What's New In Python 3.7

Editor
Elvis Pranskevichus <elvis@magic.io>

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XXX Describe the transmogrify() function added to the socket module. (Contributed by P.Y. Developer in 12345.)

This saves the maintainer the effort of going through the Git log when researching a change.

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 <changelog>.

Summary -- Release Highlights

New syntax features:

• PEP 563 <whatsnew37-pep563>, postponed evaluation of type annotations.

Backwards incompatible syntax changes:

• async and await are now reserved keywords.

New library modules:

• contextvars: PEP 567 -- Context Variables <whatsnew37-pep567>
• dataclasses: PEP 557 -- Data Classes <whatsnew37-pep557>
• whatsnew37_importlib_resources

New built-in features:

• PEP 553 <whatsnew37-pep553>, the new breakpoint function.

Python data model improvements:

• PEP 562 <whatsnew37-pep562>, customization of access to module attributes.
• PEP 560 <whatsnew37-pep560>, core support for typing module and generic types.
• the insertion-order preservation nature of dict <typesmapping> 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 <whatsnew37_asyncio>.
• The time module gained support for functions with nanosecond resolution <whatsnew37-pep564>.

CPython implementation improvements:

• Avoiding the use of ASCII as a default text encoding:
  • PEP 538 <whatsnew37-pep538>, legacy C locale coercion
  • PEP 540 <whatsnew37-pep540>, forced UTF-8 runtime mode
• PEP 552 <whatsnew37-pep552>, deterministic .pycs
• New Python Development Mode <whatsnew37-devmode>
• PEP 565 <whatsnew37-pep565>, improved DeprecationWarning handling

C API improvements:

• PEP 539 <whatsnew37-pep539>, new C API for thread-local storage

Documentation improvements:

• PEP 545 <whatsnew37-pep545>, Python documentation translations
• New documentation translations: Japanese, French, and Korean.

This release features notable performance improvements in many areas. The whatsnew37-perf 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-37 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 3107 and refined further in 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.

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.

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 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 ~sys.stdin and ~sys.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 ~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 ~sys.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 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 whatsnew37-pep540.

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 <utf8-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 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 whatsnew37-pep540).

The UTF-8 mode is enabled by default when the locale is C or POSIX, and the 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).

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().

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 <thread-local-storage-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.

539 changes this by providing a new 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 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.

PEP 562: Customization of Access to Module Attributes

Python 3.7 allows defining ~module.__getattr__ on modules and will call it whenever a module attribute is otherwise not found. Defining ~module.__dir__ on modules is now also allowed.

A typical example of where this may be useful is module attribute deprecation and lazy loading.

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, 564 adds six new "nanosecond" variants of the existing timer functions to the time module:

• time.clock_gettime_ns
• time.clock_settime_ns
• time.monotonic_ns
• time.perf_counter_ns
• time.process_time_ns
• time.time_ns

The new functions return the number of nanoseconds as an integer value.

Measurements <0564#annex-clocks-resolution-in-python> show that on Linux and Windows the resolution of time.time_ns is approximately 3 times better than that of time.time.

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 to DeprecationWarning or FutureWarning.

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.

PEP 560: Core Support for typing module and Generic Types

Initially 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 ~object.__class_getitem__ and ~object.__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.

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.

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 pyc-invalidation for more information.

PEP 545: Python Documentation Translations

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/

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 <devmode>. When in development mode, Python performs additional runtime checks that are too expensive to be enabled by default. See Python Development Mode <devmode> documentation for the full description.

Other Language Changes

• An await expression and comprehensions containing an async for clause were illegal in the expressions in formatted string literals <f-strings> 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 12844 and 18896.)
• bytes.fromhex and bytearray.fromhex now ignore all ASCII whitespace, not only spaces. (Contributed by Robert Xiao in 28927.)
• str, bytes, and bytearray gained support for the new 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 32677.)
• ImportError now displays module name and module __file__ path when from ... import ... fails. (Contributed by Matthias Bussonnier in 29546.)
• Circular imports involving absolute imports with binding a submodule to a name are now supported. (Contributed by Serhiy Storchaka in 30024.)
• object.__format__(x, '') is now equivalent to str(x) rather than format(str(self), ''). (Contributed by Serhiy Storchaka in 28974.)
• In order to better support dynamic creation of stack traces, types.TracebackType can now be instantiated from Python code, and the ~traceback.tb_next attribute on tracebacks <traceback-objects> is now writable. (Contributed by Nathaniel J. Smith in 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 33053.)
• The new -X importtime option or the PYTHONPROFILEIMPORTTIME environment variable can be used to show the timing of each module import. (Contributed by Inada Naoki in 31415.)

New Modules

contextvars

The new contextvars module and a set of 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 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.

dataclasses

The new ~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 ~object.__repr__, ~object.__eq__, and ~object.__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)"

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 32248.

Improved Modules

argparse

The new ArgumentParser.parse_intermixed_args() <argparse.ArgumentParser.parse_intermixed_args> method allows intermixing options and positional arguments. (Contributed by paul.j3 in 14191.)

asyncio

The asyncio module has received many new features, usability and performance improvements <whatsnew37-asyncio-perf>. Notable changes include:

• The new provisional <provisional API> 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 32314.)

• asyncio gained support for contextvars. loop.call_soon() <asyncio.loop.call_soon>, loop.call_soon_threadsafe() <asyncio.loop.call_soon_threadsafe>, loop.call_later() <asyncio.loop.call_later>, loop.call_at() <asyncio.loop.call_at>, and Future.add_done_callback() <asyncio.Future.add_done_callback> have a new optional keyword-only context parameter. Tasks <asyncio.Task> now track their context automatically. See 567 for more details. (Contributed by Yury Selivanov in 32436.)

• The new asyncio.create_task function has been added as a shortcut to asyncio.get_event_loop().create_task(). (Contributed by Andrew Svetlov in 32311.)

• The new loop.start_tls() <asyncio.loop.start_tls> method can be used to upgrade an existing connection to TLS. (Contributed by Yury Selivanov in 23749.)

• The new 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 31819.)

• The new asyncio.current_task function returns the currently running ~asyncio.Task instance, and the new asyncio.all_tasks function returns a set of all existing Task instances in a given loop. The !Task.current_task and !Task.all_tasks methods have been deprecated. (Contributed by Andrew Svetlov in 32250.)

• The new provisional ~asyncio.BufferedProtocol class allows implementing streaming protocols with manual control over the receive buffer. (Contributed by Yury Selivanov in 32251.)

• The new asyncio.get_running_loop function returns the currently running loop, and raises a RuntimeError if no loop is running. This is in contrast with asyncio.get_event_loop, which will create a new event loop if none is running. (Contributed by Yury Selivanov in 32269.)

• The new StreamWriter.wait_closed() <asyncio.StreamWriter.wait_closed> coroutine method allows waiting until the stream writer is closed. The new StreamWriter.is_closing() <asyncio.StreamWriter.is_closing> method can be used to determine if the writer is closing. (Contributed by Andrew Svetlov in 32391.)

• The new loop.sock_sendfile() <asyncio.loop.sock_sendfile> coroutine method allows sending files using os.sendfile when possible. (Contributed by Andrew Svetlov in 32410.)

• The new 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. Server.get_loop() <asyncio.Server.get_loop> allows doing the same for asyncio.Server objects. (Contributed by Yury Selivanov in 32415 and Srinivas Reddy Thatiparthy in 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 to loop.create_server() <asyncio.loop.create_server> and loop.create_unix_server() <asyncio.loop.create_unix_server>, as well as Server.start_serving() <asyncio.Server.start_serving>, and Server.serve_forever() <asyncio.Server.serve_forever> can be used to decouple server instantiation and serving. The new Server.is_serving() <asyncio.Server.is_serving> method returns True if the server is serving. ~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 32662.)

• Callback objects returned by loop.call_later() <asyncio.loop.call_later> gained the new when() <asyncio.TimerHandle.when> method which returns an absolute scheduled callback timestamp. (Contributed by Andrew Svetlov in 32741.)

• The :meth:`loop.create_datagram_endpoint() <asyncio.loop.create_datagram_endpoint>` method gained support for Unix sockets. (Contributed by Quentin Dawans in 31245.)

• The asyncio.open_connection, asyncio.start_server functions, loop.create_connection() <asyncio.loop.create_connection>, loop.create_server() <asyncio.loop.create_server>, 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 29970.)

• The new Handle.cancelled() <asyncio.Handle.cancelled> method returns True if the callback was cancelled. (Contributed by Marat Sharafutdinov in 31943.)

• The asyncio source has been converted to use the async/await syntax. (Contributed by Andrew Svetlov in 32193.)

• The new ReadTransport.is_reading() <asyncio.ReadTransport.is_reading> method can be used to determine the reading state of the transport. Additionally, calls to ReadTransport.resume_reading() <asyncio.ReadTransport.resume_reading> and ReadTransport.pause_reading() <asyncio.ReadTransport.pause_reading> are now idempotent. (Contributed by Yury Selivanov in 32356.)

• Loop methods which accept socket paths now support passing path-like objects <path-like object>. (Contributed by Yury Selivanov in 32066.)

• In asyncio TCP sockets on Linux are now created with TCP_NODELAY flag set by default. (Contributed by Yury Selivanov and Victor Stinner in 27456.)

• Exceptions occurring in cancelled tasks are no longer logged. (Contributed by Yury Selivanov in 30508.)

• New WindowsSelectorEventLoopPolicy and WindowsProactorEventLoopPolicy classes. (Contributed by Yury Selivanov in 33792.)

Several asyncio APIs have been deprecated <whatsnew37-asyncio-deprecated>.

binascii

The ~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 30103.)

calendar

The ~calendar.HTMLCalendar class has new class attributes which ease the customization of CSS classes in the produced HTML calendar. (Contributed by Oz Tiram in 30095.)

collections

collections.namedtuple() now supports default values. (Contributed by Raymond Hettinger in 32320.)

compileall

compileall.compile_dir learned the new invalidation_mode parameter, which can be used to enable hash-based .pyc invalidation <whatsnew37-pep552>. The invalidation mode can also be specified on the command line using the new --invalidation-mode argument. (Contributed by Benjamin Peterson in 31650.)

concurrent.futures

ProcessPoolExecutor <concurrent.futures.ProcessPoolExecutor> and ThreadPoolExecutor <concurrent.futures.ThreadPoolExecutor> now support the new initializer and initargs constructor arguments. (Contributed by Antoine Pitrou in 21423.)

The ProcessPoolExecutor <concurrent.futures.ProcessPoolExecutor> can now take the multiprocessing context via the new mp_context argument. (Contributed by Thomas Moreau in 31540.)

contextlib

The new ~contextlib.nullcontext is a simpler and faster no-op context manager than ~contextlib.ExitStack. (Contributed by Jesse-Bakker in 10049.)

The new ~contextlib.asynccontextmanager, ~contextlib.AbstractAsyncContextManager, and ~contextlib.AsyncExitStack have been added to complement their synchronous counterparts. (Contributed by Jelle Zijlstra in 29679 and 30241, and by Alexander Mohr and Ilya Kulakov in 29302.)

cProfile

The !cProfile command line now accepts -m module_name as an alternative to script path. (Contributed by Sanyam Khurana in 21862.)

crypt

The !crypt module now supports the Blowfish hashing method. (Contributed by Serhiy Storchaka in 31664.)

The !mksalt function now allows specifying the number of rounds for hashing. (Contributed by Serhiy Storchaka in 31702.)

datetime

The new datetime.fromisoformat() <datetime.datetime.fromisoformat> method constructs a ~datetime.datetime object from a string in one of the formats output by datetime.isoformat() <datetime.datetime.isoformat>. (Contributed by Paul Ganssle in 15873.)

The tzinfo <datetime.tzinfo> class now supports sub-minute offsets. (Contributed by Alexander Belopolsky in 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 <whatsnew37-pep567> to store the decimal context. (Contributed by Yury Selivanov in 32630.)

dis

The ~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 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 11913.)

enum

The Enum <enum.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 31801.)

In Python 3.8, attempting to check for non-Enum objects in ~enum.Enum classes will raise a TypeError (e.g. 1 in Color); similarly, attempting to check for non-Flag objects in a ~enum.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 33217.)

functools

functools.singledispatch now supports registering implementations using type annotations. (Contributed by Łukasz Langa in 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 31558.)

hmac

The hmac module now has an optimized one-shot ~hmac.digest function, which is up to three times faster than ~hmac.HMAC. (Contributed by Christian Heimes in 32433.)

http.client

~http.client.HTTPConnection and ~http.client.HTTPSConnection now support the new blocksize argument for improved upload throughput. (Contributed by Nir Soffer in 31945.)

http.server

~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 29654.)

~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 28707.)

The new ThreadingHTTPServer <http.server.ThreadingHTTPServer> class uses threads to handle requests using ~socketserver.ThreadingMixIn. It is used when http.server is run with -m. (Contributed by Julien Palard in 31639.)

idlelib and IDLE

Multiple fixes for autocompletion. (Contributed by Louie Lu in 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 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 13802.) The sample can be edited to include other characters. (Contributed by Serhiy Storchaka in 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 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 33642, 33768, and 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 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 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 5680 and 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 17535.)

importlib

The !importlib.abc.ResourceReader ABC was introduced to support the loading of resources from packages. See also whatsnew37_importlib_resources. (Contributed by Barry Warsaw, Brett Cannon in 32248.)

importlib.reload now raises ModuleNotFoundError if the module lacks a spec. (Contributed by Garvit Khatri in 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 30436.)

The new importlib.util.source_hash can be used to compute the hash of the passed source. A hash-based .pyc file <whatsnew37-pep552> embeds the value returned by this function.

io

The new TextIOWrapper.reconfigure() <io.TextIOWrapper.reconfigure> method can be used to reconfigure the text stream with the new settings. (Contributed by Antoine Pitrou in 30526 and INADA Naoki in 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 20825.)

itertools

itertools.islice now accepts integer-like objects <object.__index__> as start, stop, and slice arguments. (Contributed by Will Roberts in 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 10379.)

The locale.getpreferredencoding function now always returns 'UTF-8' on Android or when in the forced UTF-8 mode <whatsnew37-pep540>.

logging

~logging.Logger instances can now be pickled. (Contributed by Vinay Sajip in 30520.)

The new StreamHandler.setStream() <logging.StreamHandler.setStream> method can be used to replace the logger stream after handler creation. (Contributed by Vinay Sajip in 30522.)

It is now possible to specify keyword arguments to handler constructors in configuration passed to logging.config.fileConfig. (Contributed by Preston Landers in 31080.)

math

The new math.remainder function implements the IEEE 754-style remainder operation. (Contributed by Mark Dickinson in 29962.)

mimetypes

The MIME type of .bmp has been changed from 'image/x-ms-bmp' to 'image/bmp'. (Contributed by Nitish Chandra in 22589.)

msilib

The new !Database.Close method can be used to close the MSI database. (Contributed by Berker Peksag in 20486.)

multiprocessing

The new Process.close() <multiprocessing.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 30596.)

The new Process.kill() <multiprocessing.Process.kill> method can be used to terminate the process using the ~signal.SIGKILL signal on Unix. (Contributed by Vitor Pereira in 30794.)

Non-daemonic threads created by ~multiprocessing.Process are now joined on process exit. (Contributed by Antoine Pitrou in 18966.)

os

os.fwalk now accepts the path argument as bytes. (Contributed by Serhiy Storchaka in 28682.)

os.scandir gained support for file descriptors <path_fd>. (Contributed by Serhiy Storchaka in 25996.)

The new ~os.register_at_fork function allows registering Python callbacks to be executed at process fork. (Contributed by Antoine Pitrou in 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 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 19930.)

os.dup2 now returns the new file descriptor. Previously, None was always returned. (Contributed by Benjamin Peterson in 32441.)

The structure returned by os.stat now contains the ~os.stat_result.st_fstype attribute on Solaris and its derivatives. (Contributed by Jesús Cea Avión in 32659.)

pathlib

The new 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 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 31389.)

pdb command line now accepts -m module_name as an alternative to script file. (Contributed by Mario Corchero in 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 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 31128.)

queue

The new ~queue.SimpleQueue class is an unbounded FIFO queue. (Contributed by Antoine Pitrou in 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 31690.)

re.split now supports splitting on a pattern like r'\b', '^$' or (?=-) that matches an empty string. (Contributed by Serhiy Storchaka in 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 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 30349.)

Compiled regular expression and match objects can now be copied using copy.copy and copy.deepcopy. (Contributed by Serhiy Storchaka in 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 30050.)

socket

The new socket.getblocking() <socket.socket.getblocking> method returns True if the socket is in blocking mode and False otherwise. (Contributed by Yury Selivanov in 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 32454.)

The socket module now exposes the socket.TCP_CONGESTION <socket-unix-constants> (Linux 2.6.13), socket.TCP_USER_TIMEOUT <socket-unix-constants> (Linux 2.6.37), and socket.TCP_NOTSENT_LOWAT <socket-unix-constants> (Linux 3.12) constants. (Contributed by Omar Sandoval in 26273 and Nathaniel J. Smith in 29728.)

Support for socket.AF_VSOCK sockets has been added to allow communication between virtual machines and their hosts. (Contributed by Cathy Avery in 27584.)

Sockets now auto-detect family, type and protocol from file descriptor by default. (Contributed by Christian Heimes in 28134.)

socketserver

socketserver.ThreadingMixIn.server_close <socketserver.BaseServer.server_close> now waits until all non-daemon threads complete. socketserver.ForkingMixIn.server_close <socketserver.BaseServer.server_close> now waits until all child processes complete.

Add a new socketserver.ForkingMixIn.block_on_close <socketserver.ThreadingMixIn.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 ~sqlite3.Connection.backup method when the underlying SQLite library is at version 3.6.11 or higher. (Contributed by Lele Gaifax in 27645.)

The database argument of sqlite3.connect now accepts any path-like object, instead of just a string. (Contributed by Anders Lorentsen in 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 ~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 SSLContext.hostname_checks_common_name <ssl.SSLContext.hostname_checks_common_name>. (Contributed by Christian Heimes in 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 37-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 32185.)

!match_hostname no longer supports partial wildcards like www*.example.org. (Contributed by Mandeep Singh in 23033 and Christian Heimes in 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 31429.)

Validation of server certificates containing internationalized domain names (IDNs) is now supported. As part of this change, the SSLSocket.server_hostname <ssl.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 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 ssl-tlsv1_3. (Contributed by Christian Heimes in 32947, 20995, 29136, 30622 and 33618)

~ssl.SSLSocket and ~ssl.SSLObject no longer have a public constructor. Direct instantiation was never a documented and supported feature. Instances must be created with ~ssl.SSLContext methods ~ssl.SSLContext.wrap_socket and ~ssl.SSLContext.wrap_bio. (Contributed by Christian Heimes in 32951)

OpenSSL 1.1 APIs for setting the minimum and maximum TLS protocol version are available as SSLContext.minimum_version <ssl.SSLContext.minimum_version> and SSLContext.maximum_version <ssl.SSLContext.maximum_version>. Supported protocols are indicated by several new flags, such as ~ssl.HAS_TLSv1_1. (Contributed by Christian Heimes in 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 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 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 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 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 19764.)

The subprocess module is now more graceful when handling KeyboardInterrupt during subprocess.call, subprocess.run, or in a ~subprocess.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 25942.)

sys

The new sys.breakpointhook hook function is called by the built-in breakpoint. (Contributed by Barry Warsaw in 31353.)

On Android, the new sys.getandroidapilevel returns the build-time Android API version. (Contributed by Victor Stinner in 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 32591.)

time

564 adds six new functions with nanosecond resolution to the time module:

• time.clock_gettime_ns
• time.clock_settime_ns
• time.monotonic_ns
• time.perf_counter_ns
• time.process_time_ns
• time.time_ns

New clock identifiers have been added:

• time.CLOCK_BOOTTIME (Linux): Identical to time.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 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 32585.)

tracemalloc

tracemalloc.Traceback behaves more like regular tracebacks, sorting the frames from oldest to most recent. Traceback.format() <tracemalloc.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 32121.)

types

The new ~types.WrapperDescriptorType, ~types.MethodWrapperType, ~types.MethodDescriptorType, and ~types.ClassMethodDescriptorType classes are now available. (Contributed by Manuel Krebber and Guido van Rossum in 29377, and Serhiy Storchaka in 32265.)

The new types.resolve_bases function resolves MRO entries dynamically as specified by 560. (Contributed by Ivan Levkivskyi in 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 32071.)

unittest.mock

The ~unittest.mock.sentinel attributes now preserve their identity when they are copied <copy> or pickled <pickle>. (Contributed by Serhiy Storchaka in 20804.)

The new ~unittest.mock.seal function allows sealing ~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 30541.)

urllib.parse

urllib.parse.quote has been updated from 2396 to 3986, adding ~ to the set of characters that are never quoted by default. (Contributed by Christian Theune and Ratnadeep Debnath in 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 30103.)

uuid

The new 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 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 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 the sys.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 option
  • any filters specified with the PYTHONWARNINGS environment variable
  • any 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 <debug-build>, all warnings are now displayed by default (the implicit filter list is empty)

(Contributed by Nick Coghlan and Victor Stinner in 20361, 32043, and 32230.)

Deprecation warnings are once again shown by default in single-file scripts and at the interactive prompt. See whatsnew37-pep565 for details. (Contributed by Nick Coghlan in 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 61441.)

xml.etree

ElementPath <elementtree-xpath> 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 31648.)

xmlrpc.server

!SimpleXMLRPCDispatcher.register_function can now be used as a decorator. (Contributed by Xiang Zhang in 7769.)

zipapp

Function ~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 31072.)

Function ~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 31638.)

zipfile

~zipfile.ZipFile now accepts the new compresslevel parameter to control the compression level. (Contributed by Bo Bayles in 21417.)

Subdirectories in archives created by ZipFile are now stored in alphabetical order. (Contributed by Bernhard M. Wiedemann in 30693.)

C API Changes

A new API for thread-local storage has been implemented. See whatsnew37-pep539 for an overview and thread-specific-storage-api for a complete reference. (Contributed by Masayuki Yamamoto in 25658.)

The new context variables <whatsnew37-pep567> functionality exposes a number of new C APIs <contextvarsobjects>.

The new PyImport_GetModule function returns the previously imported module with the given name. (Contributed by Eric Snow in 28411.)

The new Py_RETURN_RICHCOMPARE macro eases writing rich comparison functions. (Contributed by Petr Victorin in 23699.)

The new Py_UNREACHABLE macro can be used to mark unreachable code paths. (Contributed by Barry Warsaw in 31338.)

The tracemalloc now exposes a C API through the new PyTraceMalloc_Track and PyTraceMalloc_Untrack functions. (Contributed by Victor Stinner in 30054.)

The new import__find__load__start <static-markers> and import__find__load__done <static-markers> static markers can be used to trace module imports. (Contributed by Christian Heimes in 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 28761.)

The result of PyUnicode_AsUTF8AndSize and PyUnicode_AsUTF8 is now of type const char * rather of char *. (Contributed by Serhiy Storchaka in 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 28280.)

Added functions PySlice_Unpack and PySlice_AdjustIndices. (Contributed by Serhiy Storchaka in 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 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 22898 and 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 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 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 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-37 section in this document and the pre-init-safe 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 29102.)

Py_DecodeLocale, Py_EncodeLocale now use the UTF-8 encoding when the UTF-8 mode <whatsnew37-pep540> is enabled. (Contributed by Victor Stinner in 29240.)

PyUnicode_DecodeLocaleAndSize and PyUnicode_EncodeLocale now use the current locale encoding for surrogateescape error handler. (Contributed by Victor Stinner in 29240.)

The start and end parameters of PyUnicode_FindChar are now adjusted to behave like string slices. (Contributed by Xiang Zhang in 28822.)

Build Changes

Support for building --without-threads has been removed. The threading module is now always available. (Contributed by Antoine Pitrou in 31370.).

A full copy of libffi is no longer bundled for use when building the _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 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 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 29300, 29507, 29452, and 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 29585, and Ivan Levkivskyi in 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 26110.)

• The asyncio.get_event_loop function has been reimplemented in C to make it up to 15 times faster. (Contributed by Yury Selivanov in 32296.)
• asyncio.Future callback management has been optimized. (Contributed by Yury Selivanov in 32348.)
• asyncio.gather is now up to 15% faster. (Contributed by Yury Selivanov in 32355.)
• asyncio.sleep is now up to 2 times faster when the delay argument is zero or negative. (Contributed by Andrew Svetlov in 32351.)
• The performance overhead of asyncio debug mode has been reduced. (Contributed by Antoine Pitrou in 31970.)

As a result of PEP 560 work <whatsnew37-pep560>, 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 32226.)

sorted and list.sort have been optimized for common cases to be up to 40-75% faster. (Contributed by Elliot Gorokhovsky in 28685.)

dict.copy is now up to 5.5 times faster. (Contributed by Yury Selivanov in 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 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 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 28638.)

datetime.date.fromordinal and datetime.date.fromtimestamp are now up to 30% faster in the common case. (Contributed by Paul Ganssle in 32403.)

The os.fwalk function is now up to 2 times faster thanks to the use of os.scandir. (Contributed by Serhiy Storchaka in 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 28564.)

Optimized case-insensitive matching and searching of regular expressions <re>. Searching some patterns can now be up to 20 times faster. (Contributed by Serhiy Storchaka in 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 31671.)

The ~selectors.BaseSelector.modify methods of classes selectors.EpollSelector, selectors.PollSelector and selectors.DevpollSelector may be around 10% faster under heavy loads. (Contributed by Giampaolo Rodola' in 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 29469 and 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 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 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 24700.)

The math.erf and math.erfc functions now use the (faster) C library implementation on most platforms. (Contributed by Serhiy Storchaka in 26121.)

Other CPython Implementation Changes

• Trace hooks may now opt out of receiving the line and opt into receiving the opcode events from the interpreter by setting the corresponding new ~frame.f_trace_lines and ~frame.f_trace_opcodes attributes on the frame being traced. (Contributed by Nick Coghlan in 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__.origin is also set to None (previously the string "namespace"). See 32305. Also, the namespace module object's __spec__.loader is set to the same value as __loader__ (previously, the former was set to None). See 32303.
• The locals dictionary now displays in the lexical order that variables were defined. Previously, the order was undefined. (Contributed by Raymond Hettinger in 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 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 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 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 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 32253.)

The !asyncio.Task.current_task and !asyncio.Task.all_tasks methods have been deprecated. (Contributed by Andrew Svetlov in 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 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 28847.)

enum

In Python 3.8, attempting to check for non-Enum objects in ~enum.Enum classes will raise a TypeError (e.g. 1 in Color); similarly, attempting to check for non-Flag objects in a ~enum.Flag member will raise TypeError (e.g. 1 in Perm.RW); currently, both operations return False instead. (Contributed by Ethan Furman in 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 28692.)

importlib

Methods !MetaPathFinder.find_module (replaced by MetaPathFinder.find_spec() <importlib.abc.MetaPathFinder.find_spec>) and !PathEntryFinder.find_loader (replaced by PathEntryFinder.find_spec() <importlib.abc.PathEntryFinder.find_spec>) both deprecated in Python 3.4 now emit DeprecationWarning. (Contributed by Matthias Bussonnier in 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 10379.)

macpath

The !macpath is now deprecated and will be removed in Python 3.8. (Contributed by Chi Hsuan Yen in 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 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 28332.)

ssl

!ssl.wrap_socket is deprecated. Use ssl.SSLContext.wrap_socket instead. (Contributed by Christian Heimes in 28124.)

sunau

!sunau.openfp has been deprecated and will be removed in Python 3.9. Use !sunau.open instead. (Contributed by Brian Curtin in 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 28799.)

wave

!wave.openfp has been deprecated and will be removed in Python 3.9. Use wave.open instead. (Contributed by Brian Curtin in 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 27867.)

PyOS_AfterFork has been deprecated. Use PyOS_BeforeFork, PyOS_AfterFork_Parent or PyOS_AfterFork_Child() instead. (Contributed by Antoine Pitrou in 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) or UTF-8 (LC_CTYPE-only locale) as an alternative to the legacy ASCII-based C 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 <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:

• 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 for re.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. Use ~os.path.splitdrive 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 28638.)
• Functions bool, float, list and tuple no longer take keyword arguments. The first argument of int can now be passed only as positional argument.
• Removed previously deprecated in Python 2.4 classes Plist, Dict and _InternalDict in the plistlib module. Dict values in the result of functions !readPlist and !readPlistFromBytes 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 the socket.socketpair function instead, it is available on all platforms since Python 3.5. asyncio.windows_utils.socketpair was just an alias to socket.socketpair on Python 3.5 and newer.
• asyncio no longer exports the selectors and !_overlapped modules as asyncio.selectors and asyncio._overlapped. Replace from asyncio import selectors with import selectors.
• Direct instantiation of ssl.SSLSocket and ssl.SSLObject objects is now prohibited. The constructors were never documented, tested, or designed as public constructors. Users were supposed to use !ssl.wrap_socket or ssl.SSLContext. (Contributed by Christian Heimes in 32951.)
• The unused distutils install_misc command has been removed. (Contributed by Eric N. Vander Weele in 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 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 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 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 and await names are now reserved keywords. Code using these names as identifiers will now raise a SyntaxError. (Contributed by Jelle Zijlstra in 30406.)

• 479 is enabled for all code in Python 3.7, meaning that StopIteration exceptions raised directly or indirectly in coroutines and generators are transformed into RuntimeError exceptions. (Contributed by Yury Selivanov in 32670.)

• object.__aiter__ methods can no longer be declared as asynchronous. (Contributed by Yury Selivanov in 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 32012 and 32023.)

• When using the -m switch, the initial working directory is now added to 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.getcwd() or 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

• socketserver.ThreadingMixIn.server_close <socketserver.BaseServer.server_close> now waits until all non-daemon threads complete. Set the new socketserver.ThreadingMixIn.block_on_close class attribute to False to get the pre-3.7 behaviour. (Contributed by Victor Stinner in 31233 and 33540.)

• socketserver.ForkingMixIn.server_close <socketserver.BaseServer.server_close> now waits until all child processes complete. Set the new socketserver.ForkingMixIn.block_on_close <socketserver.ThreadingMixIn.block_on_close> class attribute to False to get the pre-3.7 behaviour. (Contributed by Victor Stinner in 31151 and 33540.)

• The locale.localeconv function now temporarily sets the LC_CTYPE locale to the value of LC_NUMERIC in some cases. (Contributed by Victor Stinner in 31900.)

• pkgutil.walk_packages now raises a ValueError if path is a string. Previously an empty list was returned. (Contributed by Sanyam Khurana in 24744.)

• A format string argument for string.Formatter.format is now positional-only <positional-only_parameter>. Passing it as a keyword argument was deprecated in Python 3.5. (Contributed by Serhiy Storchaka in 29193.)

• Attributes ~http.cookies.Morsel.key, ~http.cookies.Morsel.value and ~http.cookies.Morsel.coded_value of class http.cookies.Morsel are now read-only. Assigning to them was deprecated in Python 3.5. Use the ~http.cookies.Morsel.set method for setting them. (Contributed by Serhiy Storchaka in 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 invoking makedirs(). (Contributed by Serhiy Storchaka in 19930.)

• The struct.Struct.format type is now str instead of bytes. (Contributed by Victor Stinner in 21071.)

• !cgi.parse_multipart now accepts the encoding and errors arguments and returns the same results as !FieldStorage: for non-file fields, the value associated to a key is a list of strings, not bytes. (Contributed by Pierre Quentel in 29979.)

• Due to internal changes in socket, calling socket.fromshare on a socket created by socket.share <socket.socket.share> in older Python versions is not supported.

• repr for BaseException has changed to not include the trailing comma. Most exceptions are affected by this change. (Contributed by Serhiy Storchaka in 30399.)

• repr for datetime.timedelta has changed to include the keyword arguments in the output. (Contributed by Utkarsh Upadhyay in 30302.)

• Because shutil.rmtree is now implemented using the os.scandir function, the user specified handler onerror is now called with the first argument os.scandir instead of os.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 30349.)

• The result of splitting a string on a 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 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*?$' will not only match empty strings at positions 2 and 3, but also the string '\n' at positions 2--3. To match only blank lines, the pattern should be rewritten as r'(?m)^[^\S\n]*$'.

  re.sub now replaces empty matches adjacent to a previous non-empty match. For example re.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 25054 and 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 29995.)

• tracemalloc.Traceback frames are now sorted from oldest to most recent to be more consistent with traceback. (Contributed by Jesse Bakker in 32121.)

• On OSes that support socket.SOCK_NONBLOCK or socket.SOCK_CLOEXEC bit flags, the socket.type <socket.socket.type> no longer has them applied. Therefore, checks like if sock.type == socket.SOCK_STREAM work as expected on all platforms. (Contributed by Yury Selivanov in 32331.)

• On Windows the default for the close_fds argument of subprocess.Popen was changed from False to True when redirecting the standard handles. If you previously depended on handles being inherited when using subprocess.Popen with standard io redirection, you will have to pass close_fds=False to preserve the previous behaviour, or use STARTUPINFO.lpAttributeList <subprocess.STARTUPINFO.lpAttributeList>.

• importlib.machinery.PathFinder.invalidate_caches -- which implicitly affects importlib.invalidate_caches -- now deletes entries in sys.path_importer_cache which are set to None. (Contributed by Brett Cannon in 33169.)

• In asyncio, loop.sock_recv() <asyncio.loop.sock_recv>, loop.sock_sendall() <asyncio.loop.sock_sendall>, loop.sock_accept() <asyncio.loop.sock_accept>, loop.getaddrinfo() <asyncio.loop.getaddrinfo>, loop.getnameinfo() <asyncio.loop.getnameinfo> have been changed to be proper coroutine methods to match their documentation. Previously, these methods returned asyncio.Future instances. (Contributed by Yury Selivanov in 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 32662.)

• Struct.format <struct.Struct.format> is now a str instance instead of a bytes instance. (Contributed by Victor Stinner in 21071.)

• argparse subparsers can now be made mandatory by passing required=True to ArgumentParser.add_subparsers() <argparse.ArgumentParser.add_subparsers>. (Contributed by Anthony Sottile in 26510.)

• ast.literal_eval is now stricter. Addition and subtraction of arbitrary numbers are no longer allowed. (Contributed by Serhiy Storchaka in 31778.)

• Calendar.itermonthdates <calendar.Calendar.itermonthdates> will now consistently raise an exception when a date falls outside of the 0001-01-01 through 9999-12-31 range. To support applications that cannot tolerate such exceptions, the new Calendar.itermonthdays3 <calendar.Calendar.itermonthdays3> and Calendar.itermonthdays4 <calendar.Calendar.itermonthdays4> can be used. The new methods return tuples and are not restricted by the range supported by datetime.date. (Contributed by Alexander Belopolsky in 28292.)

• collections.ChainMap now preserves the order of the underlying mappings. (Contributed by Raymond Hettinger in 32792.)

• The submit() method of concurrent.futures.ThreadPoolExecutor and concurrent.futures.ProcessPoolExecutor now raises a RuntimeError if called during interpreter shutdown. (Contributed by Mark Nemec in 33097.)

• The 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 23835.)

• Several undocumented internal imports were removed. One example is that os.errno is no longer available; use import 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 27867.)

CPython bytecode changes

There are two new opcodes: !LOAD_METHOD and !CALL_METHOD. (Contributed by Yury Selivanov and INADA Naoki in 26110.)

The !STORE_ANNOTATION opcode has been removed. (Contributed by Mark Shannon in 32550.)

Windows-only Changes

The file used to override sys.path is now called <python-executable>._pth instead of 'sys.path'. See windows_finding_modules for more information. (Contributed by Steve Dower in 28137.)

Other CPython implementation changes

In preparation for potential future changes to the public CPython runtime initialization API (see 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 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 calling Py_Initialize. Use !PySys_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 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 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 <contextvarsobjects_pointertype_change> to use PyObject pointers. See also 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 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 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 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 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 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 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 <int_max_str_digits> documentation. The default limit is 4300 digits in string form.