What's New In Python 3.7
|Editor:||Elvis Pranskevichus <email@example.com>|
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 :ref:`changelog <changelog>`.
Summary -- Release Highlights
New syntax features:
- :ref:`PEP 563 <whatsnew37-pep563>`, postponed evaluation of type annotations.
Backwards incompatible syntax changes:
New library modules:
- :mod:`contextvars`: :ref:`PEP 567 -- Context Variables <whatsnew37-pep567>`
- :mod:`dataclasses`: :ref:`PEP 557 -- Data Classes <whatsnew37-pep557>`
New built-in features:
Python data model improvements:
- :ref:`PEP 562 <whatsnew37-pep562>`, customization of access to module attributes.
- :ref:`PEP 560 <whatsnew37-pep560>`, core support for typing module and generic types.
- the insertion-order preservation nature of :ref:`dict <typesmapping>` objects has been declared to be an official part of the Python language spec.
Significant improvements in the standard library:
- The :mod:`asyncio` module has received new features, significant :ref:`usability and performance improvements <whatsnew37_asyncio>`.
- The :mod:`time` module gained support for :ref:`functions with nanosecond resolution <whatsnew37-pep564>`.
CPython implementation improvements:
- Avoiding the use of ASCII as a default text encoding:
- :ref:`PEP 552 <whatsnew37-pep552>`, deterministic .pycs
- :ref:`the new development runtime mode <whatsnew37-devmode>`
- :ref:`PEP 565 <whatsnew37-pep565>`, improved :exc:`DeprecationWarning` handling
C API improvements:
- :ref:`PEP 539 <whatsnew37-pep539>`, new C API for thread-local storage
- :ref:`PEP 545 <whatsnew37-pep545>`, Python documentation translations
- New documentation translations: Japanese, French, and Korean.
This release features notable performance improvements in many areas. The :ref:`whatsnew37-perf` section lists them in detail.
For a list of changes that may affect compatibility with previous Python releases please refer to the :ref:`porting-to-python-37` section.
PEP 563: Postponed Evaluation of Annotations
- 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 :func:`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 :mod:`__future__` import:
from __future__ import annotations
It will become the default in Python 4.0.
.. seealso:: :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 :envvar:`PYTHONCOERCECLOCALE`
environment variable. Automatically setting
LC_CTYPE this way means that
both the core interpreter and locale-aware C extensions (such as
:mod:`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 :data:`~sys.stdin` and
:data:`~sys.stdout` is now
surrogateescape (rather than
using any of the defined coercion target locales (currently
UTF-8). The default error handler for :data:`~sys.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
:data:`~sys.stderr`) can be requested by setting
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
.. seealso:: :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 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 :ref:`whatsnew37-pep540`).
The UTF-8 mode is enabled by default when the locale is
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).
.. seealso:: :pep:`540` -- Add a new UTF-8 mode PEP written and implemented by Victor Stinner
PEP 553: Built-in
Python 3.7 includes the new built-in :func:`breakpoint` function as an easy and consistent way to enter the Python debugger.
breakpoint() calls :func:`sys.breakpointhook`. By default, the
latter imports :mod:`pdb` and then calls
pdb.set_trace(), but by binding
sys.breakpointhook() to the function of your choosing,
enter any debugger. Additionally, the environment variable
:envvar:`PYTHONBREAKPOINT` can be set to the callable of your debugger of
PYTHONBREAKPOINT=0 to completely disable built-in
.. seealso:: :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 :ref:`Thread Local Storage (TLS) API <thread-local-storage-api>` has used :c:type:`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 :ref:`Thread Specific Storage (TSS) API <thread-specific-storage-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 :c:type:`Py_tss_t` instead of :c:type:`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 :c:type:`int`.
Note that on platforms where the native TLS key is defined in a way that cannot be safely cast to :c:type:`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.
.. seealso:: :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
A typical example of where this may be useful is module attribute deprecation and lazy loading.
.. seealso:: :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 :func:`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 :mod:`time` module:
The new functions return the number of nanoseconds as an integer value.
.. seealso:: :pep:`564` -- Add new time functions with nanosecond resolution PEP written and implemented by Victor Stinner
PEP 565: Show DeprecationWarning in
The default handling of :exc:`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 :mod:`__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:
- :exc:`FutureWarning`: always displayed by default, recommended for warnings intended to be seen by application end users (e.g. for deprecated application configuration settings).
- :exc:`DeprecationWarning`: displayed by default only in :mod:`__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.
- :exc:`PendingDeprecationWarning`: displayed by default only when running tests, intended for cases where a future version upgrade will change the warning category to :exc:`DeprecationWarning` or :exc:`FutureWarning`.
Previously both :exc:`DeprecationWarning` and :exc:`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.
.. seealso:: :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 :mod:`typing`
module are extensively used by the community, so this restriction is removed.
The PEP introduces two special methods :meth:`__class_getitem__` and
__mro_entries__, these methods are now used by most classes and special
constructs in :mod:`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 :mod:`typing` module are
.. seealso:: :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
.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
files may be generated with :mod:`py_compile` or :mod:`compileall`.
.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
.pyc files are a useful performance optimization for environments
where a system external to Python (e.g., the build system) is responsible for
.pyc files up-to-date.
See :ref:`pyc-invalidation` for more information.
.. seealso:: :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/
.. seealso:: :pep:`545` -- Python Documentation Translations PEP written and implemented by Julien Palard, Inada Naoki, and Victor Stinner.
Development Runtime Mode: -X dev
The new :option:`-X`
dev command line option or the new
:envvar:`PYTHONDEVMODE` environment variable can be used to enable
CPython's development mode. When in development mode, CPython performs
additional runtime checks that are too expensive to be enabled by default.
dev documentation for the full description of the effects
of this mode.
Other Language Changes
- 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 :issue:`12844` and :issue:`18896`.)
- :meth:`bytes.fromhex` and :meth:`bytearray.fromhex` now ignore all ASCII whitespace, not only spaces. (Contributed by Robert Xiao in :issue:`28927`.)
- :class:`str`, :class:`bytes`, and :class:`bytearray` gained support for the new :meth:`isascii() <str.isascii>` method, which can be used to test if a string or bytes contain only the ASCII characters. (Contributed by INADA Naoki in :issue:`32677`.)
- :exc:`ImportError` now displays module name and module
from ... import ...fails. (Contributed by Matthias Bussonnier in :issue:`29546`.)
- Circular imports involving absolute imports with binding a submodule to a name are now supported. (Contributed by Serhiy Storchaka in :issue:`30024`.)
object.__format__(x, '')is now equivalent to
format(str(self), ''). (Contributed by Serhiy Storchaka in :issue:`28974`.)
- In order to better support dynamic creation of stack traces,
:class:`types.TracebackType` can now be instantiated from Python code, and
tb_nextattribute on :ref:`tracebacks <traceback-objects>` is now writable. (Contributed by Nathaniel J. Smith in :issue:`30579`.)
- When using the :option:`-m` switch,
sys.pathis 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 :issue:`33053`.)
- The new :option:`-X`
importtimeoption or the :envvar:`PYTHONPROFILEIMPORTTIME` environment variable can be used to show the timing of each module import. (Contributed by Victor Stinner in :issue:`31415`.)
The new :mod:`contextvars` module and a set of :ref:`new C APIs <contextvarsobjects>` introduce support for context variables. Context variables are conceptually similar to thread-local variables. Unlike TLS, context variables support asynchronous code correctly.
The :mod:`asyncio` and :mod:`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.
.. seealso:: :pep:`567` -- Context Variables PEP written and implemented by Yury Selivanov
The new :func:`~dataclasses.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 :meth:`~object.__repr__`, :meth:`~object.__eq__`, and :meth:`~object.__hash__` are generated automatically.
@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)"
.. seealso:: :pep:`557` -- Data Classes PEP written and implemented by Eric V. Smith
The new :mod:`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 :meth:`get_resource_reader()` function which returns a :class:`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 :issue:`32248`.
.. seealso:: `importlib_resources <http://importlib-resources.readthedocs.io/en/latest/>`_ -- a PyPI backport for earlier Python versions.
The new :meth:`ArgumentParser.parse_intermixed_args() <argparse.ArgumentParser.parse_intermixed_args>` method allows intermixing options and positional arguments. (Contributed by paul.j3 in :issue:`14191`.)
The :mod:`asyncio` module has received many new features, usability and :ref:`performance improvements <whatsnew37-asyncio-perf>`. Notable changes include:
The new :term:`provisional <provisional api>` :func:`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 :issue:`32314`.)
asyncio gained support for :mod:`contextvars`. :meth:`loop.call_soon() <asyncio.loop.call_soon>`, :meth:`loop.call_soon_threadsafe() <asyncio.loop.call_soon_threadsafe>`, :meth:`loop.call_later() <asyncio.loop.call_later>`, :meth:`loop.call_at() <asyncio.loop.call_at>`, and :meth:`Future.add_done_callback() <asyncio.Future.add_done_callback>` have a new optional keyword-only context parameter. :class:`Tasks <asyncio.Task>` now track their context automatically. See PEP 567 for more details. (Contributed by Yury Selivanov in :issue:`32436`.)
The new :meth:`loop.sock_recv_into() <asyncio.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 :issue:`31819`.)
The new :func:`asyncio.current_task` function returns the currently running :class:`~asyncio.Task` instance, and the new :func:`asyncio.all_tasks` function returns a set of all existing
Taskinstances in a given loop. The :meth:`Task.current_task() <asyncio.Task.current_task>` and :meth:`Task.all_tasks() <asyncio.Task.all_tasks>` methods have been deprecated. (Contributed by Andrew Svetlov in :issue:`32250`.)
The new :func:`asyncio.get_running_loop` function returns the currently running loop, and raises a :exc:`RuntimeError` if no loop is running. This is in contrast with :func:`asyncio.get_event_loop`, which will create a new event loop if none is running. (Contributed by Yury Selivanov in :issue:`32269`.)
The new :meth:`StreamWriter.wait_closed() <asyncio.StreamWriter.wait_closed>` coroutine method allows waiting until the stream writer is closed. The new :meth:`StreamWriter.is_closing() <asyncio.StreamWriter.is_closing>` method can be used to determine if the writer is closing. (Contributed by Andrew Svetlov in :issue:`32391`.)
The new :meth:`loop.sock_sendfile() <asyncio.loop.sock_sendfile>` coroutine method allows sending files using :mod:`os.sendfile` when possible. (Contributed by Andrew Svetlov in :issue:`32410`.)
The new :meth:`Future.get_loop() <asyncio.Future.get_loop>` and
Task.get_loop()methods return the instance of the loop on which a task or a future were created. :meth:`Server.get_loop() <asyncio.Server.get_loop>` allows doing the same for :class:`asyncio.Server` objects. (Contributed by Yury Selivanov in :issue:`32415` and Srinivas Reddy Thatiparthy in :issue:`32418`.)
It is now possible to control how instances of :class:`asyncio.Server` begin serving. Previously, the server would start serving immediately when created. The new start_serving keyword argument to :meth:`loop.create_server() <asyncio.loop.create_server>` and :meth:`loop.create_unix_server() <asyncio.loop.create_unix_server>`, as well as :meth:`Server.start_serving() <asyncio.Server.start_serving>`, and :meth:`Server.serve_forever() <asyncio.Server.serve_forever>` can be used to decouple server instantiation and serving. The new :meth:`Server.is_serving() <asyncio.Server.is_serving>` method returns
Trueif the server is serving. :class:`~asyncio.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 :issue:`32662`.)
Callback objects returned by :func:`loop.call_later() <asyncio.loop.call_later>` gained the new :meth:`when() <asyncio.TimerHandle.when>` method which returns an absolute scheduled callback timestamp. (Contributed by Andrew Svetlov in :issue:`32741`.)
The :meth:`loop.create_datagram_endpoint() \ <asyncio.loop.create_datagram_endpoint>` method gained support for Unix sockets. (Contributed by Quentin Dawans in :issue:`31245`.)
The :func:`asyncio.open_connection`, :func:`asyncio.start_server` functions, :meth:`loop.create_connection() <asyncio.loop.create_connection>`, :meth:`loop.create_server() <asyncio.loop.create_server>`, :meth:`loop.create_accepted_socket() <asyncio.loop.connect_accepted_socket>` methods and their corresponding UNIX socket variants now accept the ssl_handshake_timeout keyword argument. (Contributed by Neil Aspinall in :issue:`29970`.)
The new :meth:`Handle.cancelled() <asyncio.Handle.cancelled>` method returns
Trueif the callback was cancelled. (Contributed by Marat Sharafutdinov in :issue:`31943`.)
The new :meth:`ReadTransport.is_reading() <asyncio.ReadTransport.is_reading>` method can be used to determine the reading state of the transport. Additionally, calls to :meth:`ReadTransport.resume_reading() <asyncio.ReadTransport.resume_reading>` and :meth:`ReadTransport.pause_reading() <asyncio.ReadTransport.pause_reading>` are now idempotent. (Contributed by Yury Selivanov in :issue:`32356`.)
Exceptions occurring in cancelled tasks are no longer logged. (Contributed by Yury Selivanov in :issue:`30508`.)
WindowsProactorEventLoopPolicyclasses. (Contributed by Yury Selivanov in :issue:`33792`.)
asyncio APIs have been
The :func:`~binascii.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 :issue:`30103`.)
collections.namedtuple() now supports default values.
(Contributed by Raymond Hettinger in :issue:`32320`.)
:func:`compileall.compile_dir` learned the new invalidation_mode parameter,
which can be used to enable
:ref:`hash-based .pyc invalidation <whatsnew37-pep552>`. The invalidation
mode can also be specified on the command line using the new
(Contributed by Benjamin Peterson in :issue:`31650`.)
:class:`ProcessPoolExecutor <concurrent.futures.ProcessPoolExecutor>` and :class:`ThreadPoolExecutor <concurrent.futures.ThreadPoolExecutor>` now support the new initializer and initargs constructor arguments. (Contributed by Antoine Pitrou in :issue:`21423`.)
The :class:`ProcessPoolExecutor <concurrent.futures.ProcessPoolExecutor>` can now take the multiprocessing context via the new mp_context argument. (Contributed by Thomas Moreau in :issue:`31540`.)
The new :func:`~contextlib.asynccontextmanager`, :class:`~contextlib.AbstractAsyncContextManager`, and :class:`~contextlib.AsyncExitStack` have been added to complement their synchronous counterparts. (Contributed by Jelle Zijlstra in :issue:`29679` and :issue:`30241`, and by Alexander Mohr and Ilya Kulakov in :issue:`29302`.)
The new :meth:`datetime.fromisoformat() <datetime.datetime.fromisoformat>` method constructs a :class:`~datetime.datetime` object from a string in one of the formats output by :meth:`datetime.isoformat() <datetime.datetime.isoformat>`. (Contributed by Paul Ganssle in :issue:`15873`.)
:mod:`dbm.dumb` now supports reading read-only files and no longer writes the index file when it is not changed.
The :func:`~dis.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 :issue:`11822`.)
README.rst is now included in the list of distutils standard READMEs and
therefore included in source distributions.
(Contributed by Ryan Gonzalez in :issue:`11913`.)
The :class:`Enum <enum.Enum>` learned the new
_ignore_ class property,
which allows listing the names of properties which should not become
(Contributed by Ethan Furman in :issue:`31801`.)
In Python 3.8, attempting to check for non-Enum objects in :class:`Enum`
classes will raise a :exc:`TypeError` (e.g.
1 in Color); similarly,
attempting to check for non-Flag objects in a :class:`Flag` member will
raise :exc:`TypeError` (e.g.
1 in Perm.RW); currently, both operations
return :const:`False` instead and are deprecated.
(Contributed by Ethan Furman in :issue:`33217`.)
The new :func:`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 :func:`gc.unfreeze` functions
reverses this operation. Additionally, :func:`gc.get_freeze_count` can
be used to obtain the number of frozen objects.
(Contributed by Li Zekun in :issue:`31558`.)
:class:`~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 :issue:`29654`.)
:class:`~http.server.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 :issue:`28707`.)
The new :class:`ThreadingHTTPServer <http.server.ThreadingHTTPServer>` class
uses threads to handle requests using :class:`~socketserver.ThreadingMixin`.
It is used when
http.server is run with
(Contributed by Julien Palard in :issue:`31639`.)
idlelib and IDLE
Multiple fixes for autocompletion. (Contributed by Louie Lu in :issue:`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 :issue:`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 :issue:`13802`.) The sample can be edited to include other characters. (Contributed by Serhiy Storchaka in :issue:`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 :issue:`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 :issue:`33642`, :issue:`33768`, and :issue:`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 :issue:`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 :issue:`1529353`.)
The changes above have been backported to 3.6 maintenance releases.
The :class:`importlib.abc.ResourceReader` ABC was introduced to support the loading of resources from packages. See also :ref:`whatsnew37_importlib_resources`. (Contributed by Barry Warsaw, Brett Cannon in :issue:`32248`.)
:func:`importlib.find_spec` now raises :exc:`ModuleNotFoundError` instead of
:exc:`AttributeError` if the specified parent module is not a package (i.e.
(Contributed by Milan Oberkirch in :issue:`30436`.)
The new :meth:`TextIOWrapper.reconfigure() <io.TextIOWrapper.reconfigure>` method can be used to reconfigure the text stream with the new settings. (Contributed by Antoine Pitrou in :issue:`30526` and INADA Naoki in :issue:`15216`.)
supernet_of() methods of
:class:`ipaddress.IPv6Network` and :class:`ipaddress.IPv4Network` can
be used for network containment tests.
(Contributed by Michel Albert and Cheryl Sabella in :issue:`20825`.)
The new :meth:`StreamHandler.setStream() <logging.StreamHandler.setStream>` method can be used to replace the logger stream after handler creation. (Contributed by Vinay Sajip in :issue:`30522`.)
The MIME type of .bmp has been changed from
(Contributed by Nitish Chandra in :issue:`22589`.)
The new :meth:`Process.close() <multiprocessing.Process.close>` method explicitly closes the process object and releases all resources associated with it. :exc:`ValueError` is raised if the underlying process is still running. (Contributed by Antoine Pitrou in :issue:`30596`.)
The new :meth:`Process.kill() <multiprocessing.Process.kill>` method can be used to terminate the process using the :data:`SIGKILL` signal on Unix. (Contributed by Vitor Pereira in :issue:`30794`.)
Added :func:`os.preadv` (combine the functionality of :func:`os.readv` and :func:`os.pread`) and :func:`os.pwritev` functions (combine the functionality of :func:`os.writev` and :func:`os.pwrite`). (Contributed by Pablo Galindo in :issue:`31368`.)
The new :meth:`Path.is_mount() <pathlib.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 :issue:`30897`.)
:func:`py_compile.compile` -- and by extension, :mod:`compileall` -- now
respects the :envvar:`SOURCE_DATE_EPOCH` environment variable by
.pyc files for hash-based validation.
This allows for guaranteeing
reproducible builds of
files when they are created eagerly. (Contributed by Bernhard M. Wiedemann
The pydoc server can now bind to an arbitrary hostname specified by the
-n command-line argument.
(Contributed by Feanil Patel in :issue:`31128`.)
Regular expressions compiled with the :const:`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 :issue:`30215`.)
:exc:`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 :issue:`30349`.)
The new warn_on_full_buffer argument to the :func:`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 :issue:`30050`.)
The new :func:`socket.getblocking() <socket.socket.getblocking>` method
True if the socket is in blocking mode and
(Contributed by Yury Selivanov in :issue:`32373`.)
The new :func:`socket.close` function closes the passed socket file descriptor. This function should be used instead of :func:`os.close` for better compatibility across platforms. (Contributed by Christian Heimes in :issue:`32454`.)
The :mod:`socket` module now exposes the :data:`socket.TCP_CONGESTION` (Linux 2.6.13), :data:`socket.TCP_USER_TIMEOUT` (Linux 2.6.37), and :data:`socket.TCP_NOTSENT_LOWAT` (Linux 3.12) constants. (Contributed by Omar Sandoval in :issue:`26273` and Nathaniel J. Smith in :issue:`29728`.)
Sockets now auto-detect family, type and protocol from file descriptor by default. (Contributed by Christian Heimes in :issue:`28134`.)
Add a new :attr:`socketserver.ForkingMixIn.block_on_close` class attribute to
:class:`socketserver.ForkingMixIn` and :class:`socketserver.ThreadingMixIn`
classes. Set the class attribute to
False to get the pre-3.7 behaviour.
:class:`sqlite3.Connection` now exposes the :meth:`~sqlite3.Connection.backup` method when the underlying SQLite library is at version 3.6.11 or higher. (Contributed by Lele Gaifax in :issue:`27645`.)
The :mod:`ssl` module now uses OpenSSL's builtin API instead of :func:`~ssl.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 :exc:`~ssl.SSLCertVerificationError` and aborts the handshake with a proper TLS Alert message. The new exception contains additional information. Host name validation can be customized with :attr:`SSLContext.host_flags <ssl.SSLContext.host_flags>`. (Contributed by Christian Heimes in :issue:`31399`.)
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 :ref:`37-platform-support-removals` for more details). The ssl module is mostly compatible with LibreSSL 2.7.2 and newer.
ssl module no longer sends IP addresses in SNI TLS extension.
(Contributed by Christian Heimes in :issue:`32185`.)
:func:`~ssl.match_hostname` no longer supports partial wildcards like
www*.example.org. :attr:`SSLContext.host_flags <ssl.SSLContext.host_flags>`
has partial wildcard matching disabled by default.
(Contributed by Mandeep Singh in :issue:`23033` and Christian Heimes in
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 :issue:`31429`.)
Validation of server certificates containing internationalized domain names
(IDNs) is now supported. As part of this change, the
:attr:`SSLSocket.server_hostname <ssl.SSLSocket.server_hostname>` attribute
now stores the expected hostname in A-label form (
rather than the U-label form (
"pythön.org"). (Contributed by
Nathaniel J. Smith and Christian Heimes in :issue:`28414`.)
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,
(Contributed by Christian Heimes in :issue:`32947`, :issue:`20995`,
:issue:`29136`, :issue:`30622` and :issue:`33618`)
:class:`~ssl.SSLSocket` and :class:`~ssl.SSLObject` no longer have a public constructor. Direct instantiation was never a documented and supported feature. Instances must be created with :class:`~ssl.SSLContext` methods :meth:`~ssl.SSLContext.wrap_socket` and :meth:`~ssl.SSLContext.wrap_bio`. (Contributed by Christian Heimes in :issue:`32951`)
OpenSSL 1.1 APIs for setting the minimum and maximum TLS protocol version are available as :attr:`SSLContext.minimum_version <ssl.SSLContext.minimum_version>` and :attr:`SSLContext.maximum_version <ssl.SSLContext.maximum_version>`. Supported protocols are indicated by several new flags, such as :data:`~ssl.HAS_TLSv1_1`. (Contributed by Christian Heimes in :issue:`32609`.)
:class:`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 :issue:`1198569`.)
The :func:`subprocess.run` function accepts the new capture_output keyword argument. When true, stdout and stderr will be captured. This is equivalent to passing :data:`subprocess.PIPE` as stdout and stderr arguments. (Contributed by Bo Bayles in :issue:`32102`.)
On Windows the default for close_fds was changed from
True when redirecting the standard handles. It's now possible to set
close_fds to true when redirecting the standard handles. See
:class:`subprocess.Popen`. This means that close_fds now defaults to
True on all supported platforms.
(Contributed by Segev Finer in :issue:`19764`.)
The subprocess module is now more graceful when handling
:exc:`KeyboardInterrupt` during :func:`subprocess.call`,
:func:`subprocess.run`, or in a :class:`~subprocess.Popen`
context manager. It now waits a short amount of time for the child
to exit, before continuing the handling of the
(Contributed by Gregory P. Smith in :issue:`25942`.)
The new :func:`sys.get_coroutine_origin_tracking_depth` function returns the current coroutine origin tracking depth, as set by the new :func:`sys.set_coroutine_origin_tracking_depth`. :mod:`asyncio` has been converted to use this new API instead of the deprecated :func:`sys.set_coroutine_wrapper`. (Contributed by Nathaniel J. Smith in :issue:`32591`.)
New clock identifiers have been added:
- :data:`time.CLOCK_BOOTTIME` (Linux): Identical to :data:`time.CLOCK_MONOTONIC`, except it also includes any time that the system is suspended.
- :data:`time.CLOCK_PROF` (FreeBSD, NetBSD and OpenBSD): High-resolution per-process CPU timer.
- :data:`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 :func:`time.pthread_getcpuclockid` function returns the clock ID of the thread-specific CPU-time clock.
:class:`tracemalloc.Traceback` behaves more like regular tracebacks,
sorting the frames from oldest to most recent.
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
(Contributed by Jesse Bakker in :issue:`32121`.)
The new :class:`~types.WrapperDescriptorType`, :class:`~types.MethodWrapperType`, :class:`~types.MethodDescriptorType`, and :class:`~types.ClassMethodDescriptorType` classes are now available. (Contributed by Manuel Krebber and Guido van Rossum in :issue:`29377`, and Serhiy Storchaka in :issue:`32265`.)
-k command-line option allows filtering tests by a name
substring or a Unix shell-like pattern.
python -m unittest -k foo runs
(Contributed by Jonas Haag in :issue:`32071`.)
The new :func:`~unittest.mock.seal` function allows sealing :class:`~unittest.mock.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 :issue:`30541`.)
:func:`urllib.parse.quote` has been updated from RFC 2396 to RFC 3986,
~ to the set of characters that are never quoted by default.
(Contributed by Christian Theune and Ratnadeep Debnath in :issue:`16285`.)
The new :attr:`UUID.is_safe <uuid.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 :issue:`22807`.)
:func:`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 :func:`uuid.uuid1`. If only locally administered MAC addresses are available, the first such one found is returned. (Contributed by Barry Warsaw in :issue:`32107`.)
The initialization of the default warnings filters has changed as follows:
warnings enabled via command line options (including those for :option:`-b` and the new CPython-specific :option:`-X`
devoption) are always passed to the warnings machinery via the :data:`sys.warnoptions` attribute.
warnings filters enabled via the command line or the environment now have the following order of precedence:
BytesWarningfilter for :option:`-b` (or
- any filters specified with the :option:`-W` option
- any filters specified with the :envvar:`PYTHONWARNINGS` environment variable
- any other CPython specific filters (e.g. the
defaultfilter added for the new
- 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)
:ref:`ElementPath <elementtree-xpath>` predicates in the :meth:`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 :issue:`31648`.)
:meth:`SimpleXMLRPCDispatcher.register_function <xmlrpc.server.SimpleXMLRPCDispatcher>` can now be used as a decorator. (Contributed by Xiang Zhang in :issue:`7769`.)
Function :func:`~zipapp.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 :issue:`31072`.)
Function :func:`~zipapp.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 :issue:`31638`.)
Subdirectories in archives created by
ZipFile are now stored in
(Contributed by Bernhard M. Wiedemann in :issue:`30693`.)
C API Changes
A new API for thread-local storage has been implemented. See :ref:`whatsnew37-pep539` for an overview and :ref:`thread-specific-storage-api` for a complete reference. (Contributed by Masayuki Yamamoto in :issue:`25658`.)
The fields :c:member:`name` and :c:member:`doc` of structures
and :c:type:`wrapperbase` are now of type
const char * rather of
char *. (Contributed by Serhiy Storchaka in :issue:`28761`.)
:c:func:`PyOS_AfterFork` is deprecated in favour of the new functions :c:func:`PyOS_BeforeFork`, :c:func:`PyOS_AfterFork_Parent` and :c:func:`PyOS_AfterFork_Child`. (Contributed by Antoine Pitrou in :issue:`16500`.)
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
:issue:`22898` and :issue:`30697`.
Added C API support for timezones with timezone constructors :c:func:`PyTimeZone_FromOffset` and :c:func:`PyTimeZone_FromOffsetAndName`, and access to the UTC singleton with :c:data:`PyDateTime_TimeZone_UTC`. Contributed by Paul Ganssle in :issue:`10381`.
The type of results of :c:func:`PyThread_start_new_thread` and :c:func:`PyThread_get_thread_ident`, and the id parameter of :c:func:`PyThreadState_SetAsyncExc` changed from :c:type:`long` to :c:type:`unsigned long`. (Contributed by Serhiy Storchaka in :issue:`6532`.)
:c:func:`PyUnicode_AsWideCharString` now raises a :exc:`ValueError` if the second argument is NULL and the :c:type:`wchar_t*` string contains null characters. (Contributed by Serhiy Storchaka in :issue:`30708`.)
Changes to the startup sequence and the management of dynamic memory allocators mean that the long documented requirement to call :c:func:`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 :ref:`porting-to-python-37` section in this document and the :ref:`pre-init-safe` section in the C API documentation for more details.
A full copy of libffi is no longer bundled for use when building the
:mod:`_ctypes <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 :issue:`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 :issue:`30450`.)
The :mod:`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.
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
(Contributed by Victor Stinner in :issue:`29300`, :issue:`29507`,
:issue:`29452`, and :issue:`29286`.)
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 :issue:`26110`.)
The :mod:`asyncio` module received a number of notable optimizations for commonly used functions:
- The :func:`asyncio.get_event_loop` function has been reimplemented in C to make it up to 15 times faster. (Contributed by Yury Selivanov in :issue:`32296`.)
- :class:`asyncio.Future` callback management has been optimized. (Contributed by Yury Selivanov in :issue:`32348`.)
- :func:`asyncio.gather` is now up to 15% faster. (Contributed by Yury Selivanov in :issue:`32355`.)
- :func:`asyncio.sleep` is now up to 2 times faster when the delay argument is zero or negative. (Contributed by Andrew Svetlov in :issue:`32351`.)
- The performance overhead of asyncio debug mode has been reduced. (Contributed by Antoine Pitrou in :issue:`31970`.)
As a result of :ref:`PEP 560 work <whatsnew37-pep560>`, the import time of :mod:`typing` has been reduced by a factor of 7, and many typing operations are now faster. (Contributed by Ivan Levkivskyi in :issue:`32226`.)
:func:`hasattr` and :func:`getattr` are now about 4 times faster when name is not found and obj does not override :meth:`object.__getattr__` or :meth:`object.__getattribute__`. (Contributed by INADA Naoki in :issue:`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 :issue:`24821`.)
The :func:`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 :issue:`28638`.)
:func:`re.compile` now converts
flags parameter to int object if
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 :issue:`31671`.)
The :meth:`~selectors.BaseSelector.modify` methods of classes :class:`selectors.EpollSelector`, :class:`selectors.PollSelector` and :class:`selectors.DevpollSelector` may be around 10% faster under heavy loads. (Contributed by Giampaolo Rodola' in :issue:`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 :issue:`29469` and :issue:`11549`.)
Most functions and methods in :mod:`abc` have been rewritten in C. This makes creation of abstract base classes, and calling :func:`isinstance` and :func:`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 :issue:`31333`)
Significant speed improvements to alternate constructors for :class:`datetime.date` and :class:`datetime.datetime` by using fast-path constructors when not constructing subclasses. (Contributed by Paul Ganssle in :issue:`32403`)
The speed of comparison of :class:`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 :issue:`24700`.)
Other CPython Implementation Changes
- Trace hooks may now opt out of receiving the
lineand opt into receiving the
opcodeevents from the interpreter by setting the corresponding new
f_trace_opcodesattributes on the frame being traced. (Contributed by Nick Coghlan in :issue:`31344`.)
- Fixed some consistency problems with namespace package module attributes.
Namespace module objects now have an
__file__that is set to
None(previously unset), and their
__spec__.originis also set to
None(previously the string
"namespace"). See :issue:`32305`. Also, the namespace module object's
__spec__.loaderis set to the same value as
__loader__(previously, the former was set to
None). See :issue:`32303`.
- The :func:`locals` dictionary now displays in the lexical order that variables were defined. Previously, the order was undefined. (Contributed by Raymond Hettinger in :issue:`32690`.)
- The :mod:`distutils`
uploadcommand 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 :issue:`32304`.)
Deprecated Python Behavior
Yield expressions (both
yield from clauses) are now deprecated
in comprehensions and generator expressions (aside from the iterable expression
in the leftmost :keyword:`!for` clause). This ensures that comprehensions
always immediately return a container of the appropriate type (rather than
potentially returning a :term:`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
:exc:`DeprecationWarning` when compiled, in Python 3.8 this will be a
(Contributed by Serhiy Storchaka in :issue:`10544`.)
Returning a subclass of :class:`complex` from :meth:`object.__complex__` is
deprecated and will be an error in future Python versions. This makes
__complex__() consistent with :meth:`object.__int__` and
(Contributed by Serhiy Storchaka in :issue:`28894`.)
Deprecated Python modules, functions and methods
Support for directly
await-ing instances of :class:`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 :issue:`32253`.)
In Python 3.8, the abstract base classes in :mod:`collections.abc` will no longer be exposed in the regular :mod:`collections` module. This will help create a clearer distinction between the concrete classes and the abstract base classes. (Contributed by Serhiy Storchaka in :issue:`25988`.)
:mod:`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
modes (this will be an error in future Python releases).
(Contributed by Serhiy Storchaka in :issue:`28847`.)
In Python 3.8, attempting to check for non-Enum objects in :class:`Enum`
classes will raise a :exc:`TypeError` (e.g.
1 in Color); similarly,
attempting to check for non-Flag objects in a :class:`Flag` member will
raise :exc:`TypeError` (e.g.
1 in Perm.RW); currently, both operations
return :const:`False` instead.
(Contributed by Ethan Furman in :issue:`33217`.)
Methods :meth:`MetaPathFinder.find_module() <importlib.abc.MetaPathFinder.find_module>` (replaced by :meth:`MetaPathFinder.find_spec() <importlib.abc.MetaPathFinder.find_spec>`) and :meth:`PathEntryFinder.find_loader() <importlib.abc.PathEntryFinder.find_loader>` (replaced by :meth:`PathEntryFinder.find_spec() <importlib.abc.PathEntryFinder.find_spec>`) both deprecated in Python 3.4 now emit :exc:`DeprecationWarning`. (Contributed by Matthias Bussonnier in :issue:`29576`)
:mod:`dummy_threading` and :mod:`_dummy_thread` have been deprecated. It is no longer possible to build Python with threading disabled. Use :mod:`threading` instead. (Contributed by Antoine Pitrou in :issue:`31370`.)
The silent argument value truncation in :func:`socket.htons` and :func:`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 :issue:`28332`.)
sys.callstats() function has been deprecated and
will be removed in a future Python version.
(Contributed by Victor Stinner in :issue:`28799`.)
Deprecated functions and types of the C API
Function :c:func:`PySlice_GetIndicesEx` is deprecated and replaced with
a macro if
Py_LIMITED_API is not set or set to a value in the range
0x03060000 (not inclusive), or is
or higher. (Contributed by Serhiy Storchaka in :issue:`27867`.)
:c:func:`PyOS_AfterFork` has been deprecated. Use :c:func:`PyOS_BeforeFork`, :c:func:`PyOS_AfterFork_Parent` or :c:func:`PyOS_AfterFork_Child()` instead. (Contributed by Antoine Pitrou in :issue:`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.utf8(full locale) or
LC_CTYPE-only locale) as an alternative to the legacy
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 :source:`CI configuration file <.travis.yml>` provides an example of using the SSL :source:`compatibility testing infrastructure <Tools/ssl/multissltests.py>` 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:
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 for :func:`re.sub` were deprecated in Python 3.5, and will now cause an error.
- Removed support of the exclude argument in :meth:`tarfile.TarFile.add`. It was deprecated in Python 2.7 and 3.2. Use the filter argument instead.
splitunc()function in the :mod:`ntpath` module was deprecated in Python 3.1, and has now been removed. Use the :func:`~os.path.splitdrive` function instead.
- :func:`collections.namedtuple` no longer supports the verbose parameter
_sourceattribute 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 :issue:`28638`.)
- Functions :func:`bool`, :func:`float`, :func:`list` and :func:`tuple` no longer take keyword arguments. The first argument of :func:`int` can now be passed only as positional argument.
- Removed previously deprecated in Python 2.4 classes
_InternalDictin the :mod:`plistlib` module. Dict values in the result of functions :func:`~plistlib.readPlist` and :func:`~plistlib.readPlistFromBytes` are now normal dicts. You no longer can use attribute access to access items of these dictionaries.
asyncio.windows_utils.socketpair()function has been removed. Use the :func:`socket.socketpair` function instead, it is available on all platforms since Python 3.5.
asyncio.windows_utils.socketpairwas just an alias to
socket.socketpairon Python 3.5 and newer.
- :mod:`asyncio` no longer exports the :mod:`selectors` and
:mod:`_overlapped` modules as
from asyncio import selectorswith
- Direct instantiation of :class:`ssl.SSLSocket` and :class:`ssl.SSLObject` objects is now prohibited. The constructors were never documented, tested, or designed as public constructors. Users were supposed to use :func:`ssl.wrap_socket` or :class:`ssl.SSLContext`. (Contributed by Christian Heimes in :issue:`32951`.)
- The unused :mod:`distutils`
install_misccommand has been removed. (Contributed by Eric N. Vander Weele in :issue:`29218`.)
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 :issue:`29137`.)
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
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 :issue:`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 :issue:`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
PEP 479 is enabled for all code in Python 3.7, meaning that :exc:`StopIteration` exceptions raised directly or indirectly in coroutines and generators are transformed into :exc:`RuntimeError` exceptions. (Contributed by Yury Selivanov in :issue:`32670`.)
Due to an oversight, earlier Python versions erroneously accepted the following syntax:
f(1 for x in ,) class C(1 for x in ): pass
Python 3.7 now correctly raises a :exc:`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 :issue:`32012` and :issue:`32023`.)
When using the :option:`-m` switch, the initial working directory is now added to :data:`sys.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 for
os.path.dirname(__main__.__file__), depending on why the code was checking for the empty string in the first place).
Changes in the Python API
:meth:`socketserver.ThreadingMixIn.server_close` now waits until all non-daemon threads complete. Set the new :attr:`socketserver.ThreadingMixIn.block_on_close` class attribute to
Falseto get the pre-3.7 behaviour. (Contributed by Victor Stinner in :issue:`31233` and :issue:`33540`.)
:meth:`socketserver.ForkingMixIn.server_close` now waits until all child processes complete. Set the new :attr:`socketserver.ForkingMixIn.block_on_close` class attribute to
Falseto get the pre-3.7 behaviour. (Contributed by Victor Stinner in :issue:`31151` and :issue:`33540`.)
A format string argument for :meth:`string.Formatter.format` is now :ref:`positional-only <positional-only_parameter>`. Passing it as a keyword argument was deprecated in Python 3.5. (Contributed by Serhiy Storchaka in :issue:`29193`.)
Attributes :attr:`~http.cookies.Morsel.key`, :attr:`~http.cookies.Morsel.value` and :attr:`~http.cookies.Morsel.coded_value` of class :class:`http.cookies.Morsel` are now read-only. Assigning to them was deprecated in Python 3.5. Use the :meth:`~http.cookies.Morsel.set` method for setting them. (Contributed by Serhiy Storchaka in :issue:`29192`.)
The mode argument of :func:`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 invoking
makedirs(). (Contributed by Serhiy Storchaka in :issue:`19930`.)
:func:`~cgi.parse_multipart` now accepts the encoding and errors arguments and returns the same results as :class:`~FieldStorage`: for non-file fields, the value associated to a key is a list of strings, not bytes. (Contributed by Pierre Quentel in :issue:`29979`.)
Because :func:`shutil.rmtree` is now implemented using the :func:`os.scandir` function, the user specified handler onerror is now called with the first argument
os.listdirwhen 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 :exc:`FutureWarning` will be raised in ambiguous cases for the time being. That include sets starting with a literal
'['or containing literal character sequences
'||'. To avoid a warning, escape them with a backslash. (Contributed by Serhiy Storchaka in :issue:`30349`.)
The result of splitting a string on a :mod:`regular expression <re>` that could match an empty string has been changed. For example splitting on
r'\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 to
r'\s+'. A :exc:`FutureWarning` 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 pattern
r'(?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 as
:func:`re.sub()` now replaces empty matches adjacent to a previous non-empty match. For example
re.sub('x*', '-', 'abxd')returns now
'-a-b-d-'(the first minus between 'b' and 'd' replaces 'x', and the second minus replaces an empty string between 'x' and 'd').
On OSes that support :const:`socket.SOCK_NONBLOCK` or :const:`socket.SOCK_CLOEXEC` bit flags, the :attr:`socket.type <socket.socket.type>` no longer has them applied. Therefore, checks like
if sock.type == socket.SOCK_STREAMwork as expected on all platforms. (Contributed by Yury Selivanov in :issue:`32331`.)
On Windows the default for the close_fds argument of :class:`subprocess.Popen` was changed from :const:`False` to :const:`True` when redirecting the standard handles. If you previously depended on handles being inherited when using :class:`subprocess.Popen` with standard io redirection, you will have to pass
close_fds=Falseto preserve the previous behaviour, or use :attr:`STARTUPINFO.lpAttributeList <subprocess.STARTUPINFO.lpAttributeList>`.
:meth:`importlib.machinery.PathFinder.invalidate_caches` -- which implicitly affects :func:`importlib.invalidate_caches` -- now deletes entries in :data:`sys.path_importer_cache` which are set to
None. (Contributed by Brett Cannon in :issue:`33169`.)
In :mod:`asyncio`, :meth:`loop.sock_recv() <asyncio.loop.sock_recv>`, :meth:`loop.sock_sendall() <asyncio.loop.sock_sendall>`, :meth:`loop.sock_accept() <asyncio.loop.sock_accept>`, :meth:`loop.getaddrinfo() <asyncio.loop.getaddrinfo>`, :meth:`loop.getnameinfo() <asyncio.loop.getnameinfo>` have been changed to be proper coroutine methods to match their documentation. Previously, these methods returned :class:`asyncio.Future` instances. (Contributed by Yury Selivanov in :issue:`32327`.)
:meth:`Calendar.itermonthdates <calendar.Calendar.itermonthdates>` will now consistently raise an exception when a date falls outside of the
9999-12-31range. To support applications that cannot tolerate such exceptions, the new :meth:`Calendar.itermonthdays3 <calendar.Calendar.itermonthdays3>` and :meth:`Calendar.itermonthdays4 <calendar.Calendar.itermonthdays4>` can be used. The new methods return tuples and are not restricted by the range supported by :class:`datetime.date`. (Contributed by Alexander Belopolsky in :issue:`28292`.)
submit()method of :class:`concurrent.futures.ThreadPoolExecutor` and :class:`concurrent.futures.ProcessPoolExecutor` now raises a :exc:`RuntimeError` if called during interpreter shutdown. (Contributed by Mark Nemec in :issue:`33097`.)
The :class:`configparser.ConfigParser` constructor now uses
read_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 :issue:`23835`.)
Several undocumented internal imports were removed. One example is that
os.errnois no longer available; use
import errnodirectly 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 :c:func:`PySlice_GetIndicesEx` is considered unsafe for resizable sequences. If the slice indices are not instances of :class:`int`, but objects that implement the :meth:`!__index__` method, the sequence can be resized after passing its length to :c:func:`!PySlice_GetIndicesEx`. This can lead to returning indices out of the length of the sequence. For avoiding possible problems use new functions :c:func:`PySlice_Unpack` and :c:func:`PySlice_AdjustIndices`. (Contributed by Serhiy Storchaka in :issue:`27867`.)
CPython bytecode changes
The file used to override :data:`sys.path` is now called
<python-executable>._pth instead of
See :ref:`finding_modules` for more information.
(Contributed by Steve Dower in :issue:`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 :issue:`22257`, and further updated by Nick, Eric, and Victor Stinner in a number of other issues). Some known details affected:
- :c:func:`PySys_AddWarnOptionUnicode` is not currently usable by embedding applications due to the requirement to create a Unicode object prior to calling Py_Initialize. Use :c:func:`PySys_AddWarnOption` instead.
- warnings filters added by an embedding application with :c:func:`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 :c:data:`Py_BytesWarningFlag` to a value greater than one is no longer
sufficient to both emit :exc:`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
warnings filter added to convert them to exceptions.
Due to a change in the way docstrings are handled by the compiler, the
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 :issue:`25612`.)
Notable changes in Python 3.7.1
Starting in 3.7.1, :c:func:`Py_Initialize` now consistently reads and respects all of the same environment settings as :c:func:`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 :issue:`34247`). If this behavior is unwanted, set :c:data:`Py_IgnoreEnvironmentFlag` to 1 before calling :c:func:`Py_Initialize`.
In 3.7.1 the C API for Context Variables :ref:`was updated <contextvarsobjects_pointertype_change>` to use :c:type:`PyObject` pointers. See also :issue:`34762`.
In 3.7.1 the :mod:`tokenize` module now implicitly emits a
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 :issue:`33899`.)
Notable changes in Python 3.7.2
In 3.7.2, :mod:`venv` on Windows no longer copies the original binaries, but
creates redirector scripts named
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.