What’s New In Python 3.7

What’s New In Python 3.7¶
- Editor:
Elvis Pranskevichus <elvis@magic.io>
This article explains the new features in Python 3.7, compared to 3.6. Python 3.7 was released on June 27, 2018. For full details, see the changelog.
Summary – Release Highlights¶
New syntax features:
PEP 563, postponed evaluation of type annotations.
Backwards incompatible syntax changes:
New library modules:
New built-in features:
PEP 553, the new
breakpoint()
function.
Python data model improvements:
PEP 562, customization of access to module attributes.
PEP 560, core support for typing module and generic types.
the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec.
Significant improvements in the standard library:
The
asyncio
module has received new features, significant usability and performance improvements.The
time
module gained support for functions with nanosecond resolution.
CPython implementation improvements:
Avoiding the use of ASCII as a default text encoding:
PEP 552, deterministic .pycs
PEP 565, improved
DeprecationWarning
handling
C API improvements:
PEP 539, new C API for thread-local storage
Documentation improvements:
PEP 545, Python documentation translations
New documentation translations: Japanese, French, and Korean.
This release features notable performance improvements in many areas. The Optimizations section lists them in detail.
For a list of changes that may affect compatibility with previous Python releases please refer to the Porting to Python 3.7 section.
New Features¶
PEP 563: Postponed Evaluation of Annotations¶
The advent of type hints in Python uncovered two glaring usability issues with the functionality of annotations added in PEP 3107 and refined further in PEP 526:
annotations could only use names which were already available in the current scope, in other words they didn’t support forward references of any kind; and
annotating source code had adverse effects on startup time of Python programs.
Both of these issues are fixed by postponing the evaluation of
annotations. Instead of compiling code which executes expressions in
annotations at their definition time, the compiler stores the annotation
in a string form equivalent to the AST of the expression in question.
If needed, annotations can be resolved at runtime using
typing.get_type_hints()
. In the common case where this is not
required, the annotations are cheaper to store (since short strings
are interned by the interpreter) and make startup time faster.
Usability-wise, annotations now support forward references, making the following syntax valid:
class C:
@classmethod
def from_string(cls, source: str) -> C:
...
def validate_b(self, obj: B) -> bool:
...
class B:
...
Since this change breaks compatibility, the new behavior needs to be enabled
on a per-module basis in Python 3.7 using a __future__
import:
from __future__ import annotations
It will become the default in Python 3.10.
See also
- PEP 563 – Postponed evaluation of annotations
PEP written and implemented by Łukasz Langa.
PEP 538: Legacy C Locale Coercion¶
An ongoing challenge within the Python 3 series has been determining a sensible default strategy for handling the “7-bit ASCII” text encoding assumption currently implied by the use of the default C or POSIX locale on non-Windows platforms.
PEP 538 updates the default interpreter command line interface to
automatically coerce that locale to an available UTF-8 based locale as
described in the documentation of the new PYTHONCOERCECLOCALE
environment variable. Automatically setting LC_CTYPE
this way means that
both the core interpreter and locale-aware C extensions (such as
readline
) will assume the use of UTF-8 as the default text encoding,
rather than ASCII.
The platform support definition in PEP 11 has also been updated to limit full text handling support to suitably configured non-ASCII based locales.
As part of this change, the default error handler for stdin
and
stdout
is now surrogateescape
(rather than strict
) when
using any of the defined coercion target locales (currently C.UTF-8
,
C.utf8
, and UTF-8
). The default error handler for stderr
continues to be backslashreplace
, regardless of locale.
Locale coercion is silent by default, but to assist in debugging potentially
locale related integration problems, explicit warnings (emitted directly on
stderr
) can be requested by setting PYTHONCOERCECLOCALE=warn
.
This setting will also cause the Python runtime to emit a warning if the
legacy C locale remains active when the core interpreter is initialized.
While PEP 538’s locale coercion has the benefit of also affecting extension
modules (such as GNU readline
), as well as child processes (including those
running non-Python applications and older versions of Python), it has the
downside of requiring that a suitable target locale be present on the running
system. To better handle the case where no suitable target locale is available
(as occurs on RHEL/CentOS 7, for example), Python 3.7 also implements
PEP 540: Forced UTF-8 Runtime Mode.
See also
- PEP 538 – Coercing the legacy C locale to a UTF-8 based locale
PEP written and implemented by Nick Coghlan.
PEP 540: Forced UTF-8 Runtime Mode¶
The new -X
utf8
command line option and PYTHONUTF8
environment variable can be used to enable the Python UTF-8 Mode.
When in UTF-8 mode, CPython ignores the locale settings, and uses the
UTF-8 encoding by default. The error handlers for sys.stdin
and
sys.stdout
streams are set to surrogateescape
.
The forced UTF-8 mode can be used to change the text handling behavior in an embedded Python interpreter without changing the locale settings of an embedding application.
While PEP 540’s UTF-8 mode has the benefit of working regardless of which
locales are available on the running system, it has the downside of having no
effect on extension modules (such as GNU readline
), child processes running
non-Python applications, and child processes running older versions of Python.
To reduce the risk of corrupting text data when communicating with such
components, Python 3.7 also implements PEP 540: Forced UTF-8 Runtime Mode).
The UTF-8 mode is enabled by default when the locale is C
or POSIX
, and
the PEP 538 locale coercion feature fails to change it to a UTF-8 based
alternative (whether that failure is due to PYTHONCOERCECLOCALE=0
being set,
LC_ALL
being set, or the lack of a suitable target locale).
See also
- PEP 540 – Add a new UTF-8 mode
PEP written and implemented by Victor Stinner
PEP 553: Built-in breakpoint()
¶
Python 3.7 includes the new built-in breakpoint()
function as
an easy and consistent way to enter the Python debugger.
Built-in breakpoint()
calls sys.breakpointhook()
. By default, the
latter imports pdb
and then calls pdb.set_trace()
, but by binding
sys.breakpointhook()
to the function of your choosing, breakpoint()
can
enter any debugger. Additionally, the environment variable
PYTHONBREAKPOINT
can be set to the callable of your debugger of
choice. Set PYTHONBREAKPOINT=0
to completely disable built-in
breakpoint()
.
See also
- PEP 553 – Built-in breakpoint()
PEP written and implemented by Barry Warsaw
PEP 539: New C API for Thread-Local Storage¶
While Python provides a C API for thread-local storage support; the existing Thread Local Storage (TLS) API has used int to represent TLS keys across all platforms. This has not generally been a problem for officially support platforms, but that is neither POSIX-compliant, nor portable in any practical sense.
PEP 539 changes this by providing a new Thread Specific Storage (TSS)
API to CPython which supersedes use of the
existing TLS API within the CPython interpreter, while deprecating the existing
API. The TSS API uses a new type Py_tss_t
instead of int
to represent TSS keys–an opaque type the definition of which may depend on
the underlying TLS implementation. Therefore, this will allow to build CPython
on platforms where the native TLS key is defined in a way that cannot be safely
cast to int.
Note that on platforms where the native TLS key is defined in a way that cannot be safely cast to int, all functions of the existing TLS API will be no-op and immediately return failure. This indicates clearly that the old API is not supported on platforms where it cannot be used reliably, and that no effort will be made to add such support.
See also
- PEP 539 – A New C-API for Thread-Local Storage in CPython
PEP written by Erik M. Bray; implementation by Masayuki Yamamoto.
PEP 562: Customization of Access to Module Attributes¶
Python 3.7 allows defining __getattr__()
on modules and will call
it whenever a module attribute is otherwise not found. Defining
__dir__()
on modules is now also allowed.
A typical example of where this may be useful is module attribute deprecation and lazy loading.
See also
- PEP 562 – Module
__getattr__
and__dir__
PEP written and implemented by Ivan Levkivskyi
PEP 564: New Time Functions With Nanosecond Resolution¶
The resolution of clocks in modern systems can exceed the limited precision
of a floating-point number returned by the time.time()
function
and its variants. To avoid loss of precision, PEP 564 adds six new
“nanosecond” variants of the existing timer functions to the time
module:
The new functions return the number of nanoseconds as an integer value.
Measurements
show that on Linux and Windows the resolution of time.time_ns()
is
approximately 3 times better than that of time.time()
.
See also
- PEP 564 – Add new time functions with nanosecond resolution
PEP written and implemented by Victor Stinner
PEP 565: Show DeprecationWarning in __main__
¶
The default handling of DeprecationWarning
has been changed such that
these warnings are once more shown by default, but only when the code
triggering them is running directly in the __main__
module. As a result,
developers of single file scripts and those using Python interactively should
once again start seeing deprecation warnings for the APIs they use, but
deprecation warnings triggered by imported application, library and framework
modules will continue to be hidden by default.
As a result of this change, the standard library now allows developers to choose between three different deprecation warning behaviours:
FutureWarning
: always displayed by default, recommended for warnings intended to be seen by application end users (e.g. for deprecated application configuration settings).DeprecationWarning
: displayed by default only in__main__
and when running tests, recommended for warnings intended to be seen by other Python developers where a version upgrade may result in changed behaviour or an error.PendingDeprecationWarning
: displayed by default only when running tests, intended for cases where a future version upgrade will change the warning category toDeprecationWarning
orFutureWarning
.
Previously both DeprecationWarning
and PendingDeprecationWarning
were only visible when running tests, which meant that developers primarily
writing single file scripts or using Python interactively could be surprised
by breaking changes in the APIs they used.
See also
- PEP 565 – Show DeprecationWarning in
__main__
PEP written and implemented by Nick Coghlan
PEP 560: Core Support for typing
module and Generic Types¶
Initially PEP 484 was designed in such way that it would not introduce any
changes to the core CPython interpreter. Now type hints and the typing
module are extensively used by the community, so this restriction is removed.
The PEP introduces two special methods __class_getitem__()
and
__mro_entries__()
, these methods are now used by most classes and special
constructs in typing
. As a result, the speed of various operations
with types increased up to 7 times, the generic types can be used without
metaclass conflicts, and several long standing bugs in typing
module are
fixed.
See also
- PEP 560 – Core support for typing module and generic types
PEP written and implemented by Ivan Levkivskyi
PEP 552: Hash-based .pyc Files¶
Python has traditionally checked the up-to-dateness of bytecode cache files
(i.e., .pyc
files) by comparing the source metadata (last-modified timestamp
and size) with source metadata saved in the cache file header when it was
generated. While effective, this invalidation method has its drawbacks. When
filesystem timestamps are too coarse, Python can miss source updates, leading to
user confusion. Additionally, having a timestamp in the cache file is
problematic for build reproducibility and
content-based build systems.
PEP 552 extends the pyc format to allow the hash of the source file to be
used for invalidation instead of the source timestamp. Such .pyc
files are
called “hash-based”. By default, Python still uses timestamp-based invalidation
and does not generate hash-based .pyc
files at runtime. Hash-based .pyc
files may be generated with py_compile
or compileall
.
Hash-based .pyc
files come in two variants: checked and unchecked. Python
validates checked hash-based .pyc
files against the corresponding source
files at runtime but doesn’t do so for unchecked hash-based pycs. Unchecked
hash-based .pyc
files are a useful performance optimization for environments
where a system external to Python (e.g., the build system) is responsible for
keeping .pyc
files up-to-date.
See Cached bytecode invalidation for more information.
See also
- PEP 552 – Deterministic pycs
PEP written and implemented by Benjamin Peterson
PEP 545: Python Documentation Translations¶
PEP 545 describes the process of creating and maintaining Python documentation translations.
Three new translations have been added:
Japanese: https://docs.python.org/ja/
French: https://docs.python.org/fr/
Korean: https://docs.python.org/ko/
See also
- PEP 545 – Python Documentation Translations
PEP written and implemented by Julien Palard, Inada Naoki, and Victor Stinner.
Python Development Mode (-X dev)¶
The new -X
dev
command line option or the new
PYTHONDEVMODE
environment variable can be used to enable
Python Development Mode. When in development mode, Python performs
additional runtime checks that are too expensive to be enabled by default.
See Python Development Mode documentation for the full
description.
Other Language Changes¶
An
await
expression and comprehensions containing anasync for
clause were illegal in the expressions in formatted string literals due to a problem with the implementation. In Python 3.7 this restriction was lifted.More than 255 arguments can now be passed to a function, and a function can now have more than 255 parameters. (Contributed by Serhiy Storchaka in bpo-12844 and bpo-18896.)
bytes.fromhex()
andbytearray.fromhex()
now ignore all ASCII whitespace, not only spaces. (Contributed by Robert Xiao in bpo-28927.)str
,bytes
, andbytearray
gained support for the newisascii()
method, which can be used to test if a string or bytes contain only the ASCII characters. (Contributed by INADA Naoki in bpo-32677.)ImportError
now displays module name and module__file__
path whenfrom ... import ...
fails. (Contributed by Matthias Bussonnier in bpo-29546.)Circular imports involving absolute imports with binding a submodule to a name are now supported. (Contributed by Serhiy Storchaka in bpo-30024.)
object.__format__(x, '')
is now equivalent tostr(x)
rather thanformat(str(self), '')
. (Contributed by Serhiy Storchaka in bpo-28974.)In order to better support dynamic creation of stack traces,
types.TracebackType
can now be instantiated from Python code, and thetb_next
attribute on tracebacks is now writable. (Contributed by Nathaniel J. Smith in bpo-30579.)When using the
-m
switch,sys.path[0]
is now eagerly expanded to the full starting directory path, rather than being left as the empty directory (which allows imports from the current working directory at the time when an import occurs) (Contributed by Nick Coghlan in bpo-33053.)The new
-X
importtime
option or thePYTHONPROFILEIMPORTTIME
environment variable can be used to show the timing of each module import. (Contributed by Inada Naoki in bpo-31415.)
New Modules¶
contextvars¶
The new contextvars
module and a set of
new C APIs introduce
support for context variables. Context variables are conceptually
similar to thread-local variables. Unlike TLS, context variables
support asynchronous code correctly.
The asyncio
and decimal
modules have been updated to use
and support context variables out of the box. Particularly the active
decimal context is now stored in a context variable, which allows
decimal operations to work with the correct context in asynchronous code.
See also
- PEP 567 – Context Variables
PEP written and implemented by Yury Selivanov
dataclasses¶
The new dataclass()
decorator provides a way to declare
data classes. A data class describes its attributes using class variable
annotations. Its constructor and other magic methods, such as
__repr__()
, __eq__()
, and
__hash__()
are generated automatically.
Example:
@dataclass
class Point:
x: float
y: float
z: float = 0.0
p = Point(1.5, 2.5)
print(p) # produces "Point(x=1.5, y=2.5, z=0.0)"
See also
- PEP 557 – Data Classes
PEP written and implemented by Eric V. Smith
importlib.resources¶
The new importlib.resources
module provides several new APIs and one
new ABC for access to, opening, and reading resources inside packages.
Resources are roughly similar to files inside packages, but they needn’t
be actual files on the physical file system. Module loaders can provide a
get_resource_reader()
function which returns
a importlib.abc.ResourceReader
instance to support this
new API. Built-in file path loaders and zip file loaders both support this.
Contributed by Barry Warsaw and Brett Cannon in bpo-32248.
See also
importlib_resources – a PyPI backport for earlier Python versions.
Improved Modules¶
argparse¶
The new ArgumentParser.parse_intermixed_args()
method allows intermixing options and positional arguments.
(Contributed by paul.j3 in bpo-14191.)
asyncio¶
The asyncio
module has received many new features, usability and
performance improvements. Notable changes
include:
The new provisional
asyncio.run()
function can be used to run a coroutine from synchronous code by automatically creating and destroying the event loop. (Contributed by Yury Selivanov in bpo-32314.)asyncio gained support for
contextvars
.loop.call_soon()
,loop.call_soon_threadsafe()
,loop.call_later()
,loop.call_at()
, andFuture.add_done_callback()
have a new optional keyword-only context parameter.Tasks
now track their context automatically. See PEP 567 for more details. (Contributed by Yury Selivanov in bpo-32436.)The new
asyncio.create_task()
function has been added as a shortcut toasyncio.get_event_loop().create_task()
. (Contributed by Andrew Svetlov in bpo-32311.)The new
loop.start_tls()
method can be used to upgrade an existing connection to TLS. (Contributed by Yury Selivanov in bpo-23749.)The new
loop.sock_recv_into()
method allows reading data from a socket directly into a provided buffer making it possible to reduce data copies. (Contributed by Antoine Pitrou in bpo-31819.)The new
asyncio.current_task()
function returns the currently runningTask
instance, and the newasyncio.all_tasks()
function returns a set of all existingTask
instances in a given loop. TheTask.current_task()
andTask.all_tasks()
methods have been deprecated. (Contributed by Andrew Svetlov in bpo-32250.)The new provisional
BufferedProtocol
class allows implementing streaming protocols with manual control over the receive buffer. (Contributed by Yury Selivanov in bpo-32251.)The new
asyncio.get_running_loop()
function returns the currently running loop, and raises aRuntimeError
if no loop is running. This is in contrast withasyncio.get_event_loop()
, which will create a new event loop if none is running. (Contributed by Yury Selivanov in bpo-32269.)The new
StreamWriter.wait_closed()
coroutine method allows waiting until the stream writer is closed. The newStreamWriter.is_closing()
method can be used to determine if the writer is closing. (Contributed by Andrew Svetlov in bpo-32391.)The new
loop.sock_sendfile()
coroutine method allows sending files usingos.sendfile
when possible. (Contributed by Andrew Svetlov in bpo-32410.)The new
Future.get_loop()
andTask.get_loop()
methods return the instance of the loop on which a task or a future were created.Server.get_loop()
allows doing the same forasyncio.Server
objects. (Contributed by Yury Selivanov in bpo-32415 and Srinivas Reddy Thatiparthy in bpo-32418.)It is now possible to control how instances of
asyncio.Server
begin serving. Previously, the server would start serving immediately when created. The new start_serving keyword argument toloop.create_server()
andloop.create_unix_server()
, as well asServer.start_serving()
, andServer.serve_forever()
can be used to decouple server instantiation and serving. The newServer.is_serving()
method returnsTrue
if the server is serving.Server
objects are now asynchronous context managers:srv = await loop.create_server(...) async with srv: # some code # At this point, srv is closed and no longer accepts new connections.
(Contributed by Yury Selivanov in bpo-32662.)
Callback objects returned by
loop.call_later()
gained the newwhen()
method which returns an absolute scheduled callback timestamp. (Contributed by Andrew Svetlov in bpo-32741.)The
loop.create_datagram_endpoint()
method gained support for Unix sockets. (Contributed by Quentin Dawans in bpo-31245.)The
asyncio.open_connection()
,asyncio.start_server()
functions,loop.create_connection()
,loop.create_server()
,loop.create_accepted_socket()
methods and their corresponding UNIX socket variants now accept the ssl_handshake_timeout keyword argument. (Contributed by Neil Aspinall in bpo-29970.)The new
Handle.cancelled()
method returnsTrue
if the callback was cancelled. (Contributed by Marat Sharafutdinov in bpo-31943.)The asyncio source has been converted to use the
async
/await
syntax. (Contributed by Andrew Svetlov in bpo-32193.)The new
ReadTransport.is_reading()
method can be used to determine the reading state of the transport. Additionally, calls toReadTransport.resume_reading()
andReadTransport.pause_reading()
are now idempotent. (Contributed by Yury Selivanov in bpo-32356.)Loop methods which accept socket paths now support passing path-like objects. (Contributed by Yury Selivanov in bpo-32066.)
In
asyncio
TCP sockets on Linux are now created withTCP_NODELAY
flag set by default. (Contributed by Yury Selivanov and Victor Stinner in bpo-27456.)Exceptions occurring in cancelled tasks are no longer logged. (Contributed by Yury Selivanov in bpo-30508.)
New
WindowsSelectorEventLoopPolicy
andWindowsProactorEventLoopPolicy
classes. (Contributed by Yury Selivanov in bpo-33792.)
Several asyncio
APIs have been
deprecated.
binascii¶
The b2a_uu()
function now accepts an optional backtick
keyword argument. When it’s true, zeros are represented by '`'
instead of spaces. (Contributed by Xiang Zhang in bpo-30103.)
calendar¶
The HTMLCalendar
class has new class attributes which ease
the customization of CSS classes in the produced HTML calendar.
(Contributed by Oz Tiram in bpo-30095.)
collections¶
collections.namedtuple()
now supports default values.
(Contributed by Raymond Hettinger in bpo-32320.)
compileall¶
compileall.compile_dir()
learned the new invalidation_mode parameter,
which can be used to enable
hash-based .pyc invalidation. The invalidation
mode can also be specified on the command line using the new
--invalidation-mode
argument.
(Contributed by Benjamin Peterson in bpo-31650.)
concurrent.futures¶
ProcessPoolExecutor
and
ThreadPoolExecutor
now
support the new initializer and initargs constructor arguments.
(Contributed by Antoine Pitrou in bpo-21423.)
The ProcessPoolExecutor
can now take the multiprocessing context via the new mp_context argument.
(Contributed by Thomas Moreau in bpo-31540.)
contextlib¶
The new nullcontext()
is a simpler and faster no-op
context manager than ExitStack
.
(Contributed by Jesse-Bakker in bpo-10049.)
The new asynccontextmanager()
,
AbstractAsyncContextManager
, and
AsyncExitStack
have been added to
complement their synchronous counterparts. (Contributed
by Jelle Zijlstra in bpo-29679 and bpo-30241,
and by Alexander Mohr and Ilya Kulakov in bpo-29302.)
cProfile¶
The cProfile
command line now accepts -m module_name
as an
alternative to script path. (Contributed by Sanyam Khurana in bpo-21862.)
crypt¶
The crypt
module now supports the Blowfish hashing method.
(Contributed by Serhiy Storchaka in bpo-31664.)
The mksalt()
function now allows specifying the number of rounds
for hashing. (Contributed by Serhiy Storchaka in bpo-31702.)
datetime¶
The new datetime.fromisoformat()
method constructs a datetime
object from a string
in one of the formats output by
datetime.isoformat()
.
(Contributed by Paul Ganssle in bpo-15873.)
The tzinfo
class now supports sub-minute offsets.
(Contributed by Alexander Belopolsky in bpo-5288.)
dbm¶
dbm.dumb
now supports reading read-only files and no longer writes the
index file when it is not changed.
decimal¶
The decimal
module now uses context variables
to store the decimal context.
(Contributed by Yury Selivanov in bpo-32630.)
dis¶
The dis()
function is now able to
disassemble nested code objects (the code of comprehensions, generator
expressions and nested functions, and the code used for building nested
classes). The maximum depth of disassembly recursion is controlled by
the new depth parameter.
(Contributed by Serhiy Storchaka in bpo-11822.)
distutils¶
README.rst
is now included in the list of distutils standard READMEs and
therefore included in source distributions.
(Contributed by Ryan Gonzalez in bpo-11913.)
enum¶
The Enum
learned the new _ignore_
class property,
which allows listing the names of properties which should not become
enum members.
(Contributed by Ethan Furman in bpo-31801.)
In Python 3.8, attempting to check for non-Enum objects in Enum
classes will raise a TypeError
(e.g. 1 in Color
); similarly,
attempting to check for non-Flag objects in a Flag
member will
raise TypeError
(e.g. 1 in Perm.RW
); currently, both operations
return False
instead and are deprecated.
(Contributed by Ethan Furman in bpo-33217.)
functools¶
functools.singledispatch()
now supports registering implementations
using type annotations.
(Contributed by Łukasz Langa in bpo-32227.)
gc¶
The new gc.freeze()
function allows freezing all objects tracked
by the garbage collector and excluding them from future collections.
This can be used before a POSIX fork()
call to make the GC copy-on-write
friendly or to speed up collection. The new gc.unfreeze()
functions
reverses this operation. Additionally, gc.get_freeze_count()
can
be used to obtain the number of frozen objects.
(Contributed by Li Zekun in bpo-31558.)
hmac¶
The hmac
module now has an optimized one-shot digest()
function, which is up to three times faster than HMAC()
.
(Contributed by Christian Heimes in bpo-32433.)
http.client¶
HTTPConnection
and HTTPSConnection
now support the new blocksize argument for improved upload throughput.
(Contributed by Nir Soffer in bpo-31945.)
http.server¶
SimpleHTTPRequestHandler
now supports the HTTP
If-Modified-Since
header. The server returns the 304 response status if
the target file was not modified after the time specified in the header.
(Contributed by Pierre Quentel in bpo-29654.)
SimpleHTTPRequestHandler
accepts the new directory
argument, in addition to the new --directory
command line argument.
With this parameter, the server serves the specified directory, by default it
uses the current working directory.
(Contributed by Stéphane Wirtel and Julien Palard in bpo-28707.)
The new ThreadingHTTPServer
class
uses threads to handle requests using ThreadingMixIn
.
It is used when http.server
is run with -m
.
(Contributed by Julien Palard in bpo-31639.)
idlelib and IDLE¶
Multiple fixes for autocompletion. (Contributed by Louie Lu in bpo-15786.)
Module Browser (on the File menu, formerly called Class Browser), now displays nested functions and classes in addition to top-level functions and classes. (Contributed by Guilherme Polo, Cheryl Sabella, and Terry Jan Reedy in bpo-1612262.)
The Settings dialog (Options, Configure IDLE) has been partly rewritten to improve both appearance and function. (Contributed by Cheryl Sabella and Terry Jan Reedy in multiple issues.)
The font sample now includes a selection of non-Latin characters so that users can better see the effect of selecting a particular font. (Contributed by Terry Jan Reedy in bpo-13802.) The sample can be edited to include other characters. (Contributed by Serhiy Storchaka in bpo-31860.)
The IDLE features formerly implemented as extensions have been reimplemented as normal features. Their settings have been moved from the Extensions tab to other dialog tabs. (Contributed by Charles Wohlganger and Terry Jan Reedy in bpo-27099.)
Editor code context option revised. Box displays all context lines up to maxlines. Clicking on a context line jumps the editor to that line. Context colors for custom themes is added to Highlights tab of Settings dialog. (Contributed by Cheryl Sabella and Terry Jan Reedy in bpo-33642, bpo-33768, and bpo-33679.)
On Windows, a new API call tells Windows that tk scales for DPI. On Windows 8.1+ or 10, with DPI compatibility properties of the Python binary unchanged, and a monitor resolution greater than 96 DPI, this should make text and lines sharper. It should otherwise have no effect. (Contributed by Terry Jan Reedy in bpo-33656.)
New in 3.7.1:
Output over N lines (50 by default) is squeezed down to a button. N can be changed in the PyShell section of the General page of the Settings dialog. Fewer, but possibly extra long, lines can be squeezed by right clicking on the output. Squeezed output can be expanded in place by double-clicking the button or into the clipboard or a separate window by right-clicking the button. (Contributed by Tal Einat in bpo-1529353.)
The changes above have been backported to 3.6 maintenance releases.
NEW in 3.7.4:
Add “Run Customized” to the Run menu to run a module with customized settings. Any command line arguments entered are added to sys.argv. They re-appear in the box for the next customized run. One can also suppress the normal Shell main module restart. (Contributed by Cheryl Sabella, Terry Jan Reedy, and others in bpo-5680 and bpo-37627.)
New in 3.7.5:
Add optional line numbers for IDLE editor windows. Windows open without line numbers unless set otherwise in the General tab of the configuration dialog. Line numbers for an existing window are shown and hidden in the Options menu. (Contributed by Tal Einat and Saimadhav Heblikar in bpo-17535.)
importlib¶
The importlib.abc.ResourceReader
ABC was introduced to
support the loading of resources from packages. See also
importlib.resources.
(Contributed by Barry Warsaw, Brett Cannon in bpo-32248.)
importlib.reload()
now raises ModuleNotFoundError
if the module
lacks a spec.
(Contributed by Garvit Khatri in bpo-29851.)
importlib.util.find_spec()
now raises ModuleNotFoundError
instead of
AttributeError
if the specified parent module is not a package (i.e.
lacks a __path__
attribute).
(Contributed by Milan Oberkirch in bpo-30436.)
The new importlib.util.source_hash()
can be used to compute the hash of
the passed source. A hash-based .pyc file
embeds the value returned by this function.
io¶
The new TextIOWrapper.reconfigure()
method can be used to reconfigure the text stream with the new settings.
(Contributed by Antoine Pitrou in bpo-30526 and
INADA Naoki in bpo-15216.)
ipaddress¶
The new subnet_of()
and supernet_of()
methods of
ipaddress.IPv6Network
and ipaddress.IPv4Network
can
be used for network containment tests.
(Contributed by Michel Albert and Cheryl Sabella in bpo-20825.)
itertools¶
itertools.islice()
now accepts
integer-like objects
as start, stop,
and slice arguments.
(Contributed by Will Roberts in bpo-30537.)
locale¶
The new monetary argument to locale.format_string()
can be used
to make the conversion use monetary thousands separators and
grouping strings. (Contributed by Garvit in bpo-10379.)
The locale.getpreferredencoding()
function now always returns 'UTF-8'
on Android or when in the forced UTF-8 mode.
logging¶
Logger
instances can now be pickled.
(Contributed by Vinay Sajip in bpo-30520.)
The new StreamHandler.setStream()
method can be used to replace the logger stream after handler creation.
(Contributed by Vinay Sajip in bpo-30522.)
It is now possible to specify keyword arguments to handler constructors in
configuration passed to logging.config.fileConfig()
.
(Contributed by Preston Landers in bpo-31080.)
math¶
The new math.remainder()
function implements the IEEE 754-style remainder
operation. (Contributed by Mark Dickinson in bpo-29962.)
mimetypes¶
The MIME type of .bmp has been changed from 'image/x-ms-bmp'
to
'image/bmp'
.
(Contributed by Nitish Chandra in bpo-22589.)
msilib¶
The new Database.Close()
method can be used
to close the MSI database.
(Contributed by Berker Peksag in bpo-20486.)
multiprocessing¶
The new Process.close()
method
explicitly closes the process object and releases all resources associated
with it. ValueError
is raised if the underlying process is still
running.
(Contributed by Antoine Pitrou in bpo-30596.)
The new Process.kill()
method can
be used to terminate the process using the SIGKILL
signal on Unix.
(Contributed by Vitor Pereira in bpo-30794.)
Non-daemonic threads created by Process
are now
joined on process exit.
(Contributed by Antoine Pitrou in bpo-18966.)
os¶
os.fwalk()
now accepts the path argument as bytes
.
(Contributed by Serhiy Storchaka in bpo-28682.)
os.scandir()
gained support for file descriptors.
(Contributed by Serhiy Storchaka in bpo-25996.)
The new register_at_fork()
function allows registering Python
callbacks to be executed at process fork.
(Contributed by Antoine Pitrou in bpo-16500.)
Added os.preadv()
(combine the functionality of os.readv()
and
os.pread()
) and os.pwritev()
functions (combine the functionality
of os.writev()
and os.pwrite()
). (Contributed by Pablo Galindo in
bpo-31368.)
The mode argument of os.makedirs()
no longer affects the file
permission bits of newly created intermediate-level directories.
(Contributed by Serhiy Storchaka in bpo-19930.)
os.dup2()
now returns the new file descriptor. Previously, None
was always returned.
(Contributed by Benjamin Peterson in bpo-32441.)
The structure returned by os.stat()
now contains the
st_fstype
attribute on Solaris and its derivatives.
(Contributed by Jesús Cea Avión in bpo-32659.)
pathlib¶
The new Path.is_mount()
method is now available
on POSIX systems and can be used to determine whether a path is a mount point.
(Contributed by Cooper Ry Lees in bpo-30897.)
pdb¶
pdb.set_trace()
now takes an optional header keyword-only
argument. If given, it is printed to the console just before debugging
begins. (Contributed by Barry Warsaw in bpo-31389.)
pdb
command line now accepts -m module_name
as an alternative to
script file. (Contributed by Mario Corchero in bpo-32206.)
py_compile¶
py_compile.compile()
– and by extension, compileall
– now
respects the SOURCE_DATE_EPOCH
environment variable by
unconditionally creating .pyc
files for hash-based validation.
This allows for guaranteeing
reproducible builds of .pyc
files when they are created eagerly. (Contributed by Bernhard M. Wiedemann
in bpo-29708.)
pydoc¶
The pydoc server can now bind to an arbitrary hostname specified by the
new -n
command-line argument.
(Contributed by Feanil Patel in bpo-31128.)
queue¶
The new SimpleQueue
class is an unbounded FIFO queue.
(Contributed by Antoine Pitrou in bpo-14976.)
re¶
The flags re.ASCII
, re.LOCALE
and re.UNICODE
can be set within the scope of a group.
(Contributed by Serhiy Storchaka in bpo-31690.)
re.split()
now supports splitting on a pattern like r'\b'
,
'^$'
or (?=-)
that matches an empty string.
(Contributed by Serhiy Storchaka in bpo-25054.)
Regular expressions compiled with the re.LOCALE
flag no longer
depend on the locale at compile time. Locale settings are applied only
when the compiled regular expression is used.
(Contributed by Serhiy Storchaka in bpo-30215.)
FutureWarning
is now emitted if a regular expression contains
character set constructs that will change semantically in the future,
such as nested sets and set operations.
(Contributed by Serhiy Storchaka in bpo-30349.)
Compiled regular expression and match objects can now be copied
using copy.copy()
and copy.deepcopy()
.
(Contributed by Serhiy Storchaka in bpo-10076.)
signal¶
The new warn_on_full_buffer argument to the signal.set_wakeup_fd()
function makes it possible to specify whether Python prints a warning on
stderr when the wakeup buffer overflows.
(Contributed by Nathaniel J. Smith in bpo-30050.)
socket¶
The new socket.getblocking()
method
returns True
if the socket is in blocking mode and False
otherwise.
(Contributed by Yury Selivanov in bpo-32373.)
The new socket.close()
function closes the passed socket file descriptor.
This function should be used instead of os.close()
for better
compatibility across platforms.
(Contributed by Christian Heimes in bpo-32454.)
The socket
module now exposes the socket.TCP_CONGESTION (Linux 2.6.13), socket.TCP_USER_TIMEOUT (Linux 2.6.37), and socket.TCP_NOTSENT_LOWAT (Linux 3.12) constants.
(Contributed by Omar Sandoval in bpo-26273 and
Nathaniel J. Smith in bpo-29728.)
Support for socket.AF_VSOCK
sockets has been added to allow
communication between virtual machines and their hosts.
(Contributed by Cathy Avery in bpo-27584.)
Sockets now auto-detect family, type and protocol from file descriptor by default. (Contributed by Christian Heimes in bpo-28134.)
socketserver¶
socketserver.ThreadingMixIn.server_close
now waits until all non-daemon
threads complete. socketserver.ForkingMixIn.server_close
now waits
until all child processes complete.
Add a new socketserver.ForkingMixIn.block_on_close
class attribute to
socketserver.ForkingMixIn
and socketserver.ThreadingMixIn
classes. Set the class attribute to False
to get the pre-3.7 behaviour.
sqlite3¶
sqlite3.Connection
now exposes the backup()
method when the underlying SQLite library is at version 3.6.11 or higher.
(Contributed by Lele Gaifax in bpo-27645.)
The database argument of sqlite3.connect()
now accepts any
path-like object, instead of just a string.
(Contributed by Anders Lorentsen in bpo-31843.)
ssl¶
The ssl
module now uses OpenSSL’s builtin API instead of
match_hostname()
to check a host name or an IP address. Values
are validated during TLS handshake. Any certificate validation error
including failing the host name check now raises
SSLCertVerificationError
and aborts the handshake with a proper
TLS Alert message. The new exception contains additional information.
Host name validation can be customized with
SSLContext.hostname_checks_common_name
.
(Contributed by Christian Heimes in bpo-31399.)
Note
The improved host name check requires a libssl implementation compatible with OpenSSL 1.0.2 or 1.1. Consequently, OpenSSL 0.9.8 and 1.0.1 are no longer supported (see Platform Support Removals for more details). The ssl module is mostly compatible with LibreSSL 2.7.2 and newer.
The ssl
module no longer sends IP addresses in SNI TLS extension.
(Contributed by Christian Heimes in bpo-32185.)
match_hostname()
no longer supports partial wildcards like
www*.example.org
.
(Contributed by Mandeep Singh in bpo-23033 and Christian Heimes in
bpo-31399.)
The default cipher suite selection of the ssl
module now uses a blacklist
approach rather than a hard-coded whitelist. Python no longer re-enables
ciphers that have been blocked by OpenSSL security updates. Default cipher
suite selection can be configured at compile time.
(Contributed by Christian Heimes in bpo-31429.)
Validation of server certificates containing internationalized domain names
(IDNs) is now supported. As part of this change, the
SSLSocket.server_hostname
attribute
now stores the expected hostname in A-label form ("xn--pythn-mua.org"
),
rather than the U-label form ("pythön.org"
). (Contributed by
Nathaniel J. Smith and Christian Heimes in bpo-28414.)
The ssl
module has preliminary and experimental support for TLS 1.3 and
OpenSSL 1.1.1. At the time of Python 3.7.0 release, OpenSSL 1.1.1 is still
under development and TLS 1.3 hasn’t been finalized yet. The TLS 1.3
handshake and protocol behaves slightly differently than TLS 1.2 and earlier,
see TLS 1.3.
(Contributed by Christian Heimes in bpo-32947, bpo-20995,
bpo-29136, bpo-30622 and bpo-33618)
SSLSocket
and SSLObject
no longer have a public
constructor. Direct instantiation was never a documented and supported
feature. Instances must be created with SSLContext
methods
wrap_socket()
and wrap_bio()
.
(Contributed by Christian Heimes in bpo-32951)
OpenSSL 1.1 APIs for setting the minimum and maximum TLS protocol version are
available as SSLContext.minimum_version
and SSLContext.maximum_version
.
Supported protocols are indicated by several new flags, such as
HAS_TLSv1_1
.
(Contributed by Christian Heimes in bpo-32609.)
Added ssl.SSLContext.post_handshake_auth
to enable and
ssl.SSLSocket.verify_client_post_handshake()
to initiate TLS 1.3
post-handshake authentication.
(Contributed by Christian Heimes in gh-78851.)
string¶
string.Template
now lets you to optionally modify the regular
expression pattern for braced placeholders and non-braced placeholders
separately. (Contributed by Barry Warsaw in bpo-1198569.)
subprocess¶
The subprocess.run()
function accepts the new capture_output
keyword argument. When true, stdout and stderr will be captured.
This is equivalent to passing subprocess.PIPE
as stdout and
stderr arguments.
(Contributed by Bo Bayles in bpo-32102.)
The subprocess.run
function and the subprocess.Popen
constructor
now accept the text keyword argument as an alias
to universal_newlines.
(Contributed by Andrew Clegg in bpo-31756.)
On Windows the default for close_fds was changed from False
to
True
when redirecting the standard handles. It’s now possible to set
close_fds to true when redirecting the standard handles. See
subprocess.Popen
. This means that close_fds now defaults to
True
on all supported platforms.
(Contributed by Segev Finer in bpo-19764.)
The subprocess module is now more graceful when handling
KeyboardInterrupt
during subprocess.call()
,
subprocess.run()
, or in a Popen
context manager. It now waits a short amount of time for the child
to exit, before continuing the handling of the KeyboardInterrupt
exception.
(Contributed by Gregory P. Smith in bpo-25942.)
sys¶
The new sys.breakpointhook()
hook function is called by the
built-in breakpoint()
.
(Contributed by Barry Warsaw in bpo-31353.)
On Android, the new sys.getandroidapilevel()
returns the build-time
Android API version.
(Contributed by Victor Stinner in bpo-28740.)
The new sys.get_coroutine_origin_tracking_depth()
function returns
the current coroutine origin tracking depth, as set by
the new sys.set_coroutine_origin_tracking_depth()
. asyncio
has been converted to use this new API instead of
the deprecated sys.set_coroutine_wrapper()
.
(Contributed by Nathaniel J. Smith in bpo-32591.)
time¶
PEP 564 adds six new functions with nanosecond resolution to the
time
module:
New clock identifiers have been added:
time.CLOCK_BOOTTIME
(Linux): Identical totime.CLOCK_MONOTONIC
, except it also includes any time that the system is suspended.time.CLOCK_PROF
(FreeBSD, NetBSD and OpenBSD): High-resolution per-process CPU timer.time.CLOCK_UPTIME
(FreeBSD, OpenBSD): Time whose absolute value is the time the system has been running and not suspended, providing accurate uptime measurement.
The new time.thread_time()
and time.thread_time_ns()
functions
can be used to get per-thread CPU time measurements.
(Contributed by Antoine Pitrou in bpo-32025.)
The new time.pthread_getcpuclockid()
function returns the clock ID
of the thread-specific CPU-time clock.
tkinter¶
The new tkinter.ttk.Spinbox
class is now available.
(Contributed by Alan Moore in bpo-32585.)
tracemalloc¶
tracemalloc.Traceback
behaves more like regular tracebacks,
sorting the frames from oldest to most recent.
Traceback.format()
now accepts negative limit, truncating the result to the
abs(limit)
oldest frames. To get the old behaviour, use
the new most_recent_first argument to Traceback.format()
.
(Contributed by Jesse Bakker in bpo-32121.)
types¶
The new WrapperDescriptorType
,
MethodWrapperType
, MethodDescriptorType
,
and ClassMethodDescriptorType
classes are now available.
(Contributed by Manuel Krebber and Guido van Rossum in bpo-29377,
and Serhiy Storchaka in bpo-32265.)
The new types.resolve_bases()
function resolves MRO entries
dynamically as specified by PEP 560.
(Contributed by Ivan Levkivskyi in bpo-32717.)
unicodedata¶
The internal unicodedata
database has been upgraded to use Unicode 11. (Contributed by Benjamin
Peterson.)
unittest¶
The new -k
command-line option allows filtering tests by a name
substring or a Unix shell-like pattern.
For example, python -m unittest -k foo
runs
foo_tests.SomeTest.test_something
, bar_tests.SomeTest.test_foo
,
but not bar_tests.FooTest.test_something
.
(Contributed by Jonas Haag in bpo-32071.)
unittest.mock¶
The sentinel
attributes now preserve their identity
when they are copied
or pickled
. (Contributed by
Serhiy Storchaka in bpo-20804.)
The new seal()
function allows sealing
Mock
instances, which will disallow further creation
of attribute mocks. The seal is applied recursively to all attributes that
are themselves mocks.
(Contributed by Mario Corchero in bpo-30541.)
urllib.parse¶
urllib.parse.quote()
has been updated from RFC 2396 to RFC 3986,
adding ~
to the set of characters that are never quoted by default.
(Contributed by Christian Theune and Ratnadeep Debnath in bpo-16285.)
uu¶
The uu.encode()
function now accepts an optional backtick
keyword argument. When it’s true, zeros are represented by '`'
instead of spaces. (Contributed by Xiang Zhang in bpo-30103.)
uuid¶
The new UUID.is_safe
attribute relays information
from the platform about whether generated UUIDs are generated with a
multiprocessing-safe method.
(Contributed by Barry Warsaw in bpo-22807.)
uuid.getnode()
now prefers universally administered
MAC addresses over locally administered MAC addresses.
This makes a better guarantee for global uniqueness of UUIDs returned
from uuid.uuid1()
. If only locally administered MAC addresses are
available, the first such one found is returned.
(Contributed by Barry Warsaw in bpo-32107.)
warnings¶
The initialization of the default warnings filters has changed as follows:
warnings enabled via command line options (including those for
-b
and the new CPython-specific-X
dev
option) are always passed to the warnings machinery via thesys.warnoptions
attribute.warnings filters enabled via the command line or the environment now have the following order of precedence:
the
BytesWarning
filter for-b
(or-bb
)any filters specified with the
-W
optionany filters specified with the
PYTHONWARNINGS
environment variableany other CPython specific filters (e.g. the
default
filter added for the new-X dev
mode)any implicit filters defined directly by the warnings machinery
in CPython debug builds, all warnings are now displayed by default (the implicit filter list is empty)
(Contributed by Nick Coghlan and Victor Stinner in bpo-20361, bpo-32043, and bpo-32230.)
Deprecation warnings are once again shown by default in single-file scripts and at the interactive prompt. See PEP 565: Show DeprecationWarning in __main__ for details. (Contributed by Nick Coghlan in bpo-31975.)
xml¶
As mitigation against DTD and external entity retrieval, the
xml.dom.minidom
and xml.sax
modules no longer process
external entities by default.
(Contributed by Christian Heimes in gh-61441.)
xml.etree¶
ElementPath predicates in the find()
methods can now compare text of the current node with [. = "text"]
,
not only text in children. Predicates also allow adding spaces for
better readability. (Contributed by Stefan Behnel in bpo-31648.)
xmlrpc.server¶
SimpleXMLRPCDispatcher.register_function()
can now be used as a decorator. (Contributed by Xiang Zhang in
bpo-7769.)
zipapp¶
Function create_archive()
now accepts an optional filter
argument to allow the user to select which files should be included in the
archive. (Contributed by Irmen de Jong in bpo-31072.)
Function create_archive()
now accepts an optional compressed
argument to generate a compressed archive. A command line option
--compress
has also been added to support compression.
(Contributed by Zhiming Wang in bpo-31638.)
zipfile¶
ZipFile
now accepts the new compresslevel parameter to
control the compression level.
(Contributed by Bo Bayles in bpo-21417.)
Subdirectories in archives created by ZipFile
are now stored in
alphabetical order.
(Contributed by Bernhard M. Wiedemann in bpo-30693.)
C API Changes¶
A new API for thread-local storage has been implemented. See PEP 539: New C API for Thread-Local Storage for an overview and Thread Specific Storage (TSS) API for a complete reference. (Contributed by Masayuki Yamamoto in bpo-25658.)
The new context variables functionality exposes a number of new C APIs.
The new PyImport_GetModule()
function returns the previously
imported module with the given name.
(Contributed by Eric Snow in bpo-28411.)
The new Py_RETURN_RICHCOMPARE
macro eases writing rich
comparison functions.
(Contributed by Petr Victorin in bpo-23699.)
The new Py_UNREACHABLE
macro can be used to mark unreachable
code paths.
(Contributed by Barry Warsaw in bpo-31338.)
The tracemalloc
now exposes a C API through the new
PyTraceMalloc_Track()
and PyTraceMalloc_Untrack()
functions.
(Contributed by Victor Stinner in bpo-30054.)
The new import__find__load__start and import__find__load__done static markers can be used to trace module imports. (Contributed by Christian Heimes in bpo-31574.)
The fields name
and doc
of structures
PyMemberDef
, PyGetSetDef
,
PyStructSequence_Field
, PyStructSequence_Desc
,
and wrapperbase
are now of type const char *
rather of
char *
. (Contributed by Serhiy Storchaka in bpo-28761.)
The result of PyUnicode_AsUTF8AndSize()
and PyUnicode_AsUTF8()
is now of type const char *
rather of char *
. (Contributed by Serhiy
Storchaka in bpo-28769.)
The result of PyMapping_Keys()
, PyMapping_Values()
and
PyMapping_Items()
is now always a list, rather than a list or a
tuple. (Contributed by Oren Milman in bpo-28280.)
Added functions PySlice_Unpack()
and PySlice_AdjustIndices()
.
(Contributed by Serhiy Storchaka in bpo-27867.)
PyOS_AfterFork()
is deprecated in favour of the new functions
PyOS_BeforeFork()
, PyOS_AfterFork_Parent()
and
PyOS_AfterFork_Child()
. (Contributed by Antoine Pitrou in
bpo-16500.)
The PyExc_RecursionErrorInst
singleton that was part of the public API
has been removed as its members being never cleared may cause a segfault
during finalization of the interpreter. Contributed by Xavier de Gaye in
bpo-22898 and bpo-30697.
Added C API support for timezones with timezone constructors
PyTimeZone_FromOffset()
and PyTimeZone_FromOffsetAndName()
,
and access to the UTC singleton with PyDateTime_TimeZone_UTC
.
Contributed by Paul Ganssle in bpo-10381.
The type of results of PyThread_start_new_thread()
and
PyThread_get_thread_ident()
, and the id parameter of
PyThreadState_SetAsyncExc()
changed from long to
unsigned long.
(Contributed by Serhiy Storchaka in bpo-6532.)
PyUnicode_AsWideCharString()
now raises a ValueError
if the
second argument is NULL
and the wchar_t* string contains null
characters. (Contributed by Serhiy Storchaka in bpo-30708.)
Changes to the startup sequence and the management of dynamic memory
allocators mean that the long documented requirement to call
Py_Initialize()
before calling most C API functions is now
relied on more heavily, and failing to abide by it may lead to segfaults in
embedding applications. See the Porting to Python 3.7 section in this
document and the Before Python Initialization section in the C API documentation
for more details.
The new PyInterpreterState_GetID()
returns the unique ID for a
given interpreter.
(Contributed by Eric Snow in bpo-29102.)
Py_DecodeLocale()
, Py_EncodeLocale()
now use the UTF-8
encoding when the UTF-8 mode is enabled.
(Contributed by Victor Stinner in bpo-29240.)
PyUnicode_DecodeLocaleAndSize()
and PyUnicode_EncodeLocale()
now use the current locale encoding for surrogateescape
error handler.
(Contributed by Victor Stinner in bpo-29240.)
The start and end parameters of PyUnicode_FindChar()
are
now adjusted to behave like string slices.
(Contributed by Xiang Zhang in bpo-28822.)
Build Changes¶
Support for building --without-threads
has been removed. The
threading
module is now always available.
(Contributed by Antoine Pitrou in bpo-31370.).
A full copy of libffi is no longer bundled for use when building the
_ctypes
module on non-OSX UNIX platforms. An installed copy
of libffi is now required when building _ctypes
on such platforms.
(Contributed by Zachary Ware in bpo-27979.)
The Windows build process no longer depends on Subversion to pull in external
sources, a Python script is used to download zipfiles from GitHub instead.
If Python 3.6 is not found on the system (via py -3.6
), NuGet is used to
download a copy of 32-bit Python for this purpose. (Contributed by Zachary
Ware in bpo-30450.)
The ssl
module requires OpenSSL 1.0.2 or 1.1 compatible libssl.
OpenSSL 1.0.1 has reached end of lifetime on 2016-12-31 and is no longer
supported. LibreSSL is temporarily not supported as well. LibreSSL releases
up to version 2.6.4 are missing required OpenSSL 1.0.2 APIs.
Optimizations¶
The overhead of calling many methods of various standard library classes
implemented in C has been significantly reduced by porting more code
to use the METH_FASTCALL
convention.
(Contributed by Victor Stinner in bpo-29300, bpo-29507,
bpo-29452, and bpo-29286.)
Various optimizations have reduced Python startup time by 10% on Linux and up to 30% on macOS. (Contributed by Victor Stinner, INADA Naoki in bpo-29585, and Ivan Levkivskyi in bpo-31333.)
Method calls are now up to 20% faster due to the bytecode changes which avoid creating bound method instances. (Contributed by Yury Selivanov and INADA Naoki in bpo-26110.)
The asyncio
module received a number of notable optimizations for
commonly used functions:
The
asyncio.get_event_loop()
function has been reimplemented in C to make it up to 15 times faster. (Contributed by Yury Selivanov in bpo-32296.)asyncio.Future
callback management has been optimized. (Contributed by Yury Selivanov in bpo-32348.)asyncio.gather()
is now up to 15% faster. (Contributed by Yury Selivanov in bpo-32355.)asyncio.sleep()
is now up to 2 times faster when the delay argument is zero or negative. (Contributed by Andrew Svetlov in bpo-32351.)The performance overhead of asyncio debug mode has been reduced. (Contributed by Antoine Pitrou in bpo-31970.)
As a result of PEP 560 work, the import time
of typing
has been reduced by a factor of 7, and many typing operations
are now faster.
(Contributed by Ivan Levkivskyi in bpo-32226.)
sorted()
and list.sort()
have been optimized for common cases
to be up to 40-75% faster.
(Contributed by Elliot Gorokhovsky in bpo-28685.)
dict.copy()
is now up to 5.5 times faster.
(Contributed by Yury Selivanov in bpo-31179.)
hasattr()
and getattr()
are now about 4 times faster when
name is not found and obj does not override object.__getattr__()
or object.__getattribute__()
.
(Contributed by INADA Naoki in bpo-32544.)
Searching for certain Unicode characters (like Ukrainian capital “Є”) in a string was up to 25 times slower than searching for other characters. It is now only 3 times slower in the worst case. (Contributed by Serhiy Storchaka in bpo-24821.)
The collections.namedtuple()
factory has been reimplemented to
make the creation of named tuples 4 to 6 times faster.
(Contributed by Jelle Zijlstra with further improvements by INADA Naoki,
Serhiy Storchaka, and Raymond Hettinger in bpo-28638.)
datetime.date.fromordinal()
and datetime.date.fromtimestamp()
are now up to 30% faster in the common case.
(Contributed by Paul Ganssle in bpo-32403.)
The os.fwalk()
function is now up to 2 times faster thanks to
the use of os.scandir()
.
(Contributed by Serhiy Storchaka in bpo-25996.)
The speed of the shutil.rmtree()
function has been improved by
20–40% thanks to the use of the os.scandir()
function.
(Contributed by Serhiy Storchaka in bpo-28564.)
Optimized case-insensitive matching and searching of regular
expressions
. Searching some patterns can now be up to 20 times faster.
(Contributed by Serhiy Storchaka in bpo-30285.)
re.compile()
now converts flags
parameter to int object if
it is RegexFlag
. It is now as fast as Python 3.5, and faster than
Python 3.6 by about 10% depending on the pattern.
(Contributed by INADA Naoki in bpo-31671.)
The modify()
methods of classes
selectors.EpollSelector
, selectors.PollSelector
and selectors.DevpollSelector
may be around 10% faster under
heavy loads. (Contributed by Giampaolo Rodola’ in bpo-30014)
Constant folding has been moved from the peephole optimizer to the new AST optimizer, which is able perform optimizations more consistently. (Contributed by Eugene Toder and INADA Naoki in bpo-29469 and bpo-11549.)
Most functions and methods in abc
have been rewritten in C.
This makes creation of abstract base classes, and calling isinstance()
and issubclass()
on them 1.5x faster. This also reduces Python
start-up time by up to 10%. (Contributed by Ivan Levkivskyi and INADA Naoki
in bpo-31333)
Significant speed improvements to alternate constructors for
datetime.date
and datetime.datetime
by using fast-path
constructors when not constructing subclasses. (Contributed by Paul Ganssle
in bpo-32403)
The speed of comparison of array.array
instances has been
improved considerably in certain cases. It is now from 10x to 70x faster
when comparing arrays holding values of the same integer type.
(Contributed by Adrian Wielgosik in bpo-24700.)
The math.erf()
and math.erfc()
functions now use the (faster)
C library implementation on most platforms.
(Contributed by Serhiy Storchaka in bpo-26121.)
Other CPython Implementation Changes¶
Trace hooks may now opt out of receiving the
line
and opt into receiving theopcode
events from the interpreter by setting the corresponding newf_trace_lines
andf_trace_opcodes
attributes on the frame being traced. (Contributed by Nick Coghlan in bpo-31344.)Fixed some consistency problems with namespace package module attributes. Namespace module objects now have an
__file__
that is set toNone
(previously unset), and their__spec__.origin
is also set toNone
(previously the string"namespace"
). See bpo-32305. Also, the namespace module object’s__spec__.loader
is set to the same value as__loader__
(previously, the former was set toNone
). See bpo-32303.The
locals()
dictionary now displays in the lexical order that variables were defined. Previously, the order was undefined. (Contributed by Raymond Hettinger in bpo-32690.)The
distutils
upload
command no longer tries to change CR end-of-line characters to CRLF. This fixes a corruption issue with sdists that ended with a byte equivalent to CR. (Contributed by Bo Bayles in bpo-32304.)
Deprecated Python Behavior¶
Yield expressions (both yield
and yield from
clauses) are now deprecated
in comprehensions and generator expressions (aside from the iterable expression
in the leftmost for
clause). This ensures that comprehensions
always immediately return a container of the appropriate type (rather than
potentially returning a generator iterator object), while generator
expressions won’t attempt to interleave their implicit output with the output
from any explicit yield expressions. In Python 3.7, such expressions emit
DeprecationWarning
when compiled, in Python 3.8 this will be a
SyntaxError
.
(Contributed by Serhiy Storchaka in bpo-10544.)
Returning a subclass of complex
from object.__complex__()
is
deprecated and will be an error in future Python versions. This makes
__complex__()
consistent with object.__int__()
and
object.__float__()
.
(Contributed by Serhiy Storchaka in bpo-28894.)
Deprecated Python modules, functions and methods¶
aifc¶
aifc.openfp()
has been deprecated and will be removed in Python 3.9.
Use aifc.open()
instead.
(Contributed by Brian Curtin in bpo-31985.)
asyncio¶
Support for directly await
-ing instances of asyncio.Lock
and
other asyncio synchronization primitives has been deprecated. An
asynchronous context manager must be used in order to acquire and release
the synchronization resource.
(Contributed by Andrew Svetlov in bpo-32253.)
The asyncio.Task.current_task()
and asyncio.Task.all_tasks()
methods have been deprecated.
(Contributed by Andrew Svetlov in bpo-32250.)
collections¶
In Python 3.8, the abstract base classes in collections.abc
will no
longer be exposed in the regular collections
module. This will help
create a clearer distinction between the concrete classes and the abstract
base classes.
(Contributed by Serhiy Storchaka in bpo-25988.)
dbm¶
dbm.dumb
now supports reading read-only files and no longer writes the
index file when it is not changed. A deprecation warning is now emitted
if the index file is missing and recreated in the 'r'
and 'w'
modes (this will be an error in future Python releases).
(Contributed by Serhiy Storchaka in bpo-28847.)
enum¶
In Python 3.8, attempting to check for non-Enum objects in Enum
classes will raise a TypeError
(e.g. 1 in Color
); similarly,
attempting to check for non-Flag objects in a Flag
member will
raise TypeError
(e.g. 1 in Perm.RW
); currently, both operations
return False
instead.
(Contributed by Ethan Furman in bpo-33217.)
gettext¶
Using non-integer value for selecting a plural form in gettext
is
now deprecated. It never correctly worked. (Contributed by Serhiy Storchaka
in bpo-28692.)
importlib¶
Methods
MetaPathFinder.find_module()
(replaced by
MetaPathFinder.find_spec()
)
and
PathEntryFinder.find_loader()
(replaced by
PathEntryFinder.find_spec()
)
both deprecated in Python 3.4 now emit DeprecationWarning
.
(Contributed by Matthias Bussonnier in bpo-29576.)
The importlib.abc.ResourceLoader
ABC has been deprecated in
favour of importlib.abc.ResourceReader
.
locale¶
locale.format()
has been deprecated, use locale.format_string()
instead. (Contributed by Garvit in bpo-10379.)
macpath¶
The macpath
is now deprecated and will be removed in Python 3.8.
(Contributed by Chi Hsuan Yen in bpo-9850.)
threading¶
dummy_threading
and _dummy_thread
have been deprecated. It is
no longer possible to build Python with threading disabled.
Use threading
instead.
(Contributed by Antoine Pitrou in bpo-31370.)
socket¶
The silent argument value truncation in socket.htons()
and
socket.ntohs()
has been deprecated. In future versions of Python,
if the passed argument is larger than 16 bits, an exception will be raised.
(Contributed by Oren Milman in bpo-28332.)
ssl¶
ssl.wrap_socket()
is deprecated. Use
ssl.SSLContext.wrap_socket()
instead.
(Contributed by Christian Heimes in bpo-28124.)
sunau¶
sunau.openfp()
has been deprecated and will be removed in Python 3.9.
Use sunau.open()
instead.
(Contributed by Brian Curtin in bpo-31985.)
sys¶
Deprecated sys.set_coroutine_wrapper()
and
sys.get_coroutine_wrapper()
.
The undocumented sys.callstats()
function has been deprecated and
will be removed in a future Python version.
(Contributed by Victor Stinner in bpo-28799.)
wave¶
wave.openfp()
has been deprecated and will be removed in Python 3.9.
Use wave.open()
instead.
(Contributed by Brian Curtin in bpo-31985.)
Deprecated functions and types of the C API¶
Function PySlice_GetIndicesEx()
is deprecated and replaced with
a macro if Py_LIMITED_API
is not set or set to a value in the range
between 0x03050400
and 0x03060000
(not inclusive), or is 0x03060100
or higher. (Contributed by Serhiy Storchaka in bpo-27867.)
PyOS_AfterFork()
has been deprecated. Use PyOS_BeforeFork()
,
PyOS_AfterFork_Parent()
or PyOS_AfterFork_Child()
instead.
(Contributed by Antoine Pitrou in bpo-16500.)
Platform Support Removals¶
FreeBSD 9 and older are no longer officially supported.
For full Unicode support, including within extension modules, *nix platforms are now expected to provide at least one of
C.UTF-8
(full locale),C.utf8
(full locale) orUTF-8
(LC_CTYPE
-only locale) as an alternative to the legacyASCII
-basedC
locale.OpenSSL 0.9.8 and 1.0.1 are no longer supported, which means building CPython 3.7 with SSL/TLS support on older platforms still using these versions requires custom build options that link to a more recent version of OpenSSL.
Notably, this issue affects the Debian 8 (aka “jessie”) and Ubuntu 14.04 (aka “Trusty”) LTS Linux distributions, as they still use OpenSSL 1.0.1 by default.
Debian 9 (“stretch”) and Ubuntu 16.04 (“xenial”), as well as recent releases of other LTS Linux releases (e.g. RHEL/CentOS 7.5, SLES 12-SP3), use OpenSSL 1.0.2 or later, and remain supported in the default build configuration.
CPython’s own CI configuration file provides an example of using the SSL compatibility testing infrastructure in CPython’s test suite to build and link against OpenSSL 1.1.0 rather than an outdated system provided OpenSSL.
API and Feature Removals¶
The following features and APIs have been removed from Python 3.7:
The
os.stat_float_times()
function has been removed. It was introduced in Python 2.3 for backward compatibility with Python 2.2, and was deprecated since Python 3.1.Unknown escapes consisting of
'\'
and an ASCII letter in replacement templates forre.sub()
were deprecated in Python 3.5, and will now cause an error.Removed support of the exclude argument in
tarfile.TarFile.add()
. It was deprecated in Python 2.7 and 3.2. Use the filter argument instead.The
ntpath.splitunc()
function was deprecated in Python 3.1, and has now been removed. Usesplitdrive()
instead.collections.namedtuple()
no longer supports the verbose parameter or_source
attribute which showed the generated source code for the named tuple class. This was part of an optimization designed to speed-up class creation. (Contributed by Jelle Zijlstra with further improvements by INADA Naoki, Serhiy Storchaka, and Raymond Hettinger in bpo-28638.)Functions
bool()
,float()
,list()
andtuple()
no longer take keyword arguments. The first argument ofint()
can now be passed only as positional argument.Removed previously deprecated in Python 2.4 classes
Plist
,Dict
and_InternalDict
in theplistlib
module. Dict values in the result of functionsreadPlist()
andreadPlistFromBytes()
are now normal dicts. You no longer can use attribute access to access items of these dictionaries.The
asyncio.windows_utils.socketpair()
function has been removed. Use thesocket.socketpair()
function instead, it is available on all platforms since Python 3.5.asyncio.windows_utils.socketpair
was just an alias tosocket.socketpair
on Python 3.5 and newer.asyncio
no longer exports theselectors
and_overlapped
modules asasyncio.selectors
andasyncio._overlapped
. Replacefrom asyncio import selectors
withimport selectors
.Direct instantiation of
ssl.SSLSocket
andssl.SSLObject
objects is now prohibited. The constructors were never documented, tested, or designed as public constructors. Users were supposed to usessl.wrap_socket()
orssl.SSLContext
. (Contributed by Christian Heimes in bpo-32951.)The unused
distutils
install_misc
command has been removed. (Contributed by Eric N. Vander Weele in bpo-29218.)
Module Removals¶
The fpectl
module has been removed. It was never enabled by
default, never worked correctly on x86-64, and it changed the Python
ABI in ways that caused unexpected breakage of C extensions.
(Contributed by Nathaniel J. Smith in bpo-29137.)
Windows-only Changes¶
The python launcher, (py.exe), can accept 32 & 64 bit specifiers without
having to specify a minor version as well. So py -3-32
and py -3-64
become valid as well as py -3.7-32
, also the -m-64 and -m.n-64 forms
are now accepted to force 64 bit python even if 32 bit would have otherwise
been used. If the specified version is not available py.exe will error exit.
(Contributed by Steve Barnes in bpo-30291.)
The launcher can be run as py -0
to produce a list of the installed pythons,
with default marked with an asterisk. Running py -0p
will include the paths.
If py is run with a version specifier that cannot be matched it will also print
the short form list of available specifiers.
(Contributed by Steve Barnes in bpo-30362.)
Porting to Python 3.7¶
This section lists previously described changes and other bugfixes that may require changes to your code.
Changes in Python Behavior¶
async
andawait
names are now reserved keywords. Code using these names as identifiers will now raise aSyntaxError
. (Contributed by Jelle Zijlstra in bpo-30406.)PEP 479 is enabled for all code in Python 3.7, meaning that
StopIteration
exceptions raised directly or indirectly in coroutines and generators are transformed intoRuntimeError
exceptions. (Contributed by Yury Selivanov in bpo-32670.)object.__aiter__()
methods can no longer be declared as asynchronous. (Contributed by Yury Selivanov in bpo-31709.)Due to an oversight, earlier Python versions erroneously accepted the following syntax:
f(1 for x in [1],) class C(1 for x in [1]): pass
Python 3.7 now correctly raises a
SyntaxError
, as a generator expression always needs to be directly inside a set of parentheses and cannot have a comma on either side, and the duplication of the parentheses can be omitted only on calls. (Contributed by Serhiy Storchaka in bpo-32012 and bpo-32023.)When using the
-m
switch, the initial working directory is now added tosys.path
, rather than an empty string (which dynamically denoted the current working directory at the time of each import). Any programs that are checking for the empty string, or otherwise relying on the previous behaviour, will need to be updated accordingly (e.g. by also checking foros.getcwd()
oros.path.dirname(__main__.__file__)
, depending on why the code was checking for the empty string in the first place).
Changes in the Python API¶
socketserver.ThreadingMixIn.server_close
now waits until all non-daemon threads complete. Set the newsocketserver.ThreadingMixIn.block_on_close
class attribute toFalse
to get the pre-3.7 behaviour. (Contributed by Victor Stinner in bpo-31233 and bpo-33540.)socketserver.ForkingMixIn.server_close
now waits until all child processes complete. Set the newsocketserver.ForkingMixIn.block_on_close
class attribute toFalse
to get the pre-3.7 behaviour. (Contributed by Victor Stinner in bpo-31151 and bpo-33540.)The
locale.localeconv()
function now temporarily sets theLC_CTYPE
locale to the value ofLC_NUMERIC
in some cases. (Contributed by Victor Stinner in bpo-31900.)pkgutil.walk_packages()
now raises aValueError
if path is a string. Previously an empty list was returned. (Contributed by Sanyam Khurana in bpo-24744.)A format string argument for
string.Formatter.format()
is now positional-only. Passing it as a keyword argument was deprecated in Python 3.5. (Contributed by Serhiy Storchaka in bpo-29193.)Attributes
key
,value
andcoded_value
of classhttp.cookies.Morsel
are now read-only. Assigning to them was deprecated in Python 3.5. Use theset()
method for setting them. (Contributed by Serhiy Storchaka in bpo-29192.)The mode argument of
os.makedirs()
no longer affects the file permission bits of newly created intermediate-level directories. To set their file permission bits you can set the umask before invokingmakedirs()
. (Contributed by Serhiy Storchaka in bpo-19930.)The
struct.Struct.format
type is nowstr
instead ofbytes
. (Contributed by Victor Stinner in bpo-21071.)cgi.parse_multipart()
now accepts the encoding and errors arguments and returns the same results asFieldStorage
: for non-file fields, the value associated to a key is a list of strings, not bytes. (Contributed by Pierre Quentel in bpo-29979.)Due to internal changes in
socket
, callingsocket.fromshare()
on a socket created bysocket.share
in older Python versions is not supported.repr
forBaseException
has changed to not include the trailing comma. Most exceptions are affected by this change. (Contributed by Serhiy Storchaka in bpo-30399.)repr
fordatetime.timedelta
has changed to include the keyword arguments in the output. (Contributed by Utkarsh Upadhyay in bpo-30302.)Because
shutil.rmtree()
is now implemented using theos.scandir()
function, the user specified handler onerror is now called with the first argumentos.scandir
instead ofos.listdir
when listing the directory is failed.Support for nested sets and set operations in regular expressions as in Unicode Technical Standard #18 might be added in the future. This would change the syntax. To facilitate this future change a
FutureWarning
will be raised in ambiguous cases for the time being. That include sets starting with a literal'['
or containing literal character sequences'--'
,'&&'
,'~~'
, and'||'
. To avoid a warning, escape them with a backslash. (Contributed by Serhiy Storchaka in bpo-30349.)The result of splitting a string on a
regular expression
that could match an empty string has been changed. For example splitting onr'\s*'
will now split not only on whitespaces as it did previously, but also on empty strings before all non-whitespace characters and just before the end of the string. The previous behavior can be restored by changing the pattern tor'\s+'
. AFutureWarning
was emitted for such patterns since Python 3.5.For patterns that match both empty and non-empty strings, the result of searching for all matches may also be changed in other cases. For example in the string
'a\n\n'
, the patternr'(?m)^\s*?$'
will not only match empty strings at positions 2 and 3, but also the string'\n'
at positions 2–3. To match only blank lines, the pattern should be rewritten asr'(?m)^[^\S\n]*$'
.re.sub()
now replaces empty matches adjacent to a previous non-empty match. For examplere.sub('x*', '-', 'abxd')
returns now'-a-b--d-'
instead of'-a-b-d-'
(the first minus between ‘b’ and ‘d’ replaces ‘x’, and the second minus replaces an empty string between ‘x’ and ‘d’).(Contributed by Serhiy Storchaka in bpo-25054 and bpo-32308.)
Change
re.escape()
to only escape regex special characters instead of escaping all characters other than ASCII letters, numbers, and'_'
. (Contributed by Serhiy Storchaka in bpo-29995.)tracemalloc.Traceback
frames are now sorted from oldest to most recent to be more consistent withtraceback
. (Contributed by Jesse Bakker in bpo-32121.)On OSes that support
socket.SOCK_NONBLOCK
orsocket.SOCK_CLOEXEC
bit flags, thesocket.type
no longer has them applied. Therefore, checks likeif sock.type == socket.SOCK_STREAM
work as expected on all platforms. (Contributed by Yury Selivanov in bpo-32331.)On Windows the default for the close_fds argument of
subprocess.Popen
was changed fromFalse
toTrue
when redirecting the standard handles. If you previously depended on handles being inherited when usingsubprocess.Popen
with standard io redirection, you will have to passclose_fds=False
to preserve the previous behaviour, or useSTARTUPINFO.lpAttributeList
.importlib.machinery.PathFinder.invalidate_caches()
– which implicitly affectsimportlib.invalidate_caches()
– now deletes entries insys.path_importer_cache
which are set toNone
. (Contributed by Brett Cannon in bpo-33169.)In
asyncio
,loop.sock_recv()
,loop.sock_sendall()
,loop.sock_accept()
,loop.getaddrinfo()
,loop.getnameinfo()
have been changed to be proper coroutine methods to match their documentation. Previously, these methods returnedasyncio.Future
instances. (Contributed by Yury Selivanov in bpo-32327.)asyncio.Server.sockets
now returns a copy of the internal list of server sockets, instead of returning it directly. (Contributed by Yury Selivanov in bpo-32662.)Struct.format
is now astr
instance instead of abytes
instance. (Contributed by Victor Stinner in bpo-21071.)argparse
subparsers can now be made mandatory by passingrequired=True
toArgumentParser.add_subparsers()
. (Contributed by Anthony Sottile in bpo-26510.)ast.literal_eval()
is now stricter. Addition and subtraction of arbitrary numbers are no longer allowed. (Contributed by Serhiy Storchaka in bpo-31778.)Calendar.itermonthdates
will now consistently raise an exception when a date falls outside of the0001-01-01
through9999-12-31
range. To support applications that cannot tolerate such exceptions, the newCalendar.itermonthdays3
andCalendar.itermonthdays4
can be used. The new methods return tuples and are not restricted by the range supported bydatetime.date
. (Contributed by Alexander Belopolsky in bpo-28292.)collections.ChainMap
now preserves the order of the underlying mappings. (Contributed by Raymond Hettinger in bpo-32792.)The
submit()
method ofconcurrent.futures.ThreadPoolExecutor
andconcurrent.futures.ProcessPoolExecutor
now raises aRuntimeError
if called during interpreter shutdown. (Contributed by Mark Nemec in bpo-33097.)The
configparser.ConfigParser
constructor now usesread_dict()
to process the default values, making its behavior consistent with the rest of the parser. Non-string keys and values in the defaults dictionary are now being implicitly converted to strings. (Contributed by James Tocknell in bpo-23835.)Several undocumented internal imports were removed. One example is that
os.errno
is no longer available; useimport errno
directly instead. Note that such undocumented internal imports may be removed any time without notice, even in micro version releases.
Changes in the C API¶
The function PySlice_GetIndicesEx()
is considered unsafe for
resizable sequences. If the slice indices are not instances of int
,
but objects that implement the __index__()
method, the sequence can be
resized after passing its length to PySlice_GetIndicesEx()
. This
can lead to returning indices out of the length of the sequence. For
avoiding possible problems use new functions PySlice_Unpack()
and
PySlice_AdjustIndices()
.
(Contributed by Serhiy Storchaka in bpo-27867.)
CPython bytecode changes¶
There are two new opcodes: LOAD_METHOD
and CALL_METHOD
.
(Contributed by Yury Selivanov and INADA Naoki in bpo-26110.)
The STORE_ANNOTATION
opcode has been removed.
(Contributed by Mark Shannon in bpo-32550.)
Windows-only Changes¶
The file used to override sys.path
is now called
<python-executable>._pth
instead of 'sys.path'
.
See Finding modules for more information.
(Contributed by Steve Dower in bpo-28137.)
Other CPython implementation changes¶
In preparation for potential future changes to the public CPython runtime initialization API (see PEP 432 for an initial, but somewhat outdated, draft), CPython’s internal startup and configuration management logic has been significantly refactored. While these updates are intended to be entirely transparent to both embedding applications and users of the regular CPython CLI, they’re being mentioned here as the refactoring changes the internal order of various operations during interpreter startup, and hence may uncover previously latent defects, either in embedding applications, or in CPython itself. (Initially contributed by Nick Coghlan and Eric Snow as part of bpo-22257, and further updated by Nick, Eric, and Victor Stinner in a number of other issues). Some known details affected:
PySys_AddWarnOptionUnicode()
is not currently usable by embedding applications due to the requirement to create a Unicode object prior to callingPy_Initialize
. UsePySys_AddWarnOption()
instead.warnings filters added by an embedding application with
PySys_AddWarnOption()
should now more consistently take precedence over the default filters set by the interpreter
Due to changes in the way the default warnings filters are configured,
setting Py_BytesWarningFlag
to a value greater than one is no longer
sufficient to both emit BytesWarning
messages and have them converted
to exceptions. Instead, the flag must be set (to cause the warnings to be
emitted in the first place), and an explicit error::BytesWarning
warnings filter added to convert them to exceptions.
Due to a change in the way docstrings are handled by the compiler, the
implicit return None
in a function body consisting solely of a docstring
is now marked as occurring on the same line as the docstring, not on the
function’s header line.
The current exception state has been moved from the frame object to the co-routine. This simplified the interpreter and fixed a couple of obscure bugs caused by having swap exception state when entering or exiting a generator. (Contributed by Mark Shannon in bpo-25612.)
Notable changes in Python 3.7.1¶
Starting in 3.7.1, Py_Initialize()
now consistently reads and respects
all of the same environment settings as Py_Main()
(in earlier Python
versions, it respected an ill-defined subset of those environment variables,
while in Python 3.7.0 it didn’t read any of them due to bpo-34247). If
this behavior is unwanted, set Py_IgnoreEnvironmentFlag
to 1 before
calling Py_Initialize()
.
In 3.7.1 the C API for Context Variables
was updated to use
PyObject
pointers. See also bpo-34762.
In 3.7.1 the tokenize
module now implicitly emits a NEWLINE
token
when provided with input that does not have a trailing new line. This behavior
now matches what the C tokenizer does internally.
(Contributed by Ammar Askar in bpo-33899.)
Notable changes in Python 3.7.2¶
In 3.7.2, venv
on Windows no longer copies the original binaries, but
creates redirector scripts named python.exe
and pythonw.exe
instead.
This resolves a long standing issue where all virtual environments would have
to be upgraded or recreated with each Python update. However, note that this
release will still require recreation of virtual environments in order to get
the new scripts.
Notable changes in Python 3.7.6¶
Due to significant security concerns, the reuse_address parameter of
asyncio.loop.create_datagram_endpoint()
is no longer supported. This is
because of the behavior of the socket option SO_REUSEADDR
in UDP. For more
details, see the documentation for loop.create_datagram_endpoint()
.
(Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in
bpo-37228.)
Notable changes in Python 3.7.10¶
Earlier Python versions allowed using both ;
and &
as
query parameter separators in urllib.parse.parse_qs()
and
urllib.parse.parse_qsl()
. Due to security concerns, and to conform with
newer W3C recommendations, this has been changed to allow only a single
separator key, with &
as the default. This change also affects
cgi.parse()
and cgi.parse_multipart()
as they use the affected
functions internally. For more details, please see their respective
documentation.
(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in bpo-42967.)
Notable changes in Python 3.7.11¶
A security fix alters the ftplib.FTP
behavior to not trust the
IPv4 address sent from the remote server when setting up a passive data
channel. We reuse the ftp server IP address instead. For unusual code
requiring the old behavior, set a trust_server_pasv_ipv4_address
attribute on your FTP instance to True
. (See gh-87451)
The presence of newline or tab characters in parts of a URL allows for some
forms of attacks. Following the WHATWG specification that updates RFC 3986,
ASCII newline \n
, \r
and tab \t
characters are stripped from the
URL by the parser urllib.parse()
preventing such attacks. The removal
characters are controlled by a new module level variable
urllib.parse._UNSAFE_URL_BYTES_TO_REMOVE
. (See gh-88048)
Notable security feature in 3.7.14¶
Converting between int
and str
in bases other than 2
(binary), 4, 8 (octal), 16 (hexadecimal), or 32 such as base 10 (decimal)
now raises a ValueError
if the number of digits in string form is
above a limit to avoid potential denial of service attacks due to the
algorithmic complexity. This is a mitigation for CVE 2020-10735.
This limit can be configured or disabled by environment variable, command
line flag, or sys
APIs. See the integer string conversion
length limitation documentation. The default limit
is 4300 digits in string form.


Appendix

16. Appendix¶
16.1. Interactive Mode¶
There are two variants of the interactive REPL. The classic basic interpreter is supported on all platforms with minimal line control capabilities.
Since Python 3.13, a new interactive shell is used by default.
This one supports color, multiline editing, history browsing, and
paste mode. To disable color, see Controlling color for
details. Function keys provide some additional functionality.
F1 enters the interactive help browser pydoc
.
F2 allows for browsing command-line history with neither output nor the
>>> and … prompts. F3 enters “paste mode”, which
makes pasting larger blocks of code easier. Press F3 to return to
the regular prompt.
When using the new interactive shell, exit the shell by typing exit or quit. Adding call parentheses after those commands is not required.
If the new interactive shell is not desired, it can be disabled via
the PYTHON_BASIC_REPL
environment variable.
16.1.1. Error Handling¶
When an error occurs, the interpreter prints an error message and a stack trace.
In interactive mode, it then returns to the primary prompt; when input came from
a file, it exits with a nonzero exit status after printing the stack trace.
(Exceptions handled by an except
clause in a try
statement
are not errors in this context.) Some errors are unconditionally fatal and
cause an exit with a nonzero exit status; this applies to internal inconsistencies and
some cases of running out of memory. All error messages are written to the
standard error stream; normal output from executed commands is written to
standard output.
Typing the interrupt character (usually Control-C or Delete) to the primary or
secondary prompt cancels the input and returns to the primary prompt. [1]
Typing an interrupt while a command is executing raises the
KeyboardInterrupt
exception, which may be handled by a try
statement.
16.1.2. Executable Python Scripts¶
On BSD’ish Unix systems, Python scripts can be made directly executable, like shell scripts, by putting the line
#!/usr/bin/env python3
(assuming that the interpreter is on the user’s PATH
) at the beginning
of the script and giving the file an executable mode. The #!
must be the
first two characters of the file. On some platforms, this first line must end
with a Unix-style line ending ('\n'
), not a Windows ('\r\n'
) line
ending. Note that the hash, or pound, character, '#'
, is used to start a
comment in Python.
The script can be given an executable mode, or permission, using the chmod command.
$ chmod +x myscript.py
On Windows systems, there is no notion of an “executable mode”. The Python
installer automatically associates .py
files with python.exe
so that
a double-click on a Python file will run it as a script. The extension can
also be .pyw
, in that case, the console window that normally appears is
suppressed.
16.1.3. The Interactive Startup File¶
When you use Python interactively, it is frequently handy to have some standard
commands executed every time the interpreter is started. You can do this by
setting an environment variable named PYTHONSTARTUP
to the name of a
file containing your start-up commands. This is similar to the .profile
feature of the Unix shells.
This file is only read in interactive sessions, not when Python reads commands
from a script, and not when /dev/tty
is given as the explicit source of
commands (which otherwise behaves like an interactive session). It is executed
in the same namespace where interactive commands are executed, so that objects
that it defines or imports can be used without qualification in the interactive
session. You can also change the prompts sys.ps1
and sys.ps2
in this
file.
If you want to read an additional start-up file from the current directory, you
can program this in the global start-up file using code like if
os.path.isfile('.pythonrc.py'): exec(open('.pythonrc.py').read())
.
If you want to use the startup file in a script, you must do this explicitly
in the script:
import os
filename = os.environ.get('PYTHONSTARTUP')
if filename and os.path.isfile(filename):
with open(filename) as fobj:
startup_file = fobj.read()
exec(startup_file)
16.1.4. The Customization Modules¶
Python provides two hooks to let you customize it: sitecustomize and usercustomize. To see how it works, you need first to find the location of your user site-packages directory. Start Python and run this code:
>>> import site
>>> site.getusersitepackages()
'/home/user/.local/lib/python3.x/site-packages'
Now you can create a file named usercustomize.py
in that directory and
put anything you want in it. It will affect every invocation of Python, unless
it is started with the -s
option to disable the automatic import.
sitecustomize works in the same way, but is typically created by an
administrator of the computer in the global site-packages directory, and is
imported before usercustomize. See the documentation of the site
module for more details.
Footnotes


— Access the import internals

imp
— Access the import internals¶
Deprecated since version 3.4, removed in version 3.12.
This module is no longer part of the Python standard library. It was removed in Python 3.12 after being deprecated in Python 3.4.
The removal notice includes guidance for
migrating code from imp
to importlib
.
The last version of Python that provided the imp
module was
Python 3.11.


Extending/Embedding FAQ¶
Can I create my own functions in C?¶
Yes, you can create built-in modules containing functions, variables, exceptions and even new types in C. This is explained in the document Extending and Embedding the Python Interpreter.
Most intermediate or advanced Python books will also cover this topic.
Can I create my own functions in C++?¶
Yes, using the C compatibility features found in C++. Place extern "C" {
... }
around the Python include files and put extern "C"
before each
function that is going to be called by the Python interpreter. Global or static
C++ objects with constructors are probably not a good idea.
Writing C is hard; are there any alternatives?¶
There are a number of alternatives to writing your own C extensions, depending on what you’re trying to do. Recommended third party tools offer both simpler and more sophisticated approaches to creating C and C++ extensions for Python.
How can I execute arbitrary Python statements from C?¶
The highest-level function to do this is PyRun_SimpleString()
which takes
a single string argument to be executed in the context of the module
__main__
and returns 0
for success and -1
when an exception occurred
(including SyntaxError
). If you want more control, use
PyRun_String()
; see the source for PyRun_SimpleString()
in
Python/pythonrun.c
.
How can I evaluate an arbitrary Python expression from C?¶
Call the function PyRun_String()
from the previous question with the
start symbol Py_eval_input
; it parses an expression, evaluates it and
returns its value.
How do I extract C values from a Python object?¶
That depends on the object’s type. If it’s a tuple, PyTuple_Size()
returns its length and PyTuple_GetItem()
returns the item at a specified
index. Lists have similar functions, PyList_Size()
and
PyList_GetItem()
.
For bytes, PyBytes_Size()
returns its length and
PyBytes_AsStringAndSize()
provides a pointer to its value and its
length. Note that Python bytes objects may contain null bytes so C’s
strlen()
should not be used.
To test the type of an object, first make sure it isn’t NULL
, and then use
PyBytes_Check()
, PyTuple_Check()
, PyList_Check()
, etc.
There is also a high-level API to Python objects which is provided by the
so-called ‘abstract’ interface – read Include/abstract.h
for further
details. It allows interfacing with any kind of Python sequence using calls
like PySequence_Length()
, PySequence_GetItem()
, etc. as well
as many other useful protocols such as numbers (PyNumber_Index()
et
al.) and mappings in the PyMapping APIs.
How do I use Py_BuildValue() to create a tuple of arbitrary length?¶
You can’t. Use PyTuple_Pack()
instead.
How do I call an object’s method from C?¶
The PyObject_CallMethod()
function can be used to call an arbitrary
method of an object. The parameters are the object, the name of the method to
call, a format string like that used with Py_BuildValue()
, and the
argument values:
PyObject *
PyObject_CallMethod(PyObject *object, const char *method_name,
const char *arg_format, ...);
This works for any object that has methods – whether built-in or user-defined.
You are responsible for eventually Py_DECREF()
‘ing the return value.
To call, e.g., a file object’s “seek” method with arguments 10, 0 (assuming the file object pointer is “f”):
res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
if (res == NULL) {
... an exception occurred ...
}
else {
Py_DECREF(res);
}
Note that since PyObject_CallObject()
always wants a tuple for the
argument list, to call a function without arguments, pass “()” for the format,
and to call a function with one argument, surround the argument in parentheses,
e.g. “(i)”.
How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?¶
In Python code, define an object that supports the write()
method. Assign
this object to sys.stdout
and sys.stderr
. Call print_error, or
just allow the standard traceback mechanism to work. Then, the output will go
wherever your write()
method sends it.
The easiest way to do this is to use the io.StringIO
class:
>>> import io, sys
>>> sys.stdout = io.StringIO()
>>> print('foo')
>>> print('hello world!')
>>> sys.stderr.write(sys.stdout.getvalue())
foo
hello world!
A custom object to do the same would look like this:
>>> import io, sys
>>> class StdoutCatcher(io.TextIOBase):
... def __init__(self):
... self.data = []
... def write(self, stuff):
... self.data.append(stuff)
...
>>> import sys
>>> sys.stdout = StdoutCatcher()
>>> print('foo')
>>> print('hello world!')
>>> sys.stderr.write(''.join(sys.stdout.data))
foo
hello world!
How do I access a module written in Python from C?¶
You can get a pointer to the module object as follows:
module = PyImport_ImportModule("<modulename>");
If the module hasn’t been imported yet (i.e. it is not yet present in
sys.modules
), this initializes the module; otherwise it simply returns
the value of sys.modules["<modulename>"]
. Note that it doesn’t enter the
module into any namespace – it only ensures it has been initialized and is
stored in sys.modules
.
You can then access the module’s attributes (i.e. any name defined in the module) as follows:
attr = PyObject_GetAttrString(module, "<attrname>");
Calling PyObject_SetAttrString()
to assign to variables in the module
also works.
How do I interface to C++ objects from Python?¶
Depending on your requirements, there are many approaches. To do this manually, begin by reading the “Extending and Embedding” document. Realize that for the Python run-time system, there isn’t a whole lot of difference between C and C++ – so the strategy of building a new Python type around a C structure (pointer) type will also work for C++ objects.
For C++ libraries, see Writing C is hard; are there any alternatives?.
I added a module using the Setup file and the make fails; why?¶
Setup must end in a newline, if there is no newline there, the build process fails. (Fixing this requires some ugly shell script hackery, and this bug is so minor that it doesn’t seem worth the effort.)
How do I debug an extension?¶
When using GDB with dynamically loaded extensions, you can’t set a breakpoint in your extension until your extension is loaded.
In your .gdbinit
file (or interactively), add the command:
br _PyImport_LoadDynamicModule
Then, when you run GDB:
$ gdb /local/bin/python
gdb) run myscript.py
gdb) continue # repeat until your extension is loaded
gdb) finish # so that your extension is loaded
gdb) br myfunction.c:50
gdb) continue
I want to compile a Python module on my Linux system, but some files are missing. Why?¶
Most packaged versions of Python omit some files required for compiling Python extensions.
For Red Hat, install the python3-devel RPM to get the necessary files.
For Debian, run apt-get install python3-dev
.
How do I tell “incomplete input” from “invalid input”?¶
Sometimes you want to emulate the Python interactive interpreter’s behavior, where it gives you a continuation prompt when the input is incomplete (e.g. you typed the start of an “if” statement or you didn’t close your parentheses or triple string quotes), but it gives you a syntax error message immediately when the input is invalid.
In Python you can use the codeop
module, which approximates the parser’s
behavior sufficiently. IDLE uses this, for example.
The easiest way to do it in C is to call PyRun_InteractiveLoop()
(perhaps
in a separate thread) and let the Python interpreter handle the input for
you. You can also set the PyOS_ReadlineFunctionPointer()
to point at your
custom input function. See Modules/readline.c
and Parser/myreadline.c
for more hints.
How do I find undefined g++ symbols __builtin_new or __pure_virtual?¶
To dynamically load g++ extension modules, you must recompile Python, relink it
using g++ (change LINKCC in the Python Modules Makefile), and link your
extension module using g++ (e.g., g++ -shared -o mymodule.so mymodule.o
).
Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?¶
Yes, you can inherit from built-in classes such as int
, list
,
dict
, etc.
The Boost Python Library (BPL, https://www.boost.org/libs/python/doc/index.html) provides a way of doing this from C++ (i.e. you can inherit from an extension class written in C++ using the BPL).