
                     'filename': '/logs/network.log',
                     'formatter': 'standard',
                     'level': 'INFO',
                     'maxBytes': 1000000},
              'syslog': {'class': 'logging.handlers.SysLogHandler',
                         'formatter': 'standard',
                         'level': 'ERROR'}},

 # Specify all the subordinate loggers
 'loggers': {
             'network': {
                         'handlers': ['netlog']
             }
 },
 # Specify properties of the root logger
 'root': {
          'handlers': ['syslog']
 },
}

# Set up configuration
logging.config.dictConfig(configdict)

# As an example, log two error messages
logger = logging.getLogger('/')
logger.error('Database not found')

netlogger = logging.getLogger('network')
netlogger.error('Connection failed')

Three smaller enhancements to the logging{.interpreted-text role="mod"} module, all implemented by Vinay Sajip, are:

• The ~logging.handlers.SysLogHandler{.interpreted-text role="class"} class now supports syslogging over TCP. The constructor has a socktype parameter giving the type of socket to use, either socket.SOCK_DGRAM{.interpreted-text role="const"} for UDP or socket.SOCK_STREAM{.interpreted-text role="const"} for TCP. The default protocol remains UDP.
• ~logging.Logger{.interpreted-text role="class"} instances gained a ~logging.Logger.getChild{.interpreted-text role="meth"} method that retrieves a descendant logger using a relative path. For example, once you retrieve a logger by doing log = getLogger('app'), calling log.getChild('network.listen') is equivalent to getLogger('app.network.listen').
• The ~logging.LoggerAdapter{.interpreted-text role="class"} class gained an ~logging.Logger.isEnabledFor{.interpreted-text role="meth"} method that takes a level and returns whether the underlying logger would process a message of that level of importance.

::: seealso

391{.interpreted-text role="pep"} - Dictionary-Based Configuration For Logging

: PEP written and implemented by Vinay Sajip. :::

PEP 3106: Dictionary Views

The dictionary methods ~dict.keys{.interpreted-text role="meth"}, ~dict.values{.interpreted-text role="meth"}, and ~dict.items{.interpreted-text role="meth"} are different in Python 3.x. They return an object called a view{.interpreted-text role="dfn"} instead of a fully materialized list.

It's not possible to change the return values of ~dict.keys{.interpreted-text role="meth"}, ~dict.values{.interpreted-text role="meth"}, and ~dict.items{.interpreted-text role="meth"} in Python 2.7 because too much code would break. Instead the 3.x versions were added under the new names !viewkeys{.interpreted-text role="meth"}, !viewvalues{.interpreted-text role="meth"}, and !viewitems{.interpreted-text role="meth"}.

>>> d = dict((i*10, chr(65+i)) for i in range(26))
>>> d
{0: 'A', 130: 'N', 10: 'B', 140: 'O', 20: ..., 250: 'Z'}
>>> d.viewkeys()
dict_keys([0, 130, 10, 140, 20, 150, 30, ..., 250])

Views can be iterated over, but the key and item views also behave like sets. The & operator performs intersection, and | performs a union:

>>> d1 = dict((i*10, chr(65+i)) for i in range(26))
>>> d2 = dict((i**.5, i) for i in range(1000))
>>> d1.viewkeys() & d2.viewkeys()
set([0.0, 10.0, 20.0, 30.0])
>>> d1.viewkeys() | range(0, 30)
set([0, 1, 130, 3, 4, 5, 6, ..., 120, 250])

The view keeps track of the dictionary and its contents change as the dictionary is modified:

>>> vk = d.viewkeys()
>>> vk
dict_keys([0, 130, 10, ..., 250])
>>> d[260] = '&'
>>> vk
dict_keys([0, 130, 260, 10, ..., 250])

However, note that you can't add or remove keys while you're iterating over the view:

>>> for k in vk:
...     d[k*2] = k
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration

You can use the view methods in Python 2.x code, and the 2to3 converter will change them to the standard ~dict.keys{.interpreted-text role="meth"}, ~dict.values{.interpreted-text role="meth"}, and ~dict.items{.interpreted-text role="meth"} methods.

::: seealso

3106{.interpreted-text role="pep"} - Revamping dict.keys(), .values() and .items()

: PEP written by Guido van Rossum. Backported to 2.7 by Alexandre Vassalotti; 1967{.interpreted-text role="issue"}. :::

PEP 3137: The memoryview Object

The memoryview{.interpreted-text role="class"} object provides a view of another object's memory content that matches the bytes{.interpreted-text role="class"} type's interface.

::: {.doctest options="+SKIP"} >>> import string >>> m = memoryview(string.letters) >>> m <memory at 0x37f850> >>> len(m) # Returns length of underlying object 52 >>> m[0], m[25], m[26] # Indexing returns one byte ('a', 'z', 'A') >>> m2 = m[0:26] # Slicing returns another memoryview >>> m2 <memory at 0x37f080> :::

The content of the view can be converted to a string of bytes or a list of integers:

::: {.doctest options="+SKIP"} >>> m2.tobytes() 'abcdefghijklmnopqrstuvwxyz' >>> m2.tolist() [97, 98, 99, 100, 101, 102, 103, ... 121, 122] >>> :::

memoryview{.interpreted-text role="class"} objects allow modifying the underlying object if it's a mutable object.

::: {.doctest options="+SKIP"} >>> m2[0] = 75 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: cannot modify read-only memory >>> b = bytearray(string.letters) # Creating a mutable object >>> b bytearray(b'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ') >>> mb = memoryview(b) >>> mb[0] = '*' # Assign to view, changing the bytearray. >>> b[0:5] # The bytearray has been changed. bytearray(b'*bcde') >>> :::

::: seealso

3137{.interpreted-text role="pep"} - Immutable Bytes and Mutable Buffer

: PEP written by Guido van Rossum. Implemented by Travis Oliphant, Antoine Pitrou and others. Backported to 2.7 by Antoine Pitrou; 2396{.interpreted-text role="issue"}. :::

Other Language Changes

Some smaller changes made to the core Python language are:

• The syntax for set literals has been backported from Python 3.x. Curly brackets are used to surround the contents of the resulting mutable set; set literals are distinguished from dictionaries by not containing colons and values. {} continues to represent an empty dictionary; use set() for an empty set.

  ::: {.doctest options="+SKIP"} >>> {1, 2, 3, 4, 5} set([1, 2, 3, 4, 5]) >>> set() # empty set set([]) >>> {} # empty dict {} :::

  Backported by Alexandre Vassalotti; 2335{.interpreted-text role="issue"}.

• Dictionary and set comprehensions are another feature backported from 3.x, generalizing list/generator comprehensions to use the literal syntax for sets and dictionaries.

  ::: {.doctest options="+SKIP"} >>> {x: x*x for x in range(6)} {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25} >>> {('a'*x) for x in range(6)} set(['', 'a', 'aa', 'aaa', 'aaaa', 'aaaaa']) :::

  Backported by Alexandre Vassalotti; 2333{.interpreted-text role="issue"}.

• The with{.interpreted-text role="keyword"} statement can now use multiple context managers in one statement. Context managers are processed from left to right and each one is treated as beginning a new !with{.interpreted-text role="keyword"} statement. This means that:

  with A() as a, B() as b:
      ... suite of statements ...
  

  is equivalent to:

  with A() as a:
      with B() as b:
          ... suite of statements ...
  

  The !contextlib.nested{.interpreted-text role="func"} function provides a very similar function, so it's no longer necessary and has been deprecated.

  (Proposed in https://codereview.appspot.com/53094; implemented by Georg Brandl.)

• Conversions between floating-point numbers and strings are now correctly rounded on most platforms. These conversions occur in many different places: str{.interpreted-text role="func"} on floats and complex numbers; the float{.interpreted-text role="class"} and complex{.interpreted-text role="class"} constructors; numeric formatting; serializing and deserializing floats and complex numbers using the marshal{.interpreted-text role="mod"}, pickle{.interpreted-text role="mod"} and json{.interpreted-text role="mod"} modules; parsing of float and imaginary literals in Python code; and ~decimal.Decimal{.interpreted-text role="class"}-to-float conversion.

  Related to this, the repr{.interpreted-text role="func"} of a floating-point number x now returns a result based on the shortest decimal string that's guaranteed to round back to x under correct rounding (with round-half-to-even rounding mode). Previously it gave a string based on rounding x to 17 decimal digits.

  The rounding library responsible for this improvement works on Windows and on Unix platforms using the gcc, icc, or suncc compilers. There may be a small number of platforms where correct operation of this code cannot be guaranteed, so the code is not used on such systems. You can find out which code is being used by checking sys.float_repr_style{.interpreted-text role="data"}, which will be short if the new code is in use and legacy if it isn't.

  Implemented by Eric Smith and Mark Dickinson, using David Gay's dtoa.c{.interpreted-text role="file"} library; 7117{.interpreted-text role="issue"}.

• Conversions from long integers and regular integers to floating point now round differently, returning the floating-point number closest to the number. This doesn't matter for small integers that can be converted exactly, but for large numbers that will unavoidably lose precision, Python 2.7 now approximates more closely. For example, Python 2.6 computed the following:

  >>> n = 295147905179352891391
  >>> float(n)
  2.9514790517935283e+20
  >>> n - long(float(n))
  65535L
  

  Python 2.7's floating-point result is larger, but much closer to the true value:

  >>> n = 295147905179352891391
  >>> float(n)
  2.9514790517935289e+20
  >>> n - long(float(n))
  -1L
  

  (Implemented by Mark Dickinson; 3166{.interpreted-text role="issue"}.)

  Integer division is also more accurate in its rounding behaviours. (Also implemented by Mark Dickinson; 1811{.interpreted-text role="issue"}.)

• Implicit coercion for complex numbers has been removed; the interpreter will no longer ever attempt to call a !__coerce__{.interpreted-text role="meth"} method on complex objects. (Removed by Meador Inge and Mark Dickinson; 5211{.interpreted-text role="issue"}.)

• The str.format{.interpreted-text role="meth"} method now supports automatic numbering of the replacement fields. This makes using str.format{.interpreted-text role="meth"} more closely resemble using %s formatting:

  >>> '{}:{}:{}'.format(2009, 04, 'Sunday')
  '2009:4:Sunday'
  >>> '{}:{}:{day}'.format(2009, 4, day='Sunday')
  '2009:4:Sunday'
  

  The auto-numbering takes the fields from left to right, so the first {...} specifier will use the first argument to str.format{.interpreted-text role="meth"}, the next specifier will use the next argument, and so on. You can't mix auto-numbering and explicit numbering -- either number all of your specifier fields or none of them -- but you can mix auto-numbering and named fields, as in the second example above. (Contributed by Eric Smith; 5237{.interpreted-text role="issue"}.)

  Complex numbers now correctly support usage with format{.interpreted-text role="func"}, and default to being right-aligned. Specifying a precision or comma-separation applies to both the real and imaginary parts of the number, but a specified field width and alignment is applied to the whole of the resulting 1.5+3j output. (Contributed by Eric Smith; 1588{.interpreted-text role="issue"} and 7988{.interpreted-text role="issue"}.)

  The 'F' format code now always formats its output using uppercase characters, so it will now produce 'INF' and 'NAN'. (Contributed by Eric Smith; 3382{.interpreted-text role="issue"}.)

  A low-level change: the object.__format__{.interpreted-text role="meth"} method now triggers a PendingDeprecationWarning{.interpreted-text role="exc"} if it's passed a format string, because the !__format__{.interpreted-text role="meth"} method for object{.interpreted-text role="class"} converts the object to a string representation and formats that. Previously the method silently applied the format string to the string representation, but that could hide mistakes in Python code. If you're supplying formatting information such as an alignment or precision, presumably you're expecting the formatting to be applied in some object-specific way. (Fixed by Eric Smith; 7994{.interpreted-text role="issue"}.)

• The int{.interpreted-text role="func"} and !long{.interpreted-text role="func"} types gained a bit_length method that returns the number of bits necessary to represent its argument in binary:

  >>> n = 37
  >>> bin(n)
  '0b100101'
  >>> n.bit_length()
  6
  >>> n = 2**123-1
  >>> n.bit_length()
  123
  >>> (n+1).bit_length()
  124
  

  (Contributed by Fredrik Johansson and Victor Stinner; 3439{.interpreted-text role="issue"}.)

• The import{.interpreted-text role="keyword"} statement will no longer try an absolute import if a relative import (e.g. from .os import sep) fails. This fixes a bug, but could possibly break certain !import{.interpreted-text role="keyword"} statements that were only working by accident. (Fixed by Meador Inge; 7902{.interpreted-text role="issue"}.)

• It's now possible for a subclass of the built-in !unicode{.interpreted-text role="class"} type to override the !__unicode__{.interpreted-text role="meth"} method. (Implemented by Victor Stinner; 1583863{.interpreted-text role="issue"}.)

• The bytearray{.interpreted-text role="class"} type's ~bytearray.translate{.interpreted-text role="meth"} method now accepts None as its first argument. (Fixed by Georg Brandl; 4759{.interpreted-text role="issue"}.)

• When using classmethod{.interpreted-text role="deco"} and staticmethod{.interpreted-text role="deco"} to wrap methods as class or static methods, the wrapper object now exposes the wrapped function as their ~method.__func__{.interpreted-text role="attr"} attribute. (Contributed by Amaury Forgeot d'Arc, after a suggestion by George Sakkis; 5982{.interpreted-text role="issue"}.)

• When a restricted set of attributes were set using __slots__, deleting an unset attribute would not raise AttributeError{.interpreted-text role="exc"} as you would expect. Fixed by Benjamin Peterson; 7604{.interpreted-text role="issue"}.)

• Two new encodings are now supported: "cp720", used primarily for Arabic text; and "cp858", a variant of CP 850 that adds the euro symbol. (CP720 contributed by Alexander Belchenko and Amaury Forgeot d'Arc in 1616979{.interpreted-text role="issue"}; CP858 contributed by Tim Hatch in 8016{.interpreted-text role="issue"}.)

• The !file{.interpreted-text role="class"} object will now set the !filename{.interpreted-text role="attr"} attribute on the IOError{.interpreted-text role="exc"} exception when trying to open a directory on POSIX platforms (noted by Jan Kaliszewski; 4764{.interpreted-text role="issue"}), and now explicitly checks for and forbids writing to read-only file objects instead of trusting the C library to catch and report the error (fixed by Stefan Krah; 5677{.interpreted-text role="issue"}).

• The Python tokenizer now translates line endings itself, so the compile{.interpreted-text role="func"} built-in function now accepts code using any line-ending convention. Additionally, it no longer requires that the code end in a newline.

• Extra parentheses in function definitions are illegal in Python 3.x, meaning that you get a syntax error from def f((x)): pass. In Python3-warning mode, Python 2.7 will now warn about this odd usage. (Noted by James Lingard; 7362{.interpreted-text role="issue"}.)

• It's now possible to create weak references to old-style class objects. New-style classes were always weak-referenceable. (Fixed by Antoine Pitrou; 8268{.interpreted-text role="issue"}.)

• When a module object is garbage-collected, the module's dictionary is now only cleared if no one else is holding a reference to the dictionary (7140{.interpreted-text role="issue"}).

Interpreter Changes {#new-27-interpreter}

A new environment variable, PYTHONWARNINGS{.interpreted-text role="envvar"}, allows controlling warnings. It should be set to a string containing warning settings, equivalent to those used with the -W{.interpreted-text role="option"} switch, separated by commas. (Contributed by Brian Curtin; 7301{.interpreted-text role="issue"}.)

For example, the following setting will print warnings every time they occur, but turn warnings from the Cookie <http.cookies>{.interpreted-text role="mod"} module into an error. (The exact syntax for setting an environment variable varies across operating systems and shells.)

export PYTHONWARNINGS=all,error:::Cookie:0

Optimizations

Several performance enhancements have been added:

• A new opcode was added to perform the initial setup for with{.interpreted-text role="keyword"} statements, looking up the ~object.__enter__{.interpreted-text role="meth"} and ~object.__exit__{.interpreted-text role="meth"} methods. (Contributed by Benjamin Peterson.)

• The garbage collector now performs better for one common usage pattern: when many objects are being allocated without deallocating any of them. This would previously take quadratic time for garbage collection, but now the number of full garbage collections is reduced as the number of objects on the heap grows. The new logic only performs a full garbage collection pass when the middle generation has been collected 10 times and when the number of survivor objects from the middle generation exceeds 10% of the number of objects in the oldest generation. (Suggested by Martin von Löwis and implemented by Antoine Pitrou; 4074{.interpreted-text role="issue"}.)

• The garbage collector tries to avoid tracking simple containers which can't be part of a cycle. In Python 2.7, this is now true for tuples and dicts containing atomic types (such as ints, strings, etc.). Transitively, a dict containing tuples of atomic types won't be tracked either. This helps reduce the cost of each garbage collection by decreasing the number of objects to be considered and traversed by the collector. (Contributed by Antoine Pitrou; 4688{.interpreted-text role="issue"}.)

• Long integers are now stored internally either in base 2**15 or in base 2**30, the base being determined at build time. Previously, they were always stored in base 2**15. Using base 2**30 gives significant performance improvements on 64-bit machines, but benchmark results on 32-bit machines have been mixed. Therefore, the default is to use base 2**30 on 64-bit machines and base 2**15 on 32-bit machines; on Unix, there's a new configure option !--enable-big-digits{.interpreted-text role="option"} that can be used to override this default.

  Apart from the performance improvements this change should be invisible to end users, with one exception: for testing and debugging purposes there's a new structseq !sys.long_info{.interpreted-text role="data"} that provides information about the internal format, giving the number of bits per digit and the size in bytes of the C type used to store each digit:

  >>> import sys
  >>> sys.long_info
  sys.long_info(bits_per_digit=30, sizeof_digit=4)
  

  (Contributed by Mark Dickinson; 4258{.interpreted-text role="issue"}.)

  Another set of changes made long objects a few bytes smaller: 2 bytes smaller on 32-bit systems and 6 bytes on 64-bit. (Contributed by Mark Dickinson; 5260{.interpreted-text role="issue"}.)

• The division algorithm for long integers has been made faster by tightening the inner loop, doing shifts instead of multiplications, and fixing an unnecessary extra iteration. Various benchmarks show speedups of between 50% and 150% for long integer divisions and modulo operations. (Contributed by Mark Dickinson; 5512{.interpreted-text role="issue"}.) Bitwise operations are also significantly faster (initial patch by Gregory Smith; 1087418{.interpreted-text role="issue"}).

• The implementation of % checks for the left-side operand being a Python string and special-cases it; this results in a 1--3% performance increase for applications that frequently use % with strings, such as templating libraries. (Implemented by Collin Winter; 5176{.interpreted-text role="issue"}.)

• List comprehensions with an if condition are compiled into faster bytecode. (Patch by Antoine Pitrou, back-ported to 2.7 by Jeffrey Yasskin; 4715{.interpreted-text role="issue"}.)

• Converting an integer or long integer to a decimal string was made faster by special-casing base 10 instead of using a generalized conversion function that supports arbitrary bases. (Patch by Gawain Bolton; 6713{.interpreted-text role="issue"}.)

• The !split{.interpreted-text role="meth"}, !replace{.interpreted-text role="meth"}, !rindex{.interpreted-text role="meth"}, !rpartition{.interpreted-text role="meth"}, and !rsplit{.interpreted-text role="meth"} methods of string-like types (strings, Unicode strings, and bytearray{.interpreted-text role="class"} objects) now use a fast reverse-search algorithm instead of a character-by-character scan. This is sometimes faster by a factor of 10. (Added by Florent Xicluna; 7462{.interpreted-text role="issue"} and 7622{.interpreted-text role="issue"}.)

• The pickle{.interpreted-text role="mod"} and !cPickle{.interpreted-text role="mod"} modules now automatically intern the strings used for attribute names, reducing memory usage of the objects resulting from unpickling. (Contributed by Jake McGuire; 5084{.interpreted-text role="issue"}.)

• The !cPickle{.interpreted-text role="mod"} module now special-cases dictionaries, nearly halving the time required to pickle them. (Contributed by Collin Winter; 5670{.interpreted-text role="issue"}.)

New and Improved Modules

As in every release, Python's standard library received a number of enhancements and bug fixes. Here's a partial list of the most notable changes, sorted alphabetically by module name. Consult the Misc/NEWS{.interpreted-text role="file"} file in the source tree for a more complete list of changes, or look through the Subversion logs for all the details.

• The bdb{.interpreted-text role="mod"} module's base debugging class ~bdb.Bdb{.interpreted-text role="class"} gained a feature for skipping modules. The constructor now takes an iterable containing glob-style patterns such as django.*; the debugger will not step into stack frames from a module that matches one of these patterns. (Contributed by Maru Newby after a suggestion by Senthil Kumaran; 5142{.interpreted-text role="issue"}.)

• The binascii{.interpreted-text role="mod"} module now supports the buffer API, so it can be used with memoryview{.interpreted-text role="class"} instances and other similar buffer objects. (Backported from 3.x by Florent Xicluna; 7703{.interpreted-text role="issue"}.)

• Updated module: the !bsddb{.interpreted-text role="mod"} module has been updated from 4.7.2devel9 to version 4.8.4 of the pybsddb package. The new version features better Python 3.x compatibility, various bug fixes, and adds several new BerkeleyDB flags and methods. (Updated by Jesús Cea Avión; 8156{.interpreted-text role="issue"}. The pybsddb changelog can be read at https://hg.jcea.es/pybsddb/file/tip/ChangeLog.)

• The bz2{.interpreted-text role="mod"} module's ~bz2.BZ2File{.interpreted-text role="class"} now supports the context management protocol, so you can write with bz2.BZ2File(...) as f:. (Contributed by Hagen Fürstenau; 3860{.interpreted-text role="issue"}.)

• New class: the ~collections.Counter{.interpreted-text role="class"} class in the collections{.interpreted-text role="mod"} module is useful for tallying data. ~collections.Counter{.interpreted-text role="class"} instances behave mostly like dictionaries but return zero for missing keys instead of raising a KeyError{.interpreted-text role="exc"}:

  ::: {.doctest options="+NORMALIZE_WHITESPACE"} >>> from collections import Counter >>> c = Counter() >>> for letter in 'here is a sample of english text': ... c[letter] += 1 ... >>> c # doctest: +SKIP Counter({' ': 6, 'e': 5, 's': 3, 'a': 2, 'i': 2, 'h': 2, 'l': 2, 't': 2, 'g': 1, 'f': 1, 'm': 1, 'o': 1, 'n': 1, 'p': 1, 'r': 1, 'x': 1}) >>> c['e'] 5 >>> c['z'] 0 :::

  There are three additional ~collections.Counter{.interpreted-text role="class"} methods. ~collections.Counter.most_common{.interpreted-text role="meth"} returns the N most common elements and their counts. ~collections.Counter.elements{.interpreted-text role="meth"} returns an iterator over the contained elements, repeating each element as many times as its count. ~collections.Counter.subtract{.interpreted-text role="meth"} takes an iterable and subtracts one for each element instead of adding; if the argument is a dictionary or another Counter{.interpreted-text role="class"}, the counts are subtracted. :

  >>> c.most_common(5)
  [(' ', 6), ('e', 5), ('s', 3), ('a', 2), ('i', 2)]
  >>> c.elements() ->
     'a', 'a', ' ', ' ', ' ', ' ', ' ', ' ',
     'e', 'e', 'e', 'e', 'e', 'g', 'f', 'i', 'i',
     'h', 'h', 'm', 'l', 'l', 'o', 'n', 'p', 's',
     's', 's', 'r', 't', 't', 'x'
  >>> c['e']
  5
  >>> c.subtract('very heavy on the letter e')
  >>> c['e']    # Count is now lower
  -1
  

  Contributed by Raymond Hettinger; 1696199{.interpreted-text role="issue"}.

  New class: ~collections.OrderedDict{.interpreted-text role="class"} is described in the earlier section pep-0372{.interpreted-text role="ref"}.

  New method: The ~collections.deque{.interpreted-text role="class"} data type now has a ~collections.deque.count{.interpreted-text role="meth"} method that returns the number of contained elements equal to the supplied argument x, and a ~collections.deque.reverse{.interpreted-text role="meth"} method that reverses the elements of the deque in-place. ~collections.deque{.interpreted-text role="class"} also exposes its maximum length as the read-only ~collections.deque.maxlen{.interpreted-text role="attr"} attribute. (Both features added by Raymond Hettinger.)

  The ~collections.namedtuple{.interpreted-text role="class"} class now has an optional rename parameter. If rename is true, field names that are invalid because they've been repeated or aren't legal Python identifiers will be renamed to legal names that are derived from the field's position within the list of fields:

  >>> from collections import namedtuple >>> T = namedtuple('T', ['field1', '$illegal', 'for', 'field2'], rename=True) >>> T._fields ('field1', '_1', '_2', 'field2')

  (Added by Raymond Hettinger; 1818{.interpreted-text role="issue"}.)

  Finally, the ~collections.abc.Mapping{.interpreted-text role="class"} abstract base class now returns NotImplemented{.interpreted-text role="data"} if a mapping is compared to another type that isn't a Mapping{.interpreted-text role="class"}. (Fixed by Daniel Stutzbach; 8729{.interpreted-text role="issue"}.)

• Constructors for the parsing classes in the ConfigParser <configparser>{.interpreted-text role="mod"} module now take an allow_no_value parameter, defaulting to false; if true, options without values will be allowed. For example:

  >>> import ConfigParser, StringIO
  >>> sample_config = """
  ... [mysqld]
  ... user = mysql
  ... pid-file = /var/run/mysqld/mysqld.pid
  ... skip-bdb
  ... """
  >>> config = ConfigParser.RawConfigParser(allow_no_value=True)
  >>> config.readfp(StringIO.StringIO(sample_config))
  >>> config.get('mysqld', 'user')
  'mysql'
  >>> print config.get('mysqld', 'skip-bdb')
  None
  >>> print config.get('mysqld', 'unknown')
  Traceback (most recent call last):
    ...
  NoOptionError: No option 'unknown' in section: 'mysqld'
  

  (Contributed by Mats Kindahl; 7005{.interpreted-text role="issue"}.)

• Deprecated function: !contextlib.nested{.interpreted-text role="func"}, which allows handling more than one context manager with a single with{.interpreted-text role="keyword"} statement, has been deprecated, because the !with{.interpreted-text role="keyword"} statement now supports multiple context managers.

• The cookielib <http.cookiejar>{.interpreted-text role="mod"} module now ignores cookies that have an invalid version field, one that doesn't contain an integer value. (Fixed by John J. Lee; 3924{.interpreted-text role="issue"}.)

• The copy{.interpreted-text role="mod"} module's ~copy.deepcopy{.interpreted-text role="func"} function will now correctly copy bound instance methods. (Implemented by Robert Collins; 1515{.interpreted-text role="issue"}.)

• The ctypes{.interpreted-text role="mod"} module now always converts None to a C NULL pointer for arguments declared as pointers. (Changed by Thomas Heller; 4606{.interpreted-text role="issue"}.) The underlying libffi library has been updated to version 3.0.9, containing various fixes for different platforms. (Updated by Matthias Klose; 8142{.interpreted-text role="issue"}.)

• New method: the datetime{.interpreted-text role="mod"} module's ~datetime.timedelta{.interpreted-text role="class"} class gained a ~datetime.timedelta.total_seconds{.interpreted-text role="meth"} method that returns the number of seconds in the duration. (Contributed by Brian Quinlan; 5788{.interpreted-text role="issue"}.)

• New method: the ~decimal.Decimal{.interpreted-text role="class"} class gained a ~decimal.Decimal.from_float{.interpreted-text role="meth"} class method that performs an exact conversion of a floating-point number to a !Decimal{.interpreted-text role="class"}. This exact conversion strives for the closest decimal approximation to the floating-point representation's value; the resulting decimal value will therefore still include the inaccuracy, if any. For example, Decimal.from_float(0.1) returns Decimal('0.1000000000000000055511151231257827021181583404541015625'). (Implemented by Raymond Hettinger; 4796{.interpreted-text role="issue"}.)

  Comparing instances of ~decimal.Decimal{.interpreted-text role="class"} with floating-point numbers now produces sensible results based on the numeric values of the operands. Previously such comparisons would fall back to Python's default rules for comparing objects, which produced arbitrary results based on their type. Note that you still cannot combine !Decimal{.interpreted-text role="class"} and floating point in other operations such as addition, since you should be explicitly choosing how to convert between float and !Decimal{.interpreted-text role="class"}. (Fixed by Mark Dickinson; 2531{.interpreted-text role="issue"}.)

  The constructor for ~decimal.Decimal{.interpreted-text role="class"} now accepts floating-point numbers (added by Raymond Hettinger; 8257{.interpreted-text role="issue"}) and non-European Unicode characters such as Arabic-Indic digits (contributed by Mark Dickinson; 6595{.interpreted-text role="issue"}).

  Most of the methods of the ~decimal.Context{.interpreted-text role="class"} class now accept integers as well as ~decimal.Decimal{.interpreted-text role="class"} instances; the only exceptions are the ~decimal.Context.canonical{.interpreted-text role="meth"} and ~decimal.Context.is_canonical{.interpreted-text role="meth"} methods. (Patch by Juan José Conti; 7633{.interpreted-text role="issue"}.)

  When using ~decimal.Decimal{.interpreted-text role="class"} instances with a string's ~str.format{.interpreted-text role="meth"} method, the default alignment was previously left-alignment. This has been changed to right-alignment, which is more sensible for numeric types. (Changed by Mark Dickinson; 6857{.interpreted-text role="issue"}.)

  Comparisons involving a signaling NaN value (or sNAN) now signal ~decimal.InvalidOperation{.interpreted-text role="const"} instead of silently returning a true or false value depending on the comparison operator. Quiet NaN values (or NaN) are now hashable. (Fixed by Mark Dickinson; 7279{.interpreted-text role="issue"}.)

• The difflib{.interpreted-text role="mod"} module now produces output that is more compatible with modern diff{.interpreted-text role="command"}/patch{.interpreted-text role="command"} tools through one small change, using a tab character instead of spaces as a separator in the header giving the filename. (Fixed by Anatoly Techtonik; 7585{.interpreted-text role="issue"}.)

• The Distutils sdist command now always regenerates the MANIFEST{.interpreted-text role="file"} file, since even if the MANIFEST.in{.interpreted-text role="file"} or setup.py{.interpreted-text role="file"} files haven't been modified, the user might have created some new files that should be included. (Fixed by Tarek Ziadé; 8688{.interpreted-text role="issue"}.)

• The doctest{.interpreted-text role="mod"} module's ~doctest.IGNORE_EXCEPTION_DETAIL{.interpreted-text role="const"} flag will now ignore the name of the module containing the exception being tested. (Patch by Lennart Regebro; 7490{.interpreted-text role="issue"}.)

• The email{.interpreted-text role="mod"} module's ~email.message.Message{.interpreted-text role="class"} class will now accept a Unicode-valued payload, automatically converting the payload to the encoding specified by !output_charset{.interpreted-text role="attr"}. (Added by R. David Murray; 1368247{.interpreted-text role="issue"}.)

• The ~fractions.Fraction{.interpreted-text role="class"} class now accepts a single float or ~decimal.Decimal{.interpreted-text role="class"} instance, or two rational numbers, as arguments to its constructor. (Implemented by Mark Dickinson; rationals added in 5812{.interpreted-text role="issue"}, and float/decimal in 8294{.interpreted-text role="issue"}.)

  Ordering comparisons (<, <=, >, >=) between fractions and complex numbers now raise a TypeError{.interpreted-text role="exc"}. This fixes an oversight, making the ~fractions.Fraction{.interpreted-text role="class"} match the other numeric types.

• New class: ~ftplib.FTP_TLS{.interpreted-text role="class"} in the ftplib{.interpreted-text role="mod"} module provides secure FTP connections using TLS encapsulation of authentication as well as subsequent control and data transfers. (Contributed by Giampaolo Rodola; 2054{.interpreted-text role="issue"}.)

  The ~ftplib.FTP.storbinary{.interpreted-text role="meth"} method for binary uploads can now restart uploads thanks to an added rest parameter (patch by Pablo Mouzo; 6845{.interpreted-text role="issue"}.)

• New class decorator: ~functools.total_ordering{.interpreted-text role="func"} in the functools{.interpreted-text role="mod"} module takes a class that defines an ~object.__eq__{.interpreted-text role="meth"} method and one of ~object.__lt__{.interpreted-text role="meth"}, ~object.__le__{.interpreted-text role="meth"}, ~object.__gt__{.interpreted-text role="meth"}, or ~object.__ge__{.interpreted-text role="meth"}, and generates the missing comparison methods. Since the !__cmp__{.interpreted-text role="meth"} method is being deprecated in Python 3.x, this decorator makes it easier to define ordered classes. (Added by Raymond Hettinger; 5479{.interpreted-text role="issue"}.)

  New function: ~functools.cmp_to_key{.interpreted-text role="func"} will take an old-style comparison function that expects two arguments and return a new callable that can be used as the key parameter to functions such as sorted{.interpreted-text role="func"}, min{.interpreted-text role="func"} and max{.interpreted-text role="func"}, etc. The primary intended use is to help with making code compatible with Python 3.x. (Added by Raymond Hettinger.)

• New function: the gc{.interpreted-text role="mod"} module's ~gc.is_tracked{.interpreted-text role="func"} returns true if a given instance is tracked by the garbage collector, false otherwise. (Contributed by Antoine Pitrou; 4688{.interpreted-text role="issue"}.)

• The gzip{.interpreted-text role="mod"} module's ~gzip.GzipFile{.interpreted-text role="class"} now supports the context management protocol, so you can write with gzip.GzipFile(...) as f: (contributed by Hagen Fürstenau; 3860{.interpreted-text role="issue"}), and it now implements the io.BufferedIOBase{.interpreted-text role="class"} ABC, so you can wrap it with io.BufferedReader{.interpreted-text role="class"} for faster processing (contributed by Nir Aides; 7471{.interpreted-text role="issue"}). It's also now possible to override the modification time recorded in a gzipped file by providing an optional timestamp to the constructor. (Contributed by Jacques Frechet; 4272{.interpreted-text role="issue"}.)

  Files in gzip format can be padded with trailing zero bytes; the gzip{.interpreted-text role="mod"} module will now consume these trailing bytes. (Fixed by Tadek Pietraszek and Brian Curtin; 2846{.interpreted-text role="issue"}.)

• New attribute: the hashlib{.interpreted-text role="mod"} module now has an !algorithms{.interpreted-text role="attr"} attribute containing a tuple naming the supported algorithms. In Python 2.7, hashlib.algorithms contains ('md5', 'sha1', 'sha224', 'sha256', 'sha384', 'sha512'). (Contributed by Carl Chenet; 7418{.interpreted-text role="issue"}.)

• The default ~http.client.HTTPResponse{.interpreted-text role="class"} class used by the httplib <http>{.interpreted-text role="mod"} module now supports buffering, resulting in much faster reading of HTTP responses. (Contributed by Kristján Valur Jónsson; 4879{.interpreted-text role="issue"}.)

  The ~http.client.HTTPConnection{.interpreted-text role="class"} and ~http.client.HTTPSConnection{.interpreted-text role="class"} classes now support a source_address parameter, a (host, port) 2-tuple giving the source address that will be used for the connection. (Contributed by Eldon Ziegler; 3972{.interpreted-text role="issue"}.)

• The !ihooks{.interpreted-text role="mod"} module now supports relative imports. Note that !ihooks{.interpreted-text role="mod"} is an older module for customizing imports, superseded by the !imputil{.interpreted-text role="mod"} module added in Python 2.0. (Relative import support added by Neil Schemenauer.)

• The imaplib{.interpreted-text role="mod"} module now supports IPv6 addresses. (Contributed by Derek Morr; 1655{.interpreted-text role="issue"}.)

• New function: the inspect{.interpreted-text role="mod"} module's ~inspect.getcallargs{.interpreted-text role="func"} takes a callable and its positional and keyword arguments, and figures out which of the callable's parameters will receive each argument, returning a dictionary mapping argument names to their values. For example:

  >>> from inspect import getcallargs
  >>> def f(a, b=1, *pos, **named):
  ...     pass
  ...
  >>> getcallargs(f, 1, 2, 3)
  {'a': 1, 'b': 2, 'pos': (3,), 'named': {}}
  >>> getcallargs(f, a=2, x=4)
  {'a': 2, 'b': 1, 'pos': (), 'named': {'x': 4}}
  >>> getcallargs(f)
  Traceback (most recent call last):
  ...
  TypeError: f() takes at least 1 argument (0 given)
  

  Contributed by George Sakkis; 3135{.interpreted-text role="issue"}.

• Updated module: The io{.interpreted-text role="mod"} library has been upgraded to the version shipped with Python 3.1. For 3.1, the I/O library was entirely rewritten in C and is 2 to 20 times faster depending on the task being performed. The original Python version was renamed to the !_pyio{.interpreted-text role="mod"} module.

  One minor resulting change: the io.TextIOBase{.interpreted-text role="class"} class now has an ~io.TextIOBase.errors{.interpreted-text role="attr"} attribute giving the error setting used for encoding and decoding errors (one of 'strict', 'replace', 'ignore').

  The io.FileIO{.interpreted-text role="class"} class now raises an OSError{.interpreted-text role="exc"} when passed an invalid file descriptor. (Implemented by Benjamin Peterson; 4991{.interpreted-text role="issue"}.) The ~io.IOBase.truncate{.interpreted-text role="meth"} method now preserves the file position; previously it would change the file position to the end of the new file. (Fixed by Pascal Chambon; 6939{.interpreted-text role="issue"}.)

• New function: itertools.compress(data, selectors) takes two iterators. Elements of data are returned if the corresponding value in selectors is true:

  itertools.compress('ABCDEF', [1,0,1,0,1,1]) =>
    A, C, E, F
  

  New function: itertools.combinations_with_replacement(iter, r) returns all the possible r-length combinations of elements from the iterable iter. Unlike ~itertools.combinations{.interpreted-text role="func"}, individual elements can be repeated in the generated combinations:

  itertools.combinations_with_replacement('abc', 2) =>
    ('a', 'a'), ('a', 'b'), ('a', 'c'),
    ('b', 'b'), ('b', 'c'), ('c', 'c')
  

  Note that elements are treated as unique depending on their position in the input, not their actual values.

  The itertools.count{.interpreted-text role="func"} function now has a step argument that allows incrementing by values other than 1. ~itertools.count{.interpreted-text role="func"} also now allows keyword arguments, and using non-integer values such as floats or ~decimal.Decimal{.interpreted-text role="class"} instances. (Implemented by Raymond Hettinger; 5032{.interpreted-text role="issue"}.)

  itertools.combinations{.interpreted-text role="func"} and itertools.product{.interpreted-text role="func"} previously raised ValueError{.interpreted-text role="exc"} for values of r larger than the input iterable. This was deemed a specification error, so they now return an empty iterator. (Fixed by Raymond Hettinger; 4816{.interpreted-text role="issue"}.)

• Updated module: The json{.interpreted-text role="mod"} module was upgraded to version 2.0.9 of the simplejson package, which includes a C extension that makes encoding and decoding faster. (Contributed by Bob Ippolito; 4136{.interpreted-text role="issue"}.)

  To support the new collections.OrderedDict{.interpreted-text role="class"} type, json.load{.interpreted-text role="func"} now has an optional object_pairs_hook parameter that will be called with any object literal that decodes to a list of pairs. (Contributed by Raymond Hettinger; 5381{.interpreted-text role="issue"}.)

• The mailbox{.interpreted-text role="mod"} module's ~mailbox.Maildir{.interpreted-text role="class"} class now records the timestamp on the directories it reads, and only re-reads them if the modification time has subsequently changed. This improves performance by avoiding unneeded directory scans. (Fixed by A.M. Kuchling and Antoine Pitrou; 1607951{.interpreted-text role="issue"}, 6896{.interpreted-text role="issue"}.)

• New functions: the math{.interpreted-text role="mod"} module gained ~math.erf{.interpreted-text role="func"} and ~math.erfc{.interpreted-text role="func"} for the error function and the complementary error function, ~math.expm1{.interpreted-text role="func"} which computes e**x - 1 with more precision than using ~math.exp{.interpreted-text role="func"} and subtracting 1, ~math.gamma{.interpreted-text role="func"} for the Gamma function, and ~math.lgamma{.interpreted-text role="func"} for the natural log of the Gamma function. (Contributed by Mark Dickinson and nirinA raseliarison; 3366{.interpreted-text role="issue"}.)

• The multiprocessing{.interpreted-text role="mod"} module's !Manager*{.interpreted-text role="class"} classes can now be passed a callable that will be called whenever a subprocess is started, along with a set of arguments that will be passed to the callable. (Contributed by lekma; 5585{.interpreted-text role="issue"}.)

  The ~multiprocessing.pool.Pool{.interpreted-text role="class"} class, which controls a pool of worker processes, now has an optional maxtasksperchild parameter. Worker processes will perform the specified number of tasks and then exit, causing the !Pool{.interpreted-text role="class"} to start a new worker. This is useful if tasks may leak memory or other resources, or if some tasks will cause the worker to become very large. (Contributed by Charles Cazabon; 6963{.interpreted-text role="issue"}.)

• The !nntplib{.interpreted-text role="mod"} module now supports IPv6 addresses. (Contributed by Derek Morr; 1664{.interpreted-text role="issue"}.)

• New functions: the os{.interpreted-text role="mod"} module wraps the following POSIX system calls: ~os.getresgid{.interpreted-text role="func"} and ~os.getresuid{.interpreted-text role="func"}, which return the real, effective, and saved GIDs and UIDs; ~os.setresgid{.interpreted-text role="func"} and ~os.setresuid{.interpreted-text role="func"}, which set real, effective, and saved GIDs and UIDs to new values; ~os.initgroups{.interpreted-text role="func"}, which initialize the group access list for the current process. (GID/UID functions contributed by Travis H.; 6508{.interpreted-text role="issue"}. Support for initgroups added by Jean-Paul Calderone; 7333{.interpreted-text role="issue"}.)

  The os.fork{.interpreted-text role="func"} function now re-initializes the import lock in the child process; this fixes problems on Solaris when ~os.fork{.interpreted-text role="func"} is called from a thread. (Fixed by Zsolt Cserna; 7242{.interpreted-text role="issue"}.)

• In the os.path{.interpreted-text role="mod"} module, the ~os.path.normpath{.interpreted-text role="func"} and ~os.path.abspath{.interpreted-text role="func"} functions now preserve Unicode; if their input path is a Unicode string, the return value is also a Unicode string. (~os.path.normpath{.interpreted-text role="meth"} fixed by Matt Giuca in 5827{.interpreted-text role="issue"}; ~os.path.abspath{.interpreted-text role="meth"} fixed by Ezio Melotti in 3426{.interpreted-text role="issue"}.)

• The pydoc{.interpreted-text role="mod"} module now has help for the various symbols that Python uses. You can now do help('<<') or help('@'), for example. (Contributed by David Laban; 4739{.interpreted-text role="issue"}.)

• The re{.interpreted-text role="mod"} module's ~re.split{.interpreted-text role="func"}, ~re.sub{.interpreted-text role="func"}, and ~re.subn{.interpreted-text role="func"} now accept an optional flags argument, for consistency with the other functions in the module. (Added by Gregory P. Smith.)

• New function: ~runpy.run_path{.interpreted-text role="func"} in the runpy{.interpreted-text role="mod"} module will execute the code at a provided path argument. path can be the path of a Python source file (example.py{.interpreted-text role="file"}), a compiled bytecode file (example.pyc{.interpreted-text role="file"}), a directory (./package/{.interpreted-text role="file"}), or a zip archive (example.zip{.interpreted-text role="file"}). If a directory or zip path is provided, it will be added to the front of sys.path and the module __main__{.interpreted-text role="mod"} will be imported. It's expected that the directory or zip contains a __main__.py{.interpreted-text role="file"}; if it doesn't, some other __main__.py{.interpreted-text role="file"} might be imported from a location later in sys.path. This makes more of the machinery of runpy{.interpreted-text role="mod"} available to scripts that want to mimic the way Python's command line processes an explicit path name. (Added by Nick Coghlan; 6816{.interpreted-text role="issue"}.)

• New function: in the shutil{.interpreted-text role="mod"} module, ~shutil.make_archive{.interpreted-text role="func"} takes a filename, archive type (zip or tar-format), and a directory path, and creates an archive containing the directory's contents. (Added by Tarek Ziadé.)

  shutil{.interpreted-text role="mod"}'s ~shutil.copyfile{.interpreted-text role="func"} and ~shutil.copytree{.interpreted-text role="func"} functions now raise a ~shutil.SpecialFileError{.interpreted-text role="exc"} exception when asked to copy a named pipe. Previously the code would treat named pipes like a regular file by opening them for reading, and this would block indefinitely. (Fixed by Antoine Pitrou; 3002{.interpreted-text role="issue"}.)

• The signal{.interpreted-text role="mod"} module no longer re-installs the signal handler unless this is truly necessary, which fixes a bug that could make it impossible to catch the EINTR signal robustly. (Fixed by Charles-Francois Natali; 8354{.interpreted-text role="issue"}.)

• New functions: in the site{.interpreted-text role="mod"} module, three new functions return various site- and user-specific paths. ~site.getsitepackages{.interpreted-text role="func"} returns a list containing all global site-packages directories, ~site.getusersitepackages{.interpreted-text role="func"} returns the path of the user's site-packages directory, and ~site.getuserbase{.interpreted-text role="func"} returns the value of the ~site.USER_BASE{.interpreted-text role="data"} environment variable, giving the path to a directory that can be used to store data. (Contributed by Tarek Ziadé; 6693{.interpreted-text role="issue"}.)

  The site{.interpreted-text role="mod"} module now reports exceptions occurring when the sitecustomize{.interpreted-text role="mod"} module is imported, and will no longer catch and swallow the KeyboardInterrupt{.interpreted-text role="exc"} exception. (Fixed by Victor Stinner; 3137{.interpreted-text role="issue"}.)

• The ~socket.create_connection{.interpreted-text role="func"} function gained a source_address parameter, a (host, port) 2-tuple giving the source address that will be used for the connection. (Contributed by Eldon Ziegler; 3972{.interpreted-text role="issue"}.)

  The ~socket.socket.recv_into{.interpreted-text role="meth"} and ~socket.socket.recvfrom_into{.interpreted-text role="meth"} methods will now write into objects that support the buffer API, most usefully the bytearray{.interpreted-text role="class"} and memoryview{.interpreted-text role="class"} objects. (Implemented by Antoine Pitrou; 8104{.interpreted-text role="issue"}.)

• The SocketServer <socketserver>{.interpreted-text role="mod"} module's ~socketserver.TCPServer{.interpreted-text role="class"} class now supports socket timeouts and disabling the Nagle algorithm. The !disable_nagle_algorithm{.interpreted-text role="attr"} class attribute defaults to False; if overridden to be true, new request connections will have the TCP_NODELAY option set to prevent buffering many small sends into a single TCP packet. The ~socketserver.BaseServer.timeout{.interpreted-text role="attr"} class attribute can hold a timeout in seconds that will be applied to the request socket; if no request is received within that time, ~socketserver.BaseServer.handle_timeout{.interpreted-text role="meth"} will be called and ~socketserver.BaseServer.handle_request{.interpreted-text role="meth"} will return. (Contributed by Kristján Valur Jónsson; 6192{.interpreted-text role="issue"} and 6267{.interpreted-text role="issue"}.)

• Updated module: the sqlite3{.interpreted-text role="mod"} module has been updated to version 2.6.0 of the pysqlite package. Version 2.6.0 includes a number of bugfixes, and adds the ability to load SQLite extensions from shared libraries. Call the enable_load_extension(True) method to enable extensions, and then call ~sqlite3.Connection.load_extension{.interpreted-text role="meth"} to load a particular shared library. (Updated by Gerhard Häring.)

• The ssl{.interpreted-text role="mod"} module's ~ssl.SSLSocket{.interpreted-text role="class"} objects now support the buffer API, which fixed a test suite failure (fix by Antoine Pitrou; 7133{.interpreted-text role="issue"}) and automatically set OpenSSL's !SSL_MODE_AUTO_RETRY{.interpreted-text role="c:macro"}, which will prevent an error code being returned from !recv{.interpreted-text role="meth"} operations that trigger an SSL renegotiation (fix by Antoine Pitrou; 8222{.interpreted-text role="issue"}).

  The ~ssl.SSLContext.wrap_socket{.interpreted-text role="func"} constructor function now takes a ciphers argument that's a string listing the encryption algorithms to be allowed; the format of the string is described in the OpenSSL documentation. (Added by Antoine Pitrou; 8322{.interpreted-text role="issue"}.)

  Another change makes the extension load all of OpenSSL's ciphers and digest algorithms so that they're all available. Some SSL certificates couldn't be verified, reporting an "unknown algorithm" error. (Reported by Beda Kosata, and fixed by Antoine Pitrou; 8484{.interpreted-text role="issue"}.)

  The version of OpenSSL being used is now available as the module attributes ssl.OPENSSL_VERSION{.interpreted-text role="const"} (a string), ssl.OPENSSL_VERSION_INFO{.interpreted-text role="const"} (a 5-tuple), and ssl.OPENSSL_VERSION_NUMBER{.interpreted-text role="const"} (an integer). (Added by Antoine Pitrou; 8321{.interpreted-text role="issue"}.)

• The struct{.interpreted-text role="mod"} module will no longer silently ignore overflow errors when a value is too large for a particular integer format code (one of bBhHiIlLqQ); it now always raises a struct.error{.interpreted-text role="exc"} exception. (Changed by Mark Dickinson; 1523{.interpreted-text role="issue"}.) The ~struct.pack{.interpreted-text role="func"} function will also attempt to use ~object.__index__{.interpreted-text role="meth"} to convert and pack non-integers before trying the ~object.__int__{.interpreted-text role="meth"} method or reporting an error. (Changed by Mark Dickinson; 8300{.interpreted-text role="issue"}.)

• New function: the subprocess{.interpreted-text role="mod"} module's ~subprocess.check_output{.interpreted-text role="func"} runs a command with a specified set of arguments and returns the command's output as a string when the command runs without error, or raises a ~subprocess.CalledProcessError{.interpreted-text role="exc"} exception otherwise.

  >>> subprocess.check_output(['df', '-h', '.'])
  'Filesystem     Size   Used  Avail Capacity  Mounted on\n
  /dev/disk0s2    52G    49G   3.0G    94%    /\n'
  
  >>> subprocess.check_output(['df', '-h', '/bogus'])
    ...
  subprocess.CalledProcessError: Command '['df', '-h', '/bogus']' returned non-zero exit status 1
  

  (Contributed by Gregory P. Smith.)

  The subprocess{.interpreted-text role="mod"} module will now retry its internal system calls on receiving an ~errno.EINTR{.interpreted-text role="const"} signal. (Reported by several people; final patch by Gregory P. Smith in 1068268{.interpreted-text role="issue"}.)

• New function: ~symtable.Symbol.is_declared_global{.interpreted-text role="func"} in the symtable{.interpreted-text role="mod"} module returns true for variables that are explicitly declared to be global, false for ones that are implicitly global. (Contributed by Jeremy Hylton.)

• The syslog{.interpreted-text role="mod"} module will now use the value of sys.argv[0] as the identifier instead of the previous default value of 'python'. (Changed by Sean Reifschneider; 8451{.interpreted-text role="issue"}.)

• The sys.version_info{.interpreted-text role="data"} value is now a named tuple, with attributes named !major{.interpreted-text role="attr"}, !minor{.interpreted-text role="attr"}, !micro{.interpreted-text role="attr"}, !releaselevel{.interpreted-text role="attr"}, and !serial{.interpreted-text role="attr"}. (Contributed by Ross Light; 4285{.interpreted-text role="issue"}.)

  sys.getwindowsversion{.interpreted-text role="func"} also returns a named tuple, with attributes named !major{.interpreted-text role="attr"}, !minor{.interpreted-text role="attr"}, !build{.interpreted-text role="attr"}, !platform{.interpreted-text role="attr"}, !service_pack{.interpreted-text role="attr"}, !service_pack_major{.interpreted-text role="attr"}, !service_pack_minor{.interpreted-text role="attr"}, !suite_mask{.interpreted-text role="attr"}, and !product_type{.interpreted-text role="attr"}. (Contributed by Brian Curtin; 7766{.interpreted-text role="issue"}.)

• The tarfile{.interpreted-text role="mod"} module's default error handling has changed, to no longer suppress fatal errors. The default error level was previously 0, which meant that errors would only result in a message being written to the debug log, but because the debug log is not activated by default, these errors go unnoticed. The default error level is now 1, which raises an exception if there's an error. (Changed by Lars Gustäbel; 7357{.interpreted-text role="issue"}.)

  tarfile{.interpreted-text role="mod"} now supports filtering the ~tarfile.TarInfo{.interpreted-text role="class"} objects being added to a tar file. When you call ~tarfile.TarFile.add{.interpreted-text role="meth"}, you may supply an optional filter argument that's a callable. The filter callable will be passed the ~tarfile.TarInfo{.interpreted-text role="class"} for every file being added, and can modify and return it. If the callable returns None, the file will be excluded from the resulting archive. This is more powerful than the existing exclude argument, which has therefore been deprecated. (Added by Lars Gustäbel; 6856{.interpreted-text role="issue"}.) The ~tarfile.TarFile{.interpreted-text role="class"} class also now supports the context management protocol. (Added by Lars Gustäbel; 7232{.interpreted-text role="issue"}.)

• The ~threading.Event.wait{.interpreted-text role="meth"} method of the threading.Event{.interpreted-text role="class"} class now returns the internal flag on exit. This means the method will usually return true because ~threading.Event.wait{.interpreted-text role="meth"} is supposed to block until the internal flag becomes true. The return value will only be false if a timeout was provided and the operation timed out. (Contributed by Tim Lesher; 1674032{.interpreted-text role="issue"}.)

• The Unicode database provided by the unicodedata{.interpreted-text role="mod"} module is now used internally to determine which characters are numeric, whitespace, or represent line breaks. The database also includes information from the Unihan.txt{.interpreted-text role="file"} data file (patch by Anders Chrigström and Amaury Forgeot d'Arc; 1571184{.interpreted-text role="issue"}) and has been updated to version 5.2.0 (updated by Florent Xicluna; 8024{.interpreted-text role="issue"}).

• The urlparse <urllib.parse>{.interpreted-text role="mod"} module's ~urllib.parse.urlsplit{.interpreted-text role="func"} now handles unknown URL schemes in a fashion compliant with 3986{.interpreted-text role="rfc"}: if the URL is of the form "<something>://...", the text before the :// is treated as the scheme, even if it's a made-up scheme that the module doesn't know about. This change may break code that worked around the old behaviour. For example, Python 2.6.4 or 2.5 will return the following:

  ::: {.doctest options="+SKIP"} >>> import urlparse >>> urlparse.urlsplit('invented://host/filename?query') ('invented', '', '//host/filename?query', '', '') :::

  Python 2.7 (and Python 2.6.5) will return:

  ::: {.doctest options="+SKIP"} >>> import urlparse >>> urlparse.urlsplit('invented://host/filename?query') ('invented', 'host', '/filename?query', '', '') :::

  (Python 2.7 actually produces slightly different output, since it returns a named tuple instead of a standard tuple.)

  The urlparse <urllib.parse>{.interpreted-text role="mod"} module also supports IPv6 literal addresses as defined by 2732{.interpreted-text role="rfc"} (contributed by Senthil Kumaran; 2987{.interpreted-text role="issue"}).

  ::: {.doctest options="+SKIP"} >>> urlparse.urlparse('http://[1080::8:800:200C:417A]/foo') ParseResult(scheme='http', netloc='[1080::8:800:200C:417A]', path='/foo', params='', query='', fragment='') :::

• New class: the ~weakref.WeakSet{.interpreted-text role="class"} class in the weakref{.interpreted-text role="mod"} module is a set that only holds weak references to its elements; elements will be removed once there are no references pointing to them. (Originally implemented in Python 3.x by Raymond Hettinger, and backported to 2.7 by Michael Foord.)

• The xml.etree.ElementTree{.interpreted-text role="mod"} library, no longer escapes ampersands and angle brackets when outputting an XML processing instruction (which looks like <?xml-stylesheet href="#style1"?>) or comment (which looks like <!-- comment -->). (Patch by Neil Muller; 2746{.interpreted-text role="issue"}.)

• The XML-RPC client and server, provided by the xmlrpclib <xmlrpc.client>{.interpreted-text role="mod"} and SimpleXMLRPCServer <xmlrpc.server>{.interpreted-text role="mod"} modules, have improved performance by supporting HTTP/1.1 keep-alive and by optionally using gzip encoding to compress the XML being exchanged. The gzip compression is controlled by the !encode_threshold{.interpreted-text role="attr"} attribute of ~xmlrpc.server.SimpleXMLRPCRequestHandler{.interpreted-text role="class"}, which contains a size in bytes; responses larger than this will be compressed. (Contributed by Kristján Valur Jónsson; 6267{.interpreted-text role="issue"}.)

• The zipfile{.interpreted-text role="mod"} module's ~zipfile.ZipFile{.interpreted-text role="class"} now supports the context management protocol, so you can write with zipfile.ZipFile(...) as f:. (Contributed by Brian Curtin; 5511{.interpreted-text role="issue"}.)

  zipfile{.interpreted-text role="mod"} now also supports archiving empty directories and extracts them correctly. (Fixed by Kuba Wieczorek; 4710{.interpreted-text role="issue"}.) Reading files out of an archive is faster, and interleaving read() <io.BufferedIOBase.read>{.interpreted-text role="meth"} and readline() <io.IOBase.readline>{.interpreted-text role="meth"} now works correctly. (Contributed by Nir Aides; 7610{.interpreted-text role="issue"}.)

  The ~zipfile.is_zipfile{.interpreted-text role="func"} function now accepts a file object, in addition to the path names accepted in earlier versions. (Contributed by Gabriel Genellina; 4756{.interpreted-text role="issue"}.)

  The ~zipfile.ZipFile.writestr{.interpreted-text role="meth"} method now has an optional compress_type parameter that lets you override the default compression method specified in the ~zipfile.ZipFile{.interpreted-text role="class"} constructor. (Contributed by Ronald Oussoren; 6003{.interpreted-text role="issue"}.)

New module: importlib {#importlib-section}

Python 3.1 includes the importlib{.interpreted-text role="mod"} package, a re-implementation of the logic underlying Python's import{.interpreted-text role="keyword"} statement. importlib{.interpreted-text role="mod"} is useful for implementers of Python interpreters and to users who wish to write new importers that can participate in the import process. Python 2.7 doesn't contain the complete importlib{.interpreted-text role="mod"} package, but instead has a tiny subset that contains a single function, ~importlib.import_module{.interpreted-text role="func"}.

import_module(name, package=None) imports a module. name is a string containing the module or package's name. It's possible to do relative imports by providing a string that begins with a . character, such as ..utils.errors. For relative imports, the package argument must be provided and is the name of the package that will be used as the anchor for the relative import. ~importlib.import_module{.interpreted-text role="func"} both inserts the imported module into sys.modules and returns the module object.

Here are some examples:

>>> from importlib import import_module
>>> anydbm = import_module('anydbm')  # Standard absolute import
>>> anydbm
<module 'anydbm' from '/p/python/Lib/anydbm.py'>
>>> # Relative import
>>> file_util = import_module('..file_util', 'distutils.command')
>>> file_util
<module 'distutils.file_util' from '/python/Lib/distutils/file_util.pyc'>

importlib{.interpreted-text role="mod"} was implemented by Brett Cannon and introduced in Python 3.1.

New module: sysconfig

The sysconfig{.interpreted-text role="mod"} module has been pulled out of the Distutils package, becoming a new top-level module in its own right. sysconfig{.interpreted-text role="mod"} provides functions for getting information about Python's build process: compiler switches, installation paths, the platform name, and whether Python is running from its source directory.

Some of the functions in the module are:

• ~sysconfig.get_config_var{.interpreted-text role="func"} returns variables from Python's Makefile and the pyconfig.h{.interpreted-text role="file"} file.
• ~sysconfig.get_config_vars{.interpreted-text role="func"} returns a dictionary containing all of the configuration variables.
• ~sysconfig.get_path{.interpreted-text role="func"} returns the configured path for a particular type of module: the standard library, site-specific modules, platform-specific modules, etc.
• ~sysconfig.is_python_build{.interpreted-text role="func"} returns true if you're running a binary from a Python source tree, and false otherwise.

Consult the sysconfig{.interpreted-text role="mod"} documentation for more details and for a complete list of functions.

The Distutils package and sysconfig{.interpreted-text role="mod"} are now maintained by Tarek Ziadé, who has also started a Distutils2 package (source repository at https://hg.python.org/distutils2/) for developing a next-generation version of Distutils.

ttk: Themed Widgets for Tk

Tcl/Tk 8.5 includes a set of themed widgets that re-implement basic Tk widgets but have a more customizable appearance and can therefore more closely resemble the native platform's widgets. This widget set was originally called Tile, but was renamed to Ttk (for "themed Tk") on being added to Tcl/Tck release 8.5.

To learn more, read the ~tkinter.ttk{.interpreted-text role="mod"} module documentation. You may also wish to read the Tcl/Tk manual page describing the Ttk theme engine, available at https://www.tcl.tk/man/tcl8.5/TkCmd/ttk_intro.html. Some screenshots of the Python/Ttk code in use are at https://code.google.com/archive/p/python-ttk/wikis/Screenshots.wiki.

The tkinter.ttk{.interpreted-text role="mod"} module was written by Guilherme Polo and added in 2983{.interpreted-text role="issue"}. An alternate version called Tile.py, written by Martin Franklin and maintained by Kevin Walzer, was proposed for inclusion in 2618{.interpreted-text role="issue"}, but the authors argued that Guilherme Polo's work was more comprehensive.

Updated module: unittest {#unittest-section}

The unittest{.interpreted-text role="mod"} module was greatly enhanced; many new features were added. Most of these features were implemented by Michael Foord, unless otherwise noted. The enhanced version of the module is downloadable separately for use with Python versions 2.4 to 2.6, packaged as the !unittest2{.interpreted-text role="mod"} package, from unittest2{.interpreted-text role="pypi"}.

When used from the command line, the module can automatically discover tests. It's not as fancy as py.test or nose, but provides a simple way to run tests kept within a set of package directories. For example, the following command will search the test/{.interpreted-text role="file"} subdirectory for any importable test files named test*.py:

python -m unittest discover -s test

Consult the unittest{.interpreted-text role="mod"} module documentation for more details. (Developed in 6001{.interpreted-text role="issue"}.)

The ~unittest.main{.interpreted-text role="func"} function supports some other new options:

• -b <unittest -b>{.interpreted-text role="option"} or !--buffer{.interpreted-text role="option"} will buffer the standard output and standard error streams during each test. If the test passes, any resulting output will be discarded; on failure, the buffered output will be displayed.

• -c <unittest -c>{.interpreted-text role="option"} or !--catch{.interpreted-text role="option"} will cause the control-C interrupt to be handled more gracefully. Instead of interrupting the test process immediately, the currently running test will be completed and then the partial results up to the interruption will be reported. If you're impatient, a second press of control-C will cause an immediate interruption.

  This control-C handler tries to avoid causing problems when the code being tested or the tests being run have defined a signal handler of their own, by noticing that a signal handler was already set and calling it. If this doesn't work for you, there's a ~unittest.removeHandler{.interpreted-text role="func"} decorator that can be used to mark tests that should have the control-C handling disabled.

• -f <unittest -f>{.interpreted-text role="option"} or !--failfast{.interpreted-text role="option"} makes test execution stop immediately when a test fails instead of continuing to execute further tests. (Suggested by Cliff Dyer and implemented by Michael Foord; 8074{.interpreted-text role="issue"}.)

The progress messages now show 'x' for expected failures and 'u' for unexpected successes when run in verbose mode. (Contributed by Benjamin Peterson.)

Test cases can raise the ~unittest.SkipTest{.interpreted-text role="exc"} exception to skip a test (1034053{.interpreted-text role="issue"}).

The error messages for ~unittest.TestCase.assertEqual{.interpreted-text role="meth"}, ~unittest.TestCase.assertTrue{.interpreted-text role="meth"}, and ~unittest.TestCase.assertFalse{.interpreted-text role="meth"} failures now provide more information. If you set the ~unittest.TestCase.longMessage{.interpreted-text role="attr"} attribute of your ~unittest.TestCase{.interpreted-text role="class"} classes to true, both the standard error message and any additional message you provide will be printed for failures. (Added by Michael Foord; 5663{.interpreted-text role="issue"}.)

The ~unittest.TestCase.assertRaises{.interpreted-text role="meth"} method now returns a context handler when called without providing a callable object to run. For example, you can write this:

with self.assertRaises(KeyError):
    {}['foo']

(Implemented by Antoine Pitrou; 4444{.interpreted-text role="issue"}.)

Module- and class-level setup and teardown fixtures are now supported. Modules can contain ~unittest.setUpModule{.interpreted-text role="func"} and ~unittest.tearDownModule{.interpreted-text role="func"} functions. Classes can have ~unittest.TestCase.setUpClass{.interpreted-text role="meth"} and ~unittest.TestCase.tearDownClass{.interpreted-text role="meth"} methods that must be defined as class methods (using @classmethod or equivalent). These functions and methods are invoked when the test runner switches to a test case in a different module or class.

The methods ~unittest.TestCase.addCleanup{.interpreted-text role="meth"} and ~unittest.TestCase.doCleanups{.interpreted-text role="meth"} were added. ~unittest.TestCase.addCleanup{.interpreted-text role="meth"} lets you add cleanup functions that will be called unconditionally (after ~unittest.TestCase.setUp{.interpreted-text role="meth"} if ~unittest.TestCase.setUp{.interpreted-text role="meth"} fails, otherwise after ~unittest.TestCase.tearDown{.interpreted-text role="meth"}). This allows for much simpler resource allocation and deallocation during tests (5679{.interpreted-text role="issue"}).

A number of new methods were added that provide more specialized tests. Many of these methods were written by Google engineers for use in their test suites; Gregory P. Smith, Michael Foord, and GvR worked on merging them into Python's version of unittest{.interpreted-text role="mod"}.

• ~unittest.TestCase.assertIsNone{.interpreted-text role="meth"} and ~unittest.TestCase.assertIsNotNone{.interpreted-text role="meth"} take one expression and verify that the result is or is not None.
• ~unittest.TestCase.assertIs{.interpreted-text role="meth"} and ~unittest.TestCase.assertIsNot{.interpreted-text role="meth"} take two values and check whether the two values evaluate to the same object or not. (Added by Michael Foord; 2578{.interpreted-text role="issue"}.)
• ~unittest.TestCase.assertIsInstance{.interpreted-text role="meth"} and ~unittest.TestCase.assertNotIsInstance{.interpreted-text role="meth"} check whether the resulting object is an instance of a particular class, or of one of a tuple of classes. (Added by Georg Brandl; 7031{.interpreted-text role="issue"}.)
• ~unittest.TestCase.assertGreater{.interpreted-text role="meth"}, ~unittest.TestCase.assertGreaterEqual{.interpreted-text role="meth"}, ~unittest.TestCase.assertLess{.interpreted-text role="meth"}, and ~unittest.TestCase.assertLessEqual{.interpreted-text role="meth"} compare two quantities.
• ~unittest.TestCase.assertMultiLineEqual{.interpreted-text role="meth"} compares two strings, and if they're not equal, displays a helpful comparison that highlights the differences in the two strings. This comparison is now used by default when Unicode strings are compared with ~unittest.TestCase.assertEqual{.interpreted-text role="meth"}.
• assertRegexpMatches() <unittest.TestCase.assertRegex>{.interpreted-text role="meth"} and assertNotRegexpMatches() <unittest.TestCase.assertNotRegex>{.interpreted-text role="meth"} checks whether the first argument is a string matching or not matching the regular expression provided as the second argument (8038{.interpreted-text role="issue"}).
• assertRaisesRegexp() <unittest.TestCase.assertRaisesRegex>{.interpreted-text role="meth"} checks whether a particular exception is raised, and then also checks that the string representation of the exception matches the provided regular expression.
• ~unittest.TestCase.assertIn{.interpreted-text role="meth"} and ~unittest.TestCase.assertNotIn{.interpreted-text role="meth"} tests whether first is or is not in second.
• assertItemsEqual() <unittest.TestCase.assertCountEqual>{.interpreted-text role="meth"} tests whether two provided sequences contain the same elements.
• ~unittest.TestCase.assertSetEqual{.interpreted-text role="meth"} compares whether two sets are equal, and only reports the differences between the sets in case of error.
• Similarly, ~unittest.TestCase.assertListEqual{.interpreted-text role="meth"} and ~unittest.TestCase.assertTupleEqual{.interpreted-text role="meth"} compare the specified types and explain any differences without necessarily printing their full values; these methods are now used by default when comparing lists and tuples using ~unittest.TestCase.assertEqual{.interpreted-text role="meth"}. More generally, ~unittest.TestCase.assertSequenceEqual{.interpreted-text role="meth"} compares two sequences and can optionally check whether both sequences are of a particular type.
• ~unittest.TestCase.assertDictEqual{.interpreted-text role="meth"} compares two dictionaries and reports the differences; it's now used by default when you compare two dictionaries using ~unittest.TestCase.assertEqual{.interpreted-text role="meth"}. !assertDictContainsSubset{.interpreted-text role="meth"} checks whether all of the key/value pairs in first are found in second.
• ~unittest.TestCase.assertAlmostEqual{.interpreted-text role="meth"} and ~unittest.TestCase.assertNotAlmostEqual{.interpreted-text role="meth"} test whether first and second are approximately equal. This method can either round their difference to an optionally specified number of places (the default is 7) and compare it to zero, or require the difference to be smaller than a supplied delta value.
• ~unittest.TestLoader.loadTestsFromName{.interpreted-text role="meth"} properly honors the ~unittest.TestLoader.suiteClass{.interpreted-text role="attr"} attribute of the ~unittest.TestLoader{.interpreted-text role="class"}. (Fixed by Mark Roddy; 6866{.interpreted-text role="issue"}.)
• A new hook lets you extend the ~unittest.TestCase.assertEqual{.interpreted-text role="meth"} method to handle new data types. The ~unittest.TestCase.addTypeEqualityFunc{.interpreted-text role="meth"} method takes a type object and a function. The function will be used when both of the objects being compared are of the specified type. This function should compare the two objects and raise an exception if they don't match; it's a good idea for the function to provide additional information about why the two objects aren't matching, much as the new sequence comparison methods do.

unittest.main{.interpreted-text role="func"} now takes an optional exit argument. If false, ~unittest.main{.interpreted-text role="func"} doesn't call sys.exit{.interpreted-text role="func"}, allowing ~unittest.main{.interpreted-text role="func"} to be used from the interactive interpreter. (Contributed by J. Pablo Fernández; 3379{.interpreted-text role="issue"}.)

~unittest.TestResult{.interpreted-text role="class"} has new ~unittest.TestResult.startTestRun{.interpreted-text role="meth"} and ~unittest.TestResult.stopTestRun{.interpreted-text role="meth"} methods that are called immediately before and after a test run. (Contributed by Robert Collins; 5728{.interpreted-text role="issue"}.)

With all these changes, the unittest.py{.interpreted-text role="file"} was becoming awkwardly large, so the module was turned into a package and the code split into several files (by Benjamin Peterson). This doesn't affect how the module is imported or used.

::: seealso

https://web.archive.org/web/20210619163128/http://www.voidspace.org.uk/python/articles/unittest2.shtml

: Describes the new features, how to use them, and the rationale for various design decisions. (By Michael Foord.) :::

Updated module: ElementTree 1.3 {#elementtree-section}

The version of the ElementTree library included with Python was updated to version 1.3. Some of the new features are:

• The various parsing functions now take a parser keyword argument giving an ~xml.etree.ElementTree.XMLParser{.interpreted-text role="class"} instance that will be used. This makes it possible to override the file's internal encoding:

  p = ET.XMLParser(encoding='utf-8')
  t = ET.XML("""<root/>""", parser=p)
  

  Errors in parsing XML now raise a ~xml.etree.ElementTree.ParseError{.interpreted-text role="exc"} exception, whose instances have a !position{.interpreted-text role="attr"} attribute containing a (line, column) tuple giving the location of the problem.

• ElementTree's code for converting trees to a string has been significantly reworked, making it roughly twice as fast in many cases. The ElementTree.write() <xml.etree.ElementTree.ElementTree.write>{.interpreted-text role="meth"} and !Element.write{.interpreted-text role="meth"} methods now have a method parameter that can be "xml" (the default), "html", or "text". HTML mode will output empty elements as <empty></empty> instead of <empty/>, and text mode will skip over elements and only output the text chunks. If you set the ~xml.etree.ElementTree.Element.tag{.interpreted-text role="attr"} attribute of an element to None but leave its children in place, the element will be omitted when the tree is written out, so you don't need to do more extensive rearrangement to remove a single element.

  Namespace handling has also been improved. All xmlns:<whatever> declarations are now output on the root element, not scattered throughout the resulting XML. You can set the default namespace for a tree by setting the !default_namespace{.interpreted-text role="attr"} attribute and can register new prefixes with ~xml.etree.ElementTree.register_namespace{.interpreted-text role="meth"}. In XML mode, you can use the true/false xml_declaration parameter to suppress the XML declaration.

• New ~xml.etree.ElementTree.Element{.interpreted-text role="class"} method: ~xml.etree.ElementTree.Element.extend{.interpreted-text role="meth"} appends the items from a sequence to the element's children. Elements themselves behave like sequences, so it's easy to move children from one element to another:

  from xml.etree import ElementTree as ET
  
  t = ET.XML("""<list>
    <item>1</item> <item>2</item>  <item>3</item>
  </list>""")
  new = ET.XML('<root/>')
  new.extend(t)
  
  # Outputs <root><item>1</item>...</root>
  print ET.tostring(new)
  

• New ~xml.etree.ElementTree.Element{.interpreted-text role="class"} method: ~xml.etree.ElementTree.Element.iter{.interpreted-text role="meth"} yields the children of the element as a generator. It's also possible to write for child in elem: to loop over an element's children. The existing method !getiterator{.interpreted-text role="meth"} is now deprecated, as is !getchildren{.interpreted-text role="meth"} which constructs and returns a list of children.

• New ~xml.etree.ElementTree.Element{.interpreted-text role="class"} method: ~xml.etree.ElementTree.Element.itertext{.interpreted-text role="meth"} yields all chunks of text that are descendants of the element. For example:

  t = ET.XML("""<list>
    <item>1</item> <item>2</item>  <item>3</item>
  </list>""")
  
  # Outputs ['\n  ', '1', ' ', '2', '  ', '3', '\n']
  print list(t.itertext())
  

• Deprecated: using an element as a Boolean (i.e., if elem:) would return true if the element had any children, or false if there were no children. This behaviour is confusing -- None is false, but so is a childless element? -- so it will now trigger a FutureWarning{.interpreted-text role="exc"}. In your code, you should be explicit: write len(elem) != 0 if you're interested in the number of children, or elem is not None.

Fredrik Lundh develops ElementTree and produced the 1.3 version; you can read his article describing 1.3 at https://web.archive.org/web/20200703234532/http://effbot.org/zone/elementtree-13-intro.htm. Florent Xicluna updated the version included with Python, after discussions on python-dev and in 6472{.interpreted-text role="issue"}.)

Build and C API Changes

Changes to Python's build process and to the C API include:

• The latest release of the GNU Debugger, GDB 7, can be scripted using Python. When you begin debugging an executable program P, GDB will look for a file named P-gdb.py and automatically read it. Dave Malcolm contributed a python-gdb.py{.interpreted-text role="file"} that adds a number of commands useful when debugging Python itself. For example, py-up and py-down go up or down one Python stack frame, which usually corresponds to several C stack frames. py-print prints the value of a Python variable, and py-bt prints the Python stack trace. (Added as a result of 8032{.interpreted-text role="issue"}.)

• If you use the .gdbinit{.interpreted-text role="file"} file provided with Python, the "pyo" macro in the 2.7 version now works correctly when the thread being debugged doesn't hold the GIL; the macro now acquires it before printing. (Contributed by Victor Stinner; 3632{.interpreted-text role="issue"}.)

• Py_AddPendingCall{.interpreted-text role="c:func"} is now thread-safe, letting any worker thread submit notifications to the main Python thread. This is particularly useful for asynchronous IO operations. (Contributed by Kristján Valur Jónsson; 4293{.interpreted-text role="issue"}.)

• New function: PyCode_NewEmpty{.interpreted-text role="c:func"} creates an empty code object; only the filename, function name, and first line number are required. This is useful for extension modules that are attempting to construct a more useful traceback stack. Previously such extensions needed to call !PyCode_New{.interpreted-text role="c:func"}, which had many more arguments. (Added by Jeffrey Yasskin.)

• New function: PyErr_NewExceptionWithDoc{.interpreted-text role="c:func"} creates a new exception class, just as the existing PyErr_NewException{.interpreted-text role="c:func"} does, but takes an extra char * argument containing the docstring for the new exception class. (Added by 'lekma' on the Python bug tracker; 7033{.interpreted-text role="issue"}.)

• New function: PyFrame_GetLineNumber{.interpreted-text role="c:func"} takes a frame object and returns the line number that the frame is currently executing. Previously code would need to get the index of the bytecode instruction currently executing, and then look up the line number corresponding to that address. (Added by Jeffrey Yasskin.)

• New functions: PyLong_AsLongAndOverflow{.interpreted-text role="c:func"} and PyLong_AsLongLongAndOverflow{.interpreted-text role="c:func"} approximates a Python long integer as a C long{.interpreted-text role="c:expr"} or long long{.interpreted-text role="c:expr"}. If the number is too large to fit into the output type, an overflow flag is set and returned to the caller. (Contributed by Case Van Horsen; 7528{.interpreted-text role="issue"} and 7767{.interpreted-text role="issue"}.)

• New function: stemming from the rewrite of string-to-float conversion, a new PyOS_string_to_double{.interpreted-text role="c:func"} function was added. The old !PyOS_ascii_strtod{.interpreted-text role="c:func"} and !PyOS_ascii_atof{.interpreted-text role="c:func"} functions are now deprecated.

• New function: !PySys_SetArgvEx{.interpreted-text role="c:func"} sets the value of sys.argv and can optionally update sys.path to include the directory containing the script named by sys.argv[0] depending on the value of an updatepath parameter.

  This function was added to close a security hole for applications that embed Python. The old function, !PySys_SetArgv{.interpreted-text role="c:func"}, would always update sys.path, and sometimes it would add the current directory. This meant that, if you ran an application embedding Python in a directory controlled by someone else, attackers could put a Trojan-horse module in the directory (say, a file named os.py{.interpreted-text role="file"}) that your application would then import and run.

  If you maintain a C/C++ application that embeds Python, check whether you're calling !PySys_SetArgv{.interpreted-text role="c:func"} and carefully consider whether the application should be using !PySys_SetArgvEx{.interpreted-text role="c:func"} with updatepath set to false.

  Security issue reported as 2008-5983{.interpreted-text role="cve"}; discussed in 5753{.interpreted-text role="issue"}, and fixed by Antoine Pitrou.

• New macros: the Python header files now define the following macros: Py_ISALNUM{.interpreted-text role="c:macro"}, Py_ISALPHA{.interpreted-text role="c:macro"}, Py_ISDIGIT{.interpreted-text role="c:macro"}, Py_ISLOWER{.interpreted-text role="c:macro"}, Py_ISSPACE{.interpreted-text role="c:macro"}, Py_ISUPPER{.interpreted-text role="c:macro"}, Py_ISXDIGIT{.interpreted-text role="c:macro"}, Py_TOLOWER{.interpreted-text role="c:macro"}, and Py_TOUPPER{.interpreted-text role="c:macro"}. All of these functions are analogous to the C standard macros for classifying characters, but ignore the current locale setting, because in several places Python needs to analyze characters in a locale-independent way. (Added by Eric Smith; 5793{.interpreted-text role="issue"}.)

• Removed function: !PyEval_CallObject{.interpreted-text role="c:func"} is now only available as a macro. A function version was being kept around to preserve ABI linking compatibility, but that was in 1997; it can certainly be deleted by now. (Removed by Antoine Pitrou; 8276{.interpreted-text role="issue"}.)

• New format codes: the !PyString_FromFormat{.interpreted-text role="c:func"}, !PyString_FromFormatV{.interpreted-text role="c:func"}, and PyErr_Format{.interpreted-text role="c:func"} functions now accept %lld and %llu format codes for displaying C's long long{.interpreted-text role="c:expr"} types. (Contributed by Mark Dickinson; 7228{.interpreted-text role="issue"}.)

• The complicated interaction between threads and process forking has been changed. Previously, the child process created by os.fork{.interpreted-text role="func"} might fail because the child is created with only a single thread running, the thread performing the os.fork{.interpreted-text role="func"}. If other threads were holding a lock, such as Python's import lock, when the fork was performed, the lock would still be marked as "held" in the new process. But in the child process nothing would ever release the lock, since the other threads weren't replicated, and the child process would no longer be able to perform imports.

  Python 2.7 acquires the import lock before performing an os.fork{.interpreted-text role="func"}, and will also clean up any locks created using the threading{.interpreted-text role="mod"} module. C extension modules that have internal locks, or that call fork(){.interpreted-text role="c:func"} themselves, will not benefit from this clean-up.

  (Fixed by Thomas Wouters; 1590864{.interpreted-text role="issue"}.)

• The Py_Finalize{.interpreted-text role="c:func"} function now calls the internal !threading._shutdown{.interpreted-text role="func"} function; this prevents some exceptions from being raised when an interpreter shuts down. (Patch by Adam Olsen; 1722344{.interpreted-text role="issue"}.)

• When using the PyMemberDef{.interpreted-text role="c:type"} structure to define attributes of a type, Python will no longer let you try to delete or set a !T_STRING_INPLACE{.interpreted-text role="c:macro"} attribute.

• Global symbols defined by the ctypes{.interpreted-text role="mod"} module are now prefixed with Py, or with _ctypes. (Implemented by Thomas Heller; 3102{.interpreted-text role="issue"}.)

• New configure option: the !--with-system-expat{.interpreted-text role="option"} switch allows building the pyexpat <xml.parsers.expat>{.interpreted-text role="mod"} module to use the system Expat library. (Contributed by Arfrever Frehtes Taifersar Arahesis; 7609{.interpreted-text role="issue"}.)

• New configure option: the !--with-valgrind{.interpreted-text role="option"} option will now disable the pymalloc allocator, which is difficult for the Valgrind memory-error detector to analyze correctly. Valgrind will therefore be better at detecting memory leaks and overruns. (Contributed by James Henstridge; 2422{.interpreted-text role="issue"}.)

• New configure option: you can now supply an empty string to !--with-dbmliborder={.interpreted-text role="option"} in order to disable all of the various DBM modules. (Added by Arfrever Frehtes Taifersar Arahesis; 6491{.interpreted-text role="issue"}.)

• The configure{.interpreted-text role="program"} script now checks for floating-point rounding bugs on certain 32-bit Intel chips and defines a !X87_DOUBLE_ROUNDING{.interpreted-text role="c:macro"} preprocessor definition. No code currently uses this definition, but it's available if anyone wishes to use it. (Added by Mark Dickinson; 2937{.interpreted-text role="issue"}.)

  configure{.interpreted-text role="program"} also now sets a !LDCXXSHARED{.interpreted-text role="envvar"} Makefile variable for supporting C++ linking. (Contributed by Arfrever Frehtes Taifersar Arahesis; 1222585{.interpreted-text role="issue"}.)

• The build process now creates the necessary files for pkg-config support. (Contributed by Clinton Roy; 3585{.interpreted-text role="issue"}.)

• The build process now supports Subversion 1.7. (Contributed by Arfrever Frehtes Taifersar Arahesis; 6094{.interpreted-text role="issue"}.)

Capsules {#whatsnew27-capsules}

Python 3.1 adds a new C datatype, PyCapsule{.interpreted-text role="c:type"}, for providing a C API to an extension module. A capsule is essentially the holder of a C void * pointer, and is made available as a module attribute; for example, the socket{.interpreted-text role="mod"} module's API is exposed as socket.CAPI, and unicodedata{.interpreted-text role="mod"} exposes ucnhash_CAPI. Other extensions can import the module, access its dictionary to get the capsule object, and then get the void * pointer, which will usually point to an array of pointers to the module's various API functions.

There is an existing data type already used for this, !PyCObject{.interpreted-text role="c:type"}, but it doesn't provide type safety. Evil code written in pure Python could cause a segmentation fault by taking a !PyCObject{.interpreted-text role="c:type"} from module A and somehow substituting it for the !PyCObject{.interpreted-text role="c:type"} in module B. Capsules know their own name, and getting the pointer requires providing the name:

void *vtable;

if (!PyCapsule_IsValid(capsule, "mymodule.CAPI") {
        PyErr_SetString(PyExc_ValueError, "argument type invalid");
        return NULL;
}

vtable = PyCapsule_GetPointer(capsule, "mymodule.CAPI");

You are assured that vtable points to whatever you're expecting. If a different capsule was passed in, PyCapsule_IsValid{.interpreted-text role="c:func"} would detect the mismatched name and return false. Refer to using-capsules{.interpreted-text role="ref"} for more information on using these objects.

Python 2.7 now uses capsules internally to provide various extension-module APIs, but the !PyCObject_AsVoidPtr{.interpreted-text role="c:func"} was modified to handle capsules, preserving compile-time compatibility with the !PyCObject{.interpreted-text role="c:type"} interface. Use of !PyCObject_AsVoidPtr{.interpreted-text role="c:func"} will signal a PendingDeprecationWarning{.interpreted-text role="exc"}, which is silent by default.

Implemented in Python 3.1 and backported to 2.7 by Larry Hastings; discussed in 5630{.interpreted-text role="issue"}.

Port-Specific Changes: Windows

• The msvcrt{.interpreted-text role="mod"} module now contains some constants from the crtassem.h{.interpreted-text role="file"} header file: ~msvcrt.CRT_ASSEMBLY_VERSION{.interpreted-text role="data"}, ~msvcrt.VC_ASSEMBLY_PUBLICKEYTOKEN{.interpreted-text role="data"}, and ~msvcrt.LIBRARIES_ASSEMBLY_NAME_PREFIX{.interpreted-text role="data"}. (Contributed by David Cournapeau; 4365{.interpreted-text role="issue"}.)
• The _winreg <winreg>{.interpreted-text role="mod"} module for accessing the registry now implements the ~winreg.CreateKeyEx{.interpreted-text role="func"} and ~winreg.DeleteKeyEx{.interpreted-text role="func"} functions, extended versions of previously supported functions that take several extra arguments. The ~winreg.DisableReflectionKey{.interpreted-text role="func"}, ~winreg.EnableReflectionKey{.interpreted-text role="func"}, and ~winreg.QueryReflectionKey{.interpreted-text role="func"} were also tested and documented. (Implemented by Brian Curtin: 7347{.interpreted-text role="issue"}.)
• The new !_beginthreadex{.interpreted-text role="c:func"} API is used to start threads, and the native thread-local storage functions are now used. (Contributed by Kristján Valur Jónsson; 3582{.interpreted-text role="issue"}.)
• The os.kill{.interpreted-text role="func"} function now works on Windows. The signal value can be the constants ~signal.CTRL_C_EVENT{.interpreted-text role="const"}, ~signal.CTRL_BREAK_EVENT{.interpreted-text role="const"}, or any integer. The first two constants will send Control-C{.interpreted-text role="kbd"} and Control-Break{.interpreted-text role="kbd"} keystroke events to subprocesses; any other value will use the !TerminateProcess{.interpreted-text role="c:func"} API. (Contributed by Miki Tebeka; 1220212{.interpreted-text role="issue"}.)
• The os.listdir{.interpreted-text role="func"} function now correctly fails for an empty path. (Fixed by Hirokazu Yamamoto; 5913{.interpreted-text role="issue"}.)
• The mimetypes{.interpreted-text role="mod"} module will now read the MIME database from the Windows registry when initializing. (Patch by Gabriel Genellina; 4969{.interpreted-text role="issue"}.)

Port-Specific Changes: Mac OS X

• The path /Library/Python/2.7/site-packages is now appended to sys.path, in order to share added packages between the system installation and a user-installed copy of the same version. (Changed by Ronald Oussoren; 4865{.interpreted-text role="issue"}.)

  ::: versionchanged 2.7.13

  As of 2.7.13, this change was removed. /Library/Python/2.7/site-packages, the site-packages directory used by the Apple-supplied system Python 2.7 is no longer appended to sys.path for user-installed Pythons such as from the python.org installers. As of macOS 10.12, Apple changed how the system site-packages directory is configured, which could cause installation of pip components, like setuptools, to fail. Packages installed for the system Python will no longer be shared with user-installed Pythons. (28440{.interpreted-text role="issue"}) :::

Port-Specific Changes: FreeBSD

• FreeBSD 7.1's !SO_SETFIB{.interpreted-text role="const"} constant, used with the ~socket.socket{.interpreted-text role="func"} methods ~socket.socket.getsockopt{.interpreted-text role="func"}/~socket.socket.setsockopt{.interpreted-text role="func"} to select an alternate routing table, is now available in the socket{.interpreted-text role="mod"} module. (Added by Kyle VanderBeek; 8235{.interpreted-text role="issue"}.)

Other Changes and Fixes

• Two benchmark scripts, iobench{.interpreted-text role="file"} and ccbench{.interpreted-text role="file"}, were added to the Tools{.interpreted-text role="file"} directory. iobench{.interpreted-text role="file"} measures the speed of the built-in file I/O objects returned by open{.interpreted-text role="func"} while performing various operations, and ccbench{.interpreted-text role="file"} is a concurrency benchmark that tries to measure computing throughput, thread switching latency, and IO processing bandwidth when performing several tasks using a varying number of threads.
• The Tools/i18n/msgfmt.py{.interpreted-text role="file"} script now understands plural forms in .po{.interpreted-text role="file"} files. (Fixed by Martin von Löwis; 5464{.interpreted-text role="issue"}.)
• When importing a module from a .pyc{.interpreted-text role="file"} or .pyo{.interpreted-text role="file"} file with an existing .py{.interpreted-text role="file"} counterpart, the ~codeobject.co_filename{.interpreted-text role="attr"} attributes of the resulting code objects are overwritten when the original filename is obsolete. This can happen if the file has been renamed, moved, or is accessed through different paths. (Patch by Ziga Seilnacht and Jean-Paul Calderone; 1180193{.interpreted-text role="issue"}.)
• The regrtest.py{.interpreted-text role="file"} script now takes a !--randseed={.interpreted-text role="option"} switch that takes an integer that will be used as the random seed for the !-r{.interpreted-text role="option"} option that executes tests in random order. The !-r{.interpreted-text role="option"} option also reports the seed that was used (Added by Collin Winter.)
• Another regrtest.py{.interpreted-text role="file"} switch is !-j{.interpreted-text role="option"}, which takes an integer specifying how many tests run in parallel. This allows reducing the total runtime on multi-core machines. This option is compatible with several other options, including the !-R{.interpreted-text role="option"} switch which is known to produce long runtimes. (Added by Antoine Pitrou, 6152{.interpreted-text role="issue"}.) This can also be used with a new !-F{.interpreted-text role="option"} switch that runs selected tests in a loop until they fail. (Added by Antoine Pitrou; 7312{.interpreted-text role="issue"}.)
• When executed as a script, the py_compile.py{.interpreted-text role="file"} module now accepts '-' as an argument, which will read standard input for the list of filenames to be compiled. (Contributed by Piotr Ożarowski; 8233{.interpreted-text role="issue"}.)

Porting to Python 2.7

This section lists previously described changes and other bugfixes that may require changes to your code:

• The range{.interpreted-text role="func"} function processes its arguments more consistently; it will now call ~object.__int__{.interpreted-text role="meth"} on non-float, non-integer arguments that are supplied to it. (Fixed by Alexander Belopolsky; 1533{.interpreted-text role="issue"}.)
• The string format{.interpreted-text role="meth"} method changed the default precision used for floating-point and complex numbers from 6 decimal places to 12, which matches the precision used by str{.interpreted-text role="func"}. (Changed by Eric Smith; 5920{.interpreted-text role="issue"}.)
• Because of an optimization for the with{.interpreted-text role="keyword"} statement, the special methods ~object.__enter__{.interpreted-text role="meth"} and ~object.__exit__{.interpreted-text role="meth"} must belong to the object's type, and cannot be directly attached to the object's instance. This affects new-style classes (derived from object{.interpreted-text role="class"}) and C extension types. (6101{.interpreted-text role="issue"}.)
• Due to a bug in Python 2.6, the exc_value parameter to ~object.__exit__{.interpreted-text role="meth"} methods was often the string representation of the exception, not an instance. This was fixed in 2.7, so exc_value will be an instance as expected. (Fixed by Florent Xicluna; 7853{.interpreted-text role="issue"}.)
• When a restricted set of attributes were set using __slots__, deleting an unset attribute would not raise AttributeError{.interpreted-text role="exc"} as you would expect. Fixed by Benjamin Peterson; 7604{.interpreted-text role="issue"}.)

In the standard library:

• Operations with ~datetime.datetime{.interpreted-text role="class"} instances that resulted in a year falling outside the supported range didn't always raise OverflowError{.interpreted-text role="exc"}. Such errors are now checked more carefully and will now raise the exception. (Reported by Mark Leander, patch by Anand B. Pillai and Alexander Belopolsky; 7150{.interpreted-text role="issue"}.)

• When using ~decimal.Decimal{.interpreted-text role="class"} instances with a string's format{.interpreted-text role="meth"} method, the default alignment was previously left-alignment. This has been changed to right-alignment, which might change the output of your programs. (Changed by Mark Dickinson; 6857{.interpreted-text role="issue"}.)

  Comparisons involving a signaling NaN value (or sNAN) now signal ~decimal.InvalidOperation{.interpreted-text role="const"} instead of silently returning a true or false value depending on the comparison operator. Quiet NaN values (or NaN) are now hashable. (Fixed by Mark Dickinson; 7279{.interpreted-text role="issue"}.)

• The xml.etree.ElementTree{.interpreted-text role="mod"} library no longer escapes ampersands and angle brackets when outputting an XML processing instruction (which looks like <?xml-stylesheet href="#style1"?>) or comment (which looks like <!-- comment -->). (Patch by Neil Muller; 2746{.interpreted-text role="issue"}.)

• The !readline{.interpreted-text role="meth"} method of ~io.StringIO{.interpreted-text role="class"} objects now does nothing when a negative length is requested, as other file-like objects do. (7348{.interpreted-text role="issue"}).

• The syslog{.interpreted-text role="mod"} module will now use the value of sys.argv[0] as the identifier instead of the previous default value of 'python'. (Changed by Sean Reifschneider; 8451{.interpreted-text role="issue"}.)

• The tarfile{.interpreted-text role="mod"} module's default error handling has changed, to no longer suppress fatal errors. The default error level was previously 0, which meant that errors would only result in a message being written to the debug log, but because the debug log is not activated by default, these errors go unnoticed. The default error level is now 1, which raises an exception if there's an error. (Changed by Lars Gustäbel; 7357{.interpreted-text role="issue"}.)

• The urlparse <urllib.parse>{.interpreted-text role="mod"} module's ~urllib.parse.urlsplit{.interpreted-text role="func"} now handles unknown URL schemes in a fashion compliant with 3986{.interpreted-text role="rfc"}: if the URL is of the form "<something>://...", the text before the :// is treated as the scheme, even if it's a made-up scheme that the module doesn't know about. This change may break code that worked around the old behaviour. For example, Python 2.6.4 or 2.5 will return the following:

  ::: {.doctest options="+SKIP"} >>> import urlparse >>> urlparse.urlsplit('invented://host/filename?query') ('invented', '', '//host/filename?query', '', '') :::

  Python 2.7 (and Python 2.6.5) will return:

  ::: {.doctest options="+SKIP"} >>> import urlparse >>> urlparse.urlsplit('invented://host/filename?query') ('invented', 'host', '/filename?query', '', '') :::

  (Python 2.7 actually produces slightly different output, since it returns a named tuple instead of a standard tuple.)

For C extensions:

• C extensions that use integer format codes with the PyArg_Parse* family of functions will now raise a TypeError{.interpreted-text role="exc"} exception instead of triggering a DeprecationWarning{.interpreted-text role="exc"} (5080{.interpreted-text role="issue"}).
• Use the new PyOS_string_to_double{.interpreted-text role="c:func"} function instead of the old !PyOS_ascii_strtod{.interpreted-text role="c:func"} and !PyOS_ascii_atof{.interpreted-text role="c:func"} functions, which are now deprecated.

For applications that embed Python:

• The !PySys_SetArgvEx{.interpreted-text role="c:func"} function was added, letting applications close a security hole when the existing !PySys_SetArgv{.interpreted-text role="c:func"} function was used. Check whether you're calling !PySys_SetArgv{.interpreted-text role="c:func"} and carefully consider whether the application should be using !PySys_SetArgvEx{.interpreted-text role="c:func"} with updatepath set to false.

New Features Added to Python 2.7 Maintenance Releases {#py27-maintenance-enhancements}

New features may be added to Python 2.7 maintenance releases when the situation genuinely calls for it. Any such additions must go through the Python Enhancement Proposal process, and make a compelling case for why they can't be adequately addressed by either adding the new feature solely to Python 3, or else by publishing it on the Python Package Index.

In addition to the specific proposals listed below, there is a general exemption allowing new -3 warnings to be added in any Python 2.7 maintenance release.

Two new environment variables for debug mode

In debug mode, the [xxx refs] statistic is not written by default, the !PYTHONSHOWREFCOUNT{.interpreted-text role="envvar"} environment variable now must also be set. (Contributed by Victor Stinner; 31733{.interpreted-text role="issue"}.)

When Python is compiled with COUNT_ALLOC defined, allocation counts are no longer dumped by default anymore: the !PYTHONSHOWALLOCCOUNT{.interpreted-text role="envvar"} environment variable must now also be set. Moreover, allocation counts are now dumped into stderr, rather than stdout. (Contributed by Victor Stinner; 31692{.interpreted-text role="issue"}.)

::: versionadded 2.7.15 :::

PEP 434: IDLE Enhancement Exception for All Branches

434{.interpreted-text role="pep"} describes a general exemption for changes made to the IDLE development environment shipped along with Python. This exemption makes it possible for the IDLE developers to provide a more consistent user experience across all supported versions of Python 2 and 3.

For details of any IDLE changes, refer to the NEWS file for the specific release.

PEP 466: Network Security Enhancements for Python 2.7

466{.interpreted-text role="pep"} describes a number of network security enhancement proposals that have been approved for inclusion in Python 2.7 maintenance releases, with the first of those changes appearing in the Python 2.7.7 release.

466{.interpreted-text role="pep"} related features added in Python 2.7.7:

• hmac.compare_digest{.interpreted-text role="func"} was backported from Python 3 to make a timing attack resistant comparison operation available to Python 2 applications. (Contributed by Alex Gaynor; 21306{.interpreted-text role="issue"}.)
• OpenSSL 1.0.1g was upgraded in the official Windows installers published on python.org. (Contributed by Zachary Ware; 21462{.interpreted-text role="issue"}.)

466{.interpreted-text role="pep"} related features added in Python 2.7.8:

• hashlib.pbkdf2_hmac{.interpreted-text role="func"} was backported from Python 3 to make a hashing algorithm suitable for secure password storage broadly available to Python 2 applications. (Contributed by Alex Gaynor; 21304{.interpreted-text role="issue"}.)
• OpenSSL 1.0.1h was upgraded for the official Windows installers published on python.org. (Contributed by Zachary Ware in 21671{.interpreted-text role="issue"} for 2014-0224{.interpreted-text role="cve"}.)

466{.interpreted-text role="pep"} related features added in Python 2.7.9:

• Most of Python 3.4's ssl{.interpreted-text role="mod"} module was backported. This means ssl{.interpreted-text role="mod"} now supports Server Name Indication, TLS1.x settings, access to the platform certificate store, the ~ssl.SSLContext{.interpreted-text role="class"} class, and other features. (Contributed by Alex Gaynor and David Reid; 21308{.interpreted-text role="issue"}.)

  Refer to the "Version added: 2.7.9" notes in the module documentation for specific details.

• os.urandom{.interpreted-text role="func"} was changed to cache a file descriptor to /dev/urandom instead of reopening /dev/urandom on every call. (Contributed by Alex Gaynor; 21305{.interpreted-text role="issue"}.)

• hashlib.algorithms_guaranteed{.interpreted-text role="data"} and hashlib.algorithms_available{.interpreted-text role="data"} were backported from Python 3 to make it easier for Python 2 applications to select the strongest available hash algorithm. (Contributed by Alex Gaynor in 21307{.interpreted-text role="issue"})

PEP 477: Backport ensurepip (PEP 453) to Python 2.7

477{.interpreted-text role="pep"} approves the inclusion of the 453{.interpreted-text role="pep"} ensurepip module and the improved documentation that was enabled by it in the Python 2.7 maintenance releases, appearing first in the Python 2.7.9 release.

Bootstrapping pip By Default

The new ensurepip{.interpreted-text role="mod"} module (defined in 453{.interpreted-text role="pep"}) provides a standard cross-platform mechanism to bootstrap the pip installer into Python installations. The version of pip included with Python 2.7.9 is pip 1.5.6, and future 2.7.x maintenance releases will update the bundled version to the latest version of pip that is available at the time of creating the release candidate.

By default, the commands pip, pipX and pipX.Y will be installed on all platforms (where X.Y stands for the version of the Python installation), along with the pip Python package and its dependencies.

For CPython source builds on POSIX systems <building-python-on-unix>{.interpreted-text role="ref"}, the make install and make altinstall commands do not bootstrap pip by default. This behaviour can be controlled through configure options, and overridden through Makefile options.

On Windows and Mac OS X, the CPython installers now default to installing pip along with CPython itself (users may opt out of installing it during the installation process). Window users will need to opt in to the automatic PATH modifications to have pip available from the command line by default, otherwise it can still be accessed through the Python launcher for Windows as py -m pip.

As discussed in the PEP <0477#disabling-ensurepip-by-downstream-distributors>{.interpreted-text role="pep"}, platform packagers may choose not to install these commands by default, as long as, when invoked, they provide clear and simple directions on how to install them on that platform (usually using the system package manager).

Documentation Changes

As part of this change, the installing-index{.interpreted-text role="ref"} and distributing-index{.interpreted-text role="ref"} sections of the documentation have been completely redesigned as short getting started and FAQ documents. Most packaging documentation has now been moved out to the Python Packaging Authority maintained Python Packaging User Guide and the documentation of the individual projects.

However, as this migration is currently still incomplete, the legacy versions of those guides remaining available as install-index{.interpreted-text role="ref"} and setuptools-index{.interpreted-text role="ref"}.

::: seealso

453{.interpreted-text role="pep"} -- Explicit bootstrapping of pip in Python installations

: PEP written by Donald Stufft and Nick Coghlan, implemented by Donald Stufft, Nick Coghlan, Martin von Löwis and Ned Deily. :::

PEP 476: Enabling certificate verification by default for stdlib http clients

476{.interpreted-text role="pep"} updated httplib <http>{.interpreted-text role="mod"} and modules which use it, such as urllib2 <urllib.request>{.interpreted-text role="mod"} and xmlrpclib <xmlrpc.client>{.interpreted-text role="mod"}, to now verify that the server presents a certificate which is signed by a Certificate Authority in the platform trust store and whose hostname matches the hostname being requested by default, significantly improving security for many applications. This change was made in the Python 2.7.9 release.

For applications which require the old previous behavior, they can pass an alternate context:

import urllib2
import ssl

# This disables all verification
context = ssl._create_unverified_context()

# This allows using a specific certificate for the host, which doesn't need
# to be in the trust store
context = ssl.create_default_context(cafile="/path/to/file.crt")

urllib2.urlopen("https://invalid-cert", context=context)

PEP 493: HTTPS verification migration tools for Python 2.7

493{.interpreted-text role="pep"} provides additional migration tools to support a more incremental infrastructure upgrade process for environments containing applications and services relying on the historically permissive processing of server certificates when establishing client HTTPS connections. These additions were made in the Python 2.7.12 release.

These tools are intended for use in cases where affected applications and services can't be modified to explicitly pass a more permissive SSL context when establishing the connection.

For applications and services which can't be modified at all, the new PYTHONHTTPSVERIFY environment variable may be set to 0 to revert an entire Python process back to the default permissive behaviour of Python 2.7.8 and earlier.

For cases where the connection establishment code can't be modified, but the overall application can be, the new !ssl._https_verify_certificates{.interpreted-text role="func"} function can be used to adjust the default behaviour at runtime.

New make regen-all build target

To simplify cross-compilation, and to ensure that CPython can reliably be compiled without requiring an existing version of Python to already be available, the autotools-based build system no longer attempts to implicitly recompile generated files based on file modification times.

Instead, a new make regen-all command has been added to force regeneration of these files when desired (e.g. after an initial version of Python has already been built based on the pregenerated versions).

More selective regeneration targets are also defined - see Makefile.pre.in{.interpreted-text role="source"} for details.

(Contributed by Victor Stinner in 23404{.interpreted-text role="issue"}.)

::: versionadded 2.7.14 :::

Removal of make touch build target

The make touch build target previously used to request implicit regeneration of generated files by updating their modification times has been removed.

It has been replaced by the new make regen-all target.

(Contributed by Victor Stinner in 23404{.interpreted-text role="issue"}.)

::: versionchanged 2.7.14 :::

Acknowledgements {#acks27}

The author would like to thank the following people for offering suggestions, corrections and assistance with various drafts of this article: Nick Coghlan, Philip Jenvey, Ryan Lovett, R. David Murray, Hugh Secker-Walker.