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What's New In Python 3.1

Author : Raymond Hettinger

This article explains the new features in Python 3.1, compared to 3.0. Python 3.1 was released on June 27, 2009.

PEP 372: Ordered Dictionaries

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, a new collections.OrderedDict{.interpreted-text role="class"} class has been introduced.

The OrderedDict API is substantially the same as regular dictionaries but will iterate over keys and values in a guaranteed order depending on when a key was first inserted. If a new entry overwrites an existing entry, the original insertion position is left unchanged. Deleting an entry and reinserting it will move it to the end.

The standard library now supports use of ordered dictionaries in several modules. The configparser{.interpreted-text role="mod"} module uses them by default. This lets configuration files be read, modified, and then written back in their original order. The _asdict() 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 is being built-out with an object_pairs_hook to allow OrderedDicts to be built by the decoder. Support was also added for third-party tools like PyYAML.

::: seealso

372{.interpreted-text role="pep"} - Ordered Dictionaries

: PEP written by Armin Ronacher and Raymond Hettinger. Implementation written by Raymond Hettinger. :::

Since an ordered dictionary remembers its insertion order, it can be used in conjunction with sorting to make a sorted dictionary:

>>> # regular unsorted dictionary
>>> d = {'banana': 3, 'apple':4, 'pear': 1, 'orange': 2}

>>> # dictionary sorted by key
>>> OrderedDict(sorted(d.items(), key=lambda t: t[0]))
OrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)])

>>> # dictionary sorted by value
>>> OrderedDict(sorted(d.items(), key=lambda t: t[1]))
OrderedDict([('pear', 1), ('orange', 2), ('banana', 3), ('apple', 4)])

>>> # dictionary sorted by length of the key string
>>> OrderedDict(sorted(d.items(), key=lambda t: len(t[0])))
OrderedDict([('pear', 1), ('apple', 4), ('orange', 2), ('banana', 3)])

The new sorted dictionaries maintain their sort order when entries are deleted. But when new keys are added, the keys are appended to the end and the sort is not maintained.

PEP 378: Format Specifier for Thousands Separator

The built-in format{.interpreted-text role="func"} function and the str.format{.interpreted-text role="meth"} method use a mini-language that now includes a simple, non-locale aware way to format a number with a thousands separator. That provides a way to humanize a program's output, improving its professional appearance and readability:

>>> format(1234567, ',d')
'1,234,567'
>>> format(1234567.89, ',.2f')
'1,234,567.89'
>>> format(12345.6 + 8901234.12j, ',f')
'12,345.600000+8,901,234.120000j'
>>> format(Decimal('1234567.89'), ',f')
'1,234,567.89'

The supported types are int{.interpreted-text role="class"}, float{.interpreted-text role="class"}, complex{.interpreted-text role="class"} and decimal.Decimal{.interpreted-text role="class"}.

Discussions are underway about how to specify alternative separators like dots, spaces, apostrophes, or underscores. Locale-aware applications should use the existing n format specifier which already has some support for thousands separators.

::: seealso

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

: PEP written by Raymond Hettinger and implemented by Eric Smith and Mark Dickinson. :::

Other Language Changes

Some smaller changes made to the core Python language are:

Directories and zip archives containing a __main__.py{.interpreted-text role="file"} file can now be executed directly by passing their name to the interpreter. The directory/zipfile is automatically inserted as the first entry in sys.path. (Suggestion and initial patch by Andy Chu; revised patch by Phillip J. Eby and Nick Coghlan; 1739468{.interpreted-text role="issue"}.)
The int{.interpreted-text role="func"} type gained a bit_length method that returns the number of bits necessary to represent its argument in binary:
```
>>> n = 37
>>> bin(37)
'0b100101'
>>> n.bit_length()
6
>>> n = 2**123-1
>>> n.bit_length()
123
>>> (n+1).bit_length()
124
```
(Contributed by Fredrik Johansson, Victor Stinner, Raymond Hettinger, and Mark Dickinson; 3439{.interpreted-text role="issue"}.)
The fields in format{.interpreted-text role="func"} strings can now be automatically numbered:
```
>>> 'Sir {} of {}'.format('Gallahad', 'Camelot')
'Sir Gallahad of Camelot'
```
Formerly, the string would have required numbered fields such as: 'Sir {0} of {1}'.

(Contributed by Eric Smith; 5237{.interpreted-text role="issue"}.)
The !string.maketrans{.interpreted-text role="func"} function is deprecated and is replaced by new static methods, bytes.maketrans{.interpreted-text role="meth"} and bytearray.maketrans{.interpreted-text role="meth"}. This change solves the confusion around which types were supported by the string{.interpreted-text role="mod"} module. Now, str{.interpreted-text role="class"}, bytes{.interpreted-text role="class"}, and bytearray{.interpreted-text role="class"} each have their own maketrans and translate methods with intermediate translation tables of the appropriate type.

(Contributed by Georg Brandl; 5675{.interpreted-text role="issue"}.)
The syntax of the with{.interpreted-text role="keyword"} statement now allows multiple context managers in a single statement:
```
>>> with open('mylog.txt') as infile, open('a.out', 'w') as outfile:
...     for line in infile:
...         if '<critical>' in line:
...             outfile.write(line)
```
With the new syntax, the !contextlib.nested{.interpreted-text role="func"} function is no longer needed and is now deprecated.

(Contributed by Georg Brandl and Mattias Brändström; appspot issue 53094.)
round(x, n) now returns an integer if x is an integer. Previously it returned a float:
```
>>> round(1123, -2)
1100
```
(Contributed by Mark Dickinson; 4707{.interpreted-text role="issue"}.)
Python now uses David Gay's algorithm for finding the shortest floating-point representation that doesn't change its value. This should help mitigate some of the confusion surrounding binary floating-point numbers.

The significance is easily seen with a number like 1.1 which does not have an exact equivalent in binary floating point. Since there is no exact equivalent, an expression like float('1.1') evaluates to the nearest representable value which is 0x1.199999999999ap+0 in hex or 1.100000000000000088817841970012523233890533447265625 in decimal. That nearest value was and still is used in subsequent floating-point calculations.

What is new is how the number gets displayed. Formerly, Python used a simple approach. The value of repr(1.1) was computed as format(1.1, '.17g') which evaluated to '1.1000000000000001'. The advantage of using 17 digits was that it relied on IEEE-754 guarantees to assure that eval(repr(1.1)) would round-trip exactly to its original value. The disadvantage is that many people found the output to be confusing (mistaking intrinsic limitations of binary floating-point representation as being a problem with Python itself).

The new algorithm for repr(1.1) is smarter and returns '1.1'. Effectively, it searches all equivalent string representations (ones that get stored with the same underlying float value) and returns the shortest representation.

The new algorithm tends to emit cleaner representations when possible, but it does not change the underlying values. So, it is still the case that 1.1 + 2.2 != 3.3 even though the representations may suggest otherwise.

The new algorithm depends on certain features in the underlying floating-point implementation. If the required features are not found, the old algorithm will continue to be used. Also, the text pickle protocols assure cross-platform portability by using the old algorithm.

(Contributed by Eric Smith and Mark Dickinson; 1580{.interpreted-text role="issue"})

New, Improved, and Deprecated Modules

Added a collections.Counter{.interpreted-text role="class"} class to support convenient counting of unique items in a sequence or iterable:
```
>>> Counter(['red', 'blue', 'red', 'green', 'blue', 'blue'])
Counter({'blue': 3, 'red': 2, 'green': 1})
```
(Contributed by Raymond Hettinger; 1696199{.interpreted-text role="issue"}.)
Added a new module, tkinter.ttk{.interpreted-text role="mod"} for access to the Tk themed widget set. The basic idea of ttk is to separate, to the extent possible, the code implementing a widget's behavior from the code implementing its appearance.

(Contributed by Guilherme Polo; 2983{.interpreted-text role="issue"}.)
The gzip.GzipFile{.interpreted-text role="class"} and bz2.BZ2File{.interpreted-text role="class"} classes now support the context management protocol:
```
>>> # Automatically close file after writing
>>> with gzip.GzipFile(filename, "wb") as f:
...     f.write(b"xxx")
```
(Contributed by Antoine Pitrou.)
The decimal{.interpreted-text role="mod"} module now supports methods for creating a decimal object from a binary float{.interpreted-text role="class"}. The conversion is exact but can sometimes be surprising:
```
>>> Decimal.from_float(1.1)
Decimal('1.100000000000000088817841970012523233890533447265625')
```
The long decimal result shows the actual binary fraction being stored for 1.1. The fraction has many digits because 1.1 cannot be exactly represented in binary.

(Contributed by Raymond Hettinger and Mark Dickinson.)
The itertools{.interpreted-text role="mod"} module grew two new functions. The itertools.combinations_with_replacement{.interpreted-text role="func"} function is one of four for generating combinatorics including permutations and Cartesian products. The itertools.compress{.interpreted-text role="func"} function mimics its namesake from APL. Also, the existing itertools.count{.interpreted-text role="func"} function now has an optional step argument and can accept any type of counting sequence including fractions.Fraction{.interpreted-text role="class"} and decimal.Decimal{.interpreted-text role="class"}:
```
>>> [p+q for p,q in combinations_with_replacement('LOVE', 2)]
['LL', 'LO', 'LV', 'LE', 'OO', 'OV', 'OE', 'VV', 'VE', 'EE']

>>> list(compress(data=range(10), selectors=[0,0,1,1,0,1,0,1,0,0]))
[2, 3, 5, 7]

>>> c = count(start=Fraction(1,2), step=Fraction(1,6))
>>> [next(c), next(c), next(c), next(c)]
[Fraction(1, 2), Fraction(2, 3), Fraction(5, 6), Fraction(1, 1)]
```
(Contributed by Raymond Hettinger.)

collections.namedtuple{.interpreted-text role="func"} now supports a keyword argument rename which lets invalid fieldnames be automatically converted to positional names in the form 0, 1, etc. This is useful when the field names are being created by an external source such as a CSV header, SQL field list, or user input:

>>> query = input()
SELECT region, dept, count(*) FROM main GROUPBY region, dept

>>> cursor.execute(query)
>>> query_fields = [desc[0] for desc in cursor.description]
>>> UserQuery = namedtuple('UserQuery', query_fields, rename=True)
>>> pprint.pprint([UserQuery(*row) for row in cursor])
[UserQuery(region='South', dept='Shipping', _2=185),
 UserQuery(region='North', dept='Accounting', _2=37),
 UserQuery(region='West', dept='Sales', _2=419)]

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

The re.sub{.interpreted-text role="func"}, re.subn{.interpreted-text role="func"} and re.split{.interpreted-text role="func"} functions now accept a flags parameter.

(Contributed by Gregory Smith.)
The logging{.interpreted-text role="mod"} module now implements a simple logging.NullHandler{.interpreted-text role="class"} class for applications that are not using logging but are calling library code that does. Setting-up a null handler will suppress spurious warnings such as "No handlers could be found for logger foo":
```
>>> h = logging.NullHandler()
>>> logging.getLogger("foo").addHandler(h)
```
(Contributed by Vinay Sajip; 4384{.interpreted-text role="issue"}).
The runpy{.interpreted-text role="mod"} module which supports the -m command line switch now supports the execution of packages by looking for and executing a __main__ submodule when a package name is supplied.

(Contributed by Andi Vajda; 4195{.interpreted-text role="issue"}.)
The pdb{.interpreted-text role="mod"} module can now access and display source code loaded via zipimport{.interpreted-text role="mod"} (or any other conformant 302{.interpreted-text role="pep"} loader).

(Contributed by Alexander Belopolsky; 4201{.interpreted-text role="issue"}.)
functools.partial{.interpreted-text role="class"} objects can now be pickled.

(Suggested by Antoine Pitrou and Jesse Noller. Implemented by Jack Diederich; 5228{.interpreted-text role="issue"}.)

Add pydoc{.interpreted-text role="mod"} help topics for symbols so that help('@') works as expected in the interactive environment.

(Contributed by David Laban; 4739{.interpreted-text role="issue"}.)
The unittest{.interpreted-text role="mod"} module now supports skipping individual tests or classes of tests. And it supports marking a test as an expected failure, a test that is known to be broken, but shouldn't be counted as a failure on a TestResult:
```
class TestGizmo(unittest.TestCase):

    @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
    def test_gizmo_on_windows(self):
        ...

    @unittest.expectedFailure
    def test_gimzo_without_required_library(self):
        ...
```
Also, tests for exceptions have been builtout to work with context managers using the with{.interpreted-text role="keyword"} statement:
```
def test_division_by_zero(self):
    with self.assertRaises(ZeroDivisionError):
        x / 0
```
In addition, several new assertion methods were added including ~unittest.TestCase.assertSetEqual{.interpreted-text role="meth"}, ~unittest.TestCase.assertDictEqual{.interpreted-text role="meth"}, !assertDictContainsSubset{.interpreted-text role="meth"}, ~unittest.TestCase.assertListEqual{.interpreted-text role="meth"}, ~unittest.TestCase.assertTupleEqual{.interpreted-text role="meth"}, ~unittest.TestCase.assertSequenceEqual{.interpreted-text role="meth"}, assertRaisesRegexp() <unittest.TestCase.assertRaisesRegex>{.interpreted-text role="meth"}, ~unittest.TestCase.assertIsNone{.interpreted-text role="meth"}, and ~unittest.TestCase.assertIsNotNone{.interpreted-text role="meth"}.

(Contributed by Benjamin Peterson and Antoine Pitrou.)
The io{.interpreted-text role="mod"} module has three new constants for the ~io.IOBase.seek{.interpreted-text role="meth"} method: ~os.SEEK_SET{.interpreted-text role="data"}, ~os.SEEK_CUR{.interpreted-text role="data"}, and ~os.SEEK_END{.interpreted-text role="data"}.
The sys.version_info{.interpreted-text role="data"} tuple is now a named tuple:
```
>>> sys.version_info
sys.version_info(major=3, minor=1, micro=0, releaselevel='alpha', serial=2)
```
(Contributed by Ross Light; 4285{.interpreted-text role="issue"}.)
The !nntplib{.interpreted-text role="mod"} and imaplib{.interpreted-text role="mod"} modules now support IPv6.

(Contributed by Derek Morr; 1655{.interpreted-text role="issue"} and 1664{.interpreted-text role="issue"}.)
The pickle{.interpreted-text role="mod"} module has been adapted for better interoperability with Python 2.x when used with protocol 2 or lower. The reorganization of the standard library changed the formal reference for many objects. For example, __builtin__.set in Python 2 is called builtins.set in Python

3. This change confounded efforts to share data between different versions of Python. But now when protocol 2 or lower is selected, the pickler will automatically use the old Python 2 names for both loading and dumping. This remapping is turned-on by default but can be disabled with the fix_imports option:
```
>>> s = {1, 2, 3}
>>> pickle.dumps(s, protocol=0)
b'c__builtin__\nset\np0\n((lp1\nL1L\naL2L\naL3L\natp2\nRp3\n.'
>>> pickle.dumps(s, protocol=0, fix_imports=False)
b'cbuiltins\nset\np0\n((lp1\nL1L\naL2L\naL3L\natp2\nRp3\n.'
```
An unfortunate but unavoidable side-effect of this change is that protocol 2 pickles produced by Python 3.1 won't be readable with Python 3.0. The latest pickle protocol, protocol 3, should be used when migrating data between Python 3.x implementations, as it doesn't attempt to remain compatible with Python 2.x.

(Contributed by Alexandre Vassalotti and Antoine Pitrou, 6137{.interpreted-text role="issue"}.)
A new module, importlib{.interpreted-text role="mod"} was added. It provides a complete, portable, pure Python reference implementation of the import{.interpreted-text role="keyword"} statement and its counterpart, the __import__{.interpreted-text role="func"} function. It represents a substantial step forward in documenting and defining the actions that take place during imports.

(Contributed by Brett Cannon.)

Optimizations

Major performance enhancements have been added:

The new I/O library (as defined in 3116{.interpreted-text role="pep"}) was mostly written in Python and quickly proved to be a problematic bottleneck in Python 3.0. In Python 3.1, the I/O library has been entirely rewritten in C and is 2 to 20 times faster depending on the task at hand. The pure Python version is still available for experimentation purposes through the _pyio module.

(Contributed by Amaury Forgeot d'Arc and Antoine Pitrou.)
Added a heuristic so that tuples and dicts containing only untrackable objects are not tracked by the garbage collector. This can reduce the size of collections and therefore the garbage collection overhead on long-running programs, depending on their particular use of datatypes.

(Contributed by Antoine Pitrou, 4688{.interpreted-text role="issue"}.)
Enabling a configure option named --with-computed-gotos on compilers that support it (notably: gcc, SunPro, icc), the bytecode evaluation loop is compiled with a new dispatch mechanism which gives speedups of up to 20%, depending on the system, the compiler, and the benchmark.

(Contributed by Antoine Pitrou along with a number of other participants, 4753{.interpreted-text role="issue"}).
The decoding of UTF-8, UTF-16 and LATIN-1 is now two to four times faster.

(Contributed by Antoine Pitrou and Amaury Forgeot d'Arc, 4868{.interpreted-text role="issue"}.)
The json{.interpreted-text role="mod"} module now has a C extension to substantially improve its performance. In addition, the API was modified so that json works only with str{.interpreted-text role="class"}, not with bytes{.interpreted-text role="class"}. That change makes the module closely match the JSON specification which is defined in terms of Unicode.

(Contributed by Bob Ippolito and converted to Py3.1 by Antoine Pitrou and Benjamin Peterson; 4136{.interpreted-text role="issue"}.)
Unpickling now interns the attribute names of pickled objects. This saves memory and allows pickles to be smaller.

(Contributed by Jake McGuire and Antoine Pitrou; 5084{.interpreted-text role="issue"}.)

IDLE

IDLE's format menu now provides an option to strip trailing whitespace from a source file.

(Contributed by Roger D. Serwy; 5150{.interpreted-text role="issue"}.)

Build and C API Changes

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

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 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 sys.int_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.int_info
sys.int_info(bits_per_digit=30, sizeof_digit=4)
```
(Contributed by Mark Dickinson; 4258{.interpreted-text role="issue"}.)
The PyLong_AsUnsignedLongLong(){.interpreted-text role="c:func"} function now handles a negative pylong by raising OverflowError{.interpreted-text role="exc"} instead of TypeError{.interpreted-text role="exc"}.

(Contributed by Mark Dickinson and Lisandro Dalcrin; 5175{.interpreted-text role="issue"}.)
Deprecated !PyNumber_Int{.interpreted-text role="c:func"}. Use PyNumber_Long{.interpreted-text role="c:func"} instead.

(Contributed by Mark Dickinson; 4910{.interpreted-text role="issue"}.)
Added a new PyOS_string_to_double{.interpreted-text role="c:func"} function to replace the deprecated functions !PyOS_ascii_strtod{.interpreted-text role="c:func"} and !PyOS_ascii_atof{.interpreted-text role="c:func"}.

(Contributed by Mark Dickinson; 5914{.interpreted-text role="issue"}.)
Added PyCapsule{.interpreted-text role="c:type"} as a replacement for the !PyCObject{.interpreted-text role="c:type"} API. The principal difference is that the new type has a well defined interface for passing typing safety information and a less complicated signature for calling a destructor. The old type had a problematic API and is now deprecated.

(Contributed by Larry Hastings; 5630{.interpreted-text role="issue"}.)

Porting to Python 3.1

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

The new floating-point string representations can break existing doctests. For example:

def e():
    '''Compute the base of natural logarithms.

    >>> e()
    2.7182818284590451

    '''
    return sum(1/math.factorial(x) for x in reversed(range(30)))

doctest.testmod()

**********************************************************************
Failed example:
    e()
Expected:
    2.7182818284590451
Got:
    2.718281828459045
**********************************************************************

The automatic name remapping in the pickle module for protocol 2 or lower can make Python 3.1 pickles unreadable in Python 3.0. One solution is to use protocol 3. Another solution is to set the fix_imports option to False. See the discussion above for more details.