****************************
  What's New In Python 3.7
****************************

:Release: |release|
:Date: |today|

.. Rules for maintenance:

   * Anyone can add text to this document.  Do not spend very much time
   on the wording of your changes, because your text will probably
   get rewritten to some degree.

   * The maintainer will go through Misc/NEWS periodically and add
   changes; it's therefore more important to add your changes to
   Misc/NEWS than to this file.

   * This is not a complete list of every single change; completeness
   is the purpose of Misc/NEWS.  Some changes I consider too small
   or esoteric to include.  If such a change is added to the text,
   I'll just remove it.  (This is another reason you shouldn't spend
   too much time on writing your addition.)

   * If you want to draw your new text to the attention of the
   maintainer, add 'XXX' to the beginning of the paragraph or
   section.

   * It's OK to just add a fragmentary note about a change.  For
   example: "XXX Describe the transmogrify() function added to the
   socket module."  The maintainer will research the change and
   write the necessary text.

   * You can comment out your additions if you like, but it's not
   necessary (especially when a final release is some months away).

   * Credit the author of a patch or bugfix.   Just the name is
   sufficient; the e-mail address isn't necessary.

   * It's helpful to add the bug/patch number as a comment:

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

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

This article explains the new features in Python 3.7, compared to 3.6.

For full details, see the :ref:`changelog <changelog>`.

.. note::

   Prerelease users should be aware that this document is currently in draft
   form. It will be updated substantially as Python 3.7 moves towards release,
   so it's worth checking back even after reading earlier versions.


Summary -- Release highlights
=============================

.. This section singles out the most important changes in Python 3.7.
   Brevity is key.


.. PEP-sized items next.



New Features
============


.. _whatsnew37-pep538:

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 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 ``stdin`` and ``stdout``
is now ``surrogateescape`` (rather than ``strict``) when using any of the
defined coercion target locales (currently ``C.UTF-8``, ``C.utf8``, and
``UTF-8``). The default error handler for ``stderr`` continues to be
``backslashreplace``, regardless of locale.

Locale coercion is silent by default, but to assist in debugging potentially
locale related integration problems, explicit warnings (emitted directly on
``stderr`` can be requested by setting ``PYTHONCOERCECLOCALE=warn``. This
setting will also cause the Python runtime to emit a warning if the legacy C
locale remains active when the core interpreter is initialized.

.. seealso::

    :pep:`538` -- Coercing the legacy C locale to a UTF-8 based locale
       PEP written and implemented by Nick Coghlan.


.. _whatsnew37-pep553:

PEP 553: Built-in breakpoint()
------------------------------

:pep:`553` describes a new built-in called ``breakpoint()`` which makes it
easy and consistent to enter the Python debugger.  Built-in ``breakpoint()``
calls ``sys.breakpointhook()``.  By default, this 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.  Or, the
environment variable :envvar:`PYTHONBREAKPOINT` can be set to the callable of
your debugger of choice.  Set ``PYTHONBREAKPOINT=0`` to completely disable
built-in ``breakpoint()``.

.. seealso::

    :pep:`553` -- Built-in breakpoint()
      PEP written and implemented by Barry Warsaw


.. _whatsnew37-pep539:

PEP 539: A New C-API for Thread-Local Storage in CPython
--------------------------------------------------------

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
-----------------------------------------------------

It is sometimes convenient to customize or otherwise have control over access
to module attributes. A typical example is managing deprecation warnings.
Typical workarounds are assigning ``__class__`` of a module object to
a custom subclass of :class:`types.ModuleType` or replacing the ``sys.modules``
item with a custom wrapper instance. This procedure is now simplified by
recognizing ``__getattr__`` defined directly in a module that would act like
a normal ``__getattr__`` method, except that it will be defined on module
*instances*.

.. seealso::

    :pep:`562` -- Module ``__getattr__`` and ``__dir__``
       PEP written and implemented by Ivan Levkivskyi


PEP 564: Add new time functions with nanosecond resolution
----------------------------------------------------------

Add six new "nanosecond" variants of existing functions to the :mod:`time`
module:

* :func:`time.clock_gettime_ns`
* :func:`time.clock_settime_ns`
* :func:`time.monotonic_ns`
* :func:`time.perf_counter_ns`
* :func:`time.process_time_ns`
* :func:`time.time_ns`

While similar to the existing functions without the ``_ns`` suffix, they
provide nanosecond resolution: they return a number of nanoseconds as a Python
``int``.

The ``time.time_ns()`` resolution is 3 times better than the ``time.time()``
resolution on Linux and Windows.

.. seealso::

    :pep:`564` -- Add new time functions with nanosecond resolution
       PEP written and implemented by Victor Stinner


.. _whatsnew37-pep565:

PEP 565: Show DeprecationWarning in ``__main__``
------------------------------------------------

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 ``__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 ``__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 540: Add a new UTF-8 mode
-----------------------------

Add a new UTF-8 mode to ignore the locale, use the UTF-8 encoding, and change
:data:`sys.stdin` and :data:`sys.stdout` error handlers to ``surrogateescape``.
This mode is enabled by default in the POSIX locale, but otherwise disabled by
default.

The new :option:`-X` ``utf8`` command line option and :envvar:`PYTHONUTF8`
environment variable are added to control the UTF-8 mode.

.. seealso::

    :pep:`540` -- Add a new UTF-8 mode
       PEP written and implemented by Victor Stinner


New Development Mode: -X dev
----------------------------

Add a new "development mode": :option:`-X` ``dev`` command line option and
:envvar:`PYTHONDEVMODE` environment variable to enable CPython's "development
mode", introducing additional runtime checks which are too expensive to be
enabled by default. See :option:`-X` ``dev`` documentation for the effects of
the development mode.

Hash-based pycs
---------------

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 reproduciblity <https://reproducible-builds.org/>`_ and
content-based build systems.

:pep:`552` extends the pyc format to allow the hash of the source file to be
used for invalidation instead of the source timestamp. Such ``.pyc`` files are
called "hash-based". By default, Python still uses timestamp-based invalidation
and does not generate hash-based ``.pyc`` files at runtime. Hash-based ``.pyc``
files may be generated with :mod:`py_compile` or :mod:`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 :ref:`pyc-invalidation` for more information.


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`.)

* :exc:`ImportError` now displays module name and module ``__file__`` path when
  ``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 ``str(x)`` rather than
  ``format(str(self), '')``.
  (Contributed by Serhiy Storchaka in :issue:`28974`.)


New Modules
===========

importlib.resources
-------------------

This module provides several new APIs and one new ABC for access to, opening,
and reading *resources* inside packages.  Resources are roughly akin to files
inside of packages, but they needn't be actual files on the physical file
system.  Module loaders can implement the
:class:`importlib.abc.ResourceReader` ABC to support this new module's API.


Improved Modules
================


argparse
--------

The :meth:`~argparse.ArgumentParser.parse_intermixed_args` supports letting
the user intermix options and positional arguments on the command line,
as is possible in many unix commands.  It supports most but not all
argparse features.  (Contributed by paul.j3 in :issue:`14191`.)


binascii
--------

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`.)


calendar
--------

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

cgi
---

:func:`~cgi.parse_multipart` 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`.)

contextlib
----------

:func:`~contextlib.asynccontextmanager` and
:class:`~contextlib.AbstractAsyncContextManager` have been added. (Contributed
by Jelle Zijlstra in :issue:`29679` and :issue:`30241`.)

cProfile
--------

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

crypt
-----

Added support for the Blowfish method.
(Contributed by Serhiy Storchaka in :issue:`31664`.)

The :func:`~crypt.mksalt` function now allows to specify the number of rounds
for hashing.  (Contributed by Serhiy Storchaka in :issue:`31702`.)

dis
---

The :func:`~dis.dis` function now is able to
disassemble nested code objects (the code of comprehensions, generator
expressions and nested functions, and the code used for building nested
classes).  (Contributed by Serhiy Storchaka in :issue:`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 :issue:`11913`.)

:class:`distutils.core.setup` now warns if the ``classifiers``, ``keywords``
and ``platforms`` fields are not specified as a list or a string.
(Contributed by Berker Peksag in :issue:`19610`.)

http.client
-----------

Add Configurable *blocksize* to ``HTTPConnection`` and
``HTTPSConnection`` for improved upload throughput.
(Contributed by Nir Soffer in :issue:`31945`.)

http.server
-----------

:class:`~http.server.SimpleHTTPRequestHandler` 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`.)

Add the parameter ``directory`` to the :class:`~http.server.SimpleHTTPRequestHandler`
and the ``--directory`` to the command line of the module :mod:`~http.server`.
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`.)

locale
------

Added another argument *monetary* in :meth:`format_string` of :mod:`locale`.
If *monetary* is true, the conversion uses monetary thousands separator and
grouping strings. (Contributed by Garvit in :issue:`10379`.)

The :func:`locale.getpreferredencoding` function now always returns ``'UTF-8'``
on Android or in the UTF-8 mode  (:option:`-X` ``utf8`` option), the locale and
the *do_setlocale* argument are ignored.

math
----

New :func:`~math.remainder` function, implementing the IEEE 754-style remainder
operation. (Contributed by Mark Dickinson in :issue:`29962`.)

os
--

Added support for :class:`bytes` paths in :func:`~os.fwalk`. (Contributed by
Serhiy Storchaka in :issue:`28682`.)

Added support for :ref:`file descriptors <path_fd>` in :func:`~os.scandir`
on Unix.  (Contributed by Serhiy Storchaka in :issue:`25996`.)

New function :func:`os.register_at_fork` allows registering Python callbacks
to be executed on a process fork.  (Contributed by Antoine Pitrou in
:issue:`16500`.)

pdb
---

:func:`~pdb.set_trace` now takes an optional *header* keyword-only
argument.  If given, this is printed to the console just before debugging
begins.  (Contributed by Barry Warsaw in :issue:`31389`.)

pdb command line now accepts `-m module_name` as an alternative to
script file. (Contributed by Mario Corchero in :issue:`32206`.)


re
--

The flags :const:`re.ASCII`, :const:`re.LOCALE` and :const:`re.UNICODE`
can be set within the scope of a group.
(Contributed by Serhiy Storchaka in :issue:`31690`.)

:func:`re.split` now supports splitting on a pattern like ``r'\b'``,
``'^$'`` or ``(?=-)`` that matches an empty string.
(Contributed by Serhiy Storchaka in :issue:`25054`.)

string
------

: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`.)

subprocess
----------

On Windows the default for *close_fds* was changed from :const:`False` to
:const:`True` when redirecting the standard handles. It's now possible to set
*close_fds* to :const:`True` when redirecting the standard handles. See
:class:`subprocess.Popen`.

This means that *close_fds* now defaults to :const:`True` on all supported
platforms.

sys
---

Added :attr:`sys.flags.dev_mode` flag for the new development mode.

time
----

The :pep:`564` added six new functions with nanosecond resolution:

* :func:`time.clock_gettime_ns`
* :func:`time.clock_settime_ns`
* :func:`time.monotonic_ns`
* :func:`time.perf_counter_ns`
* :func:`time.process_time_ns`
* :func:`time.time_ns`

Add new clock identifiers:

* :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 timer from the CPU.
* :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, both absolute and interval.

Added functions :func:`time.thread_time` and :func:`time.thread_time_ns`
to get per-thread CPU time measurements.
(Contributed by Antoine Pitrou in :issue:`32025`.)


unittest
--------
Added new command-line option ``-k`` to filter tests to run with a substring or
Unix shell-like pattern.  For example, ``python -m unittest -k foo`` runs the
tests ``foo_tests.SomeTest.test_something``, ``bar_tests.SomeTest.test_foo``,
but not ``bar_tests.FooTest.test_something``.


unittest.mock
-------------

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

New function :const:`~unittest.mock.seal` will disable the creation of mock
children by preventing to get or set any new attribute on the sealed mock.
The sealing process is performed recursively. (Contributed by Mario Corchero
in :issue:`30541`.)

xmlrpc.server
-------------

:meth:`register_function` of :class:`xmlrpc.server.SimpleXMLRPCDispatcher` and
its subclasses can be used as a decorator. (Contributed by Xiang Zhang in
:issue:`7769`.)

unicodedata
-----------

The internal :mod:`unicodedata` database has been upgraded to use `Unicode 10
<http://www.unicode.org/versions/Unicode10.0.0/>`_. (Contributed by Benjamin
Peterson.)

urllib.parse
------------

:func:`urllib.parse.quote` has been updated from RFC 2396 to RFC 3986,
adding `~` to the set of characters that is never quoted by default.
(Contributed by Christian Theune and Ratnadeep Debnath in :issue:`16285`.)

uu
--

Function :func:`~uu.encode` 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`.)

warnings
--------

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 ``-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 precedence order:

     * the ``BytesWarning`` filter for :option:`-b` (or ``-bb``)
     * any filters specified with :option:`-W`
     * any filters specified with :envvar:`PYTHONWARNINGS`
     * 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, all warnings are now displayed by default (the
  implicit filter list is empty)

(Contributed by Nick Coghlan and Victor Stinner in :issue:`20361`,
:issue:`32043`, and :issue:`32230`)

xml.etree
---------

: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`.)

zipapp
------

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, and an optional *compressed* argument to generate a compressed
archive.

A command line option ``--compress`` has also been added to support
compression.


Optimizations
=============

* Added two new opcodes: ``LOAD_METHOD`` and ``CALL_METHOD`` to avoid
  instantiation of bound method objects for method calls, which results
  in method calls being faster up to 20%. (Contributed by Yury Selivanov and
  INADA Naoki in :issue:`26110`.)

* Searching some unlucky Unicode characters (like Ukrainian capital "Є")
  in a string was up to 25 times slower than searching other characters.
  Now it is slower only by 3 times in the worst case.
  (Contributed by Serhiy Storchaka in :issue:`24821`.)

* Fast implementation from standard C library is now used for functions
  :func:`~math.erf` and :func:`~math.erfc` in the :mod:`math` module.
  (Contributed by Serhiy Storchaka in :issue:`26121`.)

* The :func:`os.fwalk` function has been sped up by 2 times.  This was done
  using the :func:`os.scandir` function.
  (Contributed by Serhiy Storchaka in :issue:`25996`.)

* The :func:`shutil.rmtree` function has been sped up to 20--40%.
  This was done using the :func:`os.scandir` function.
  (Contributed by Serhiy Storchaka in :issue:`28564`.)

* Optimized case-insensitive matching and searching of :mod:`regular
  expressions <re>`.  Searching some patterns can now be up to 20 times faster.
  (Contributed by Serhiy Storchaka in :issue:`30285`.)

* :func:`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 :issue:`31671`.)

* :meth:`selectors.EpollSelector.modify`, :meth:`selectors.PollSelector.modify`
  and :meth:`selectors.DevpollSelector.modify` may be around 10% faster under
  heavy loads. (Contributed by Giampaolo Rodola' in :issue:`30014`)

* Constant folding is moved from peephole optimizer to new AST optimizer.
  (Contributed by Eugene Toder and INADA Naoki in :issue:`29469`)

Build and C API Changes
=======================

* 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 fields :c:member:`name` and :c:member:`doc` of structures
  :c:type:`PyMemberDef`, :c:type:`PyGetSetDef`,
  :c:type:`PyStructSequence_Field`, :c:type:`PyStructSequence_Desc`,
  and :c:type:`wrapperbase` are now of type ``const char *`` rather of
  ``char *``.  (Contributed by Serhiy Storchaka in :issue:`28761`.)

* The result of :c:func:`PyUnicode_AsUTF8AndSize` and :c:func:`PyUnicode_AsUTF8`
  is now of type ``const char *`` rather of ``char *``. (Contributed by Serhiy
  Storchaka in :issue:`28769`.)

* The result of :c:func:`PyMapping_Keys`, :c:func:`PyMapping_Values` and
  :c:func:`PyMapping_Items` is now always a list, rather than a list or a
  tuple. (Contributed by Oren Milman in :issue:`28280`.)

* Added functions :c:func:`PySlice_Unpack` and :c:func:`PySlice_AdjustIndices`.
  (Contributed by Serhiy Storchaka in :issue:`27867`.)

* :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`.)

* 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 ``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`.

* Support for building ``--without-threads`` is removed.
  (Contributed by Antoine Pitrou in :issue:`31370`.).


Other CPython Implementation Changes
====================================

* Trace hooks may now opt out of receiving ``line`` events from the interpreter
  by setting the new ``f_trace_lines`` attribute to :const:`False` on the frame
  being traced. (Contributed by Nick Coghlan in :issue:`31344`.)

* Trace hooks may now opt in to receiving ``opcode`` events from the interpreter
  by setting the new ``f_trace_opcodes`` attribute to :const:`True` on the frame
  being traced. (Contributed by Nick Coghlan in :issue:`31344`.)


Deprecated
==========

* Yield expressions (both ``yield`` and ``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+ they will emit :exc:`SyntaxError`. (Contributed by Serhiy
  Storchaka in :issue:`10544`.)

- Function :c:func:`PySlice_GetIndicesEx` is deprecated and replaced with
  a macro if ``Py_LIMITED_API`` is not set or set to the value between
  ``0x03050400`` and ``0x03060000`` (not including) or ``0x03060100`` or
  higher.  (Contributed by Serhiy Storchaka in :issue:`27867`.)

- Deprecated :meth:`format` from :mod:`locale`, use the :meth:`format_string`
  instead. (Contributed by Garvit in :issue:`10379`.)

- 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`)

- Using non-integer value for selecting a plural form in :mod:`gettext` is
  now deprecated.  It never correctly worked. (Contributed by Serhiy Storchaka
  in :issue:`28692`.)

- The :mod:`macpath` is now deprecated and will be removed in Python 3.8.


Changes in the C API
--------------------

- 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`.)


Windows Only
------------
- 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 :issue:`30291`.)

- The launcher can be run as "py -0" to produce a list of the installed pythons,
  *with default marked with an asterix*. 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`.)

Removed
=======

API and Feature Removals
------------------------

* 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 :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.

* The ``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
  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 :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 ``Plist``, ``Dict`` and
  ``_InternalDict`` in 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.


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
--------------------------

* 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 :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`.)


Changes in the Python API
-------------------------

* The ``asyncio.windows_utils.socketpair()`` function has been
  removed: use directly :func:`socket.socketpair` which is available on all
  platforms since Python 3.5 (before, it wasn't available on Windows).
  ``asyncio.windows_utils.socketpair()`` was just an alias to
  ``socket.socketpair`` on Python 3.5 and newer.

* :mod:`asyncio`: The module doesn't export :mod:`selectors` and
  :mod:`_overlapped` modules as ``asyncio.selectors`` and
  ``asyncio._overlapped``. Replace ``from asyncio import selectors`` with
  ``import selectors`` for example.

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

* 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`.)

* ``Module``, ``FunctionDef``, ``AsyncFunctionDef``, and
  ``ClassDef`` AST nodes now have a new ``docstring`` field.
  The first statement in their body is not considered as a docstring
  anymore.  ``co_firstlineno`` and ``co_lnotab`` of code object for class
  and module are affected by this change. (Contributed by INADA Naoki and
  Eugene Toder in :issue:`29463`.)

* 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`.)

* The :attr:`struct.Struct.format` type is now :class:`str` instead of
  :class:`bytes`. (Contributed by Victor Stinner in :issue:`21071`.)

* Due to internal changes in :mod:`socket` you won't be able to
  :func:`socket.fromshare` a socket :func:`~socket.socket.share`-ed in older
  Python versions.

* ``repr`` for :class:`datetime.timedelta` has changed to include keyword arguments
  in the output. (Contributed by Utkarsh Upadhyay in :issue:`30302`.)

* 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.scandir`` instead of ``os.listdir`` when listing the direcory
  is failed.

* Support of 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, so to facilitate this 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 ``'--'``, ``'&&'``, ``'~~'``, and ``'||'``.  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*?$'`` will not only
  match empty strings at positions 2 and 3, but also the string ``'\n'`` at
  positions 2--3.  To match only blank lines, the pattern should be rewritten
  as ``r'(?m)^[^\S\n]*$'``.

  :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-'`` 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 :issue:`25054` and :issue:`32308`.)

* :class:`tracemalloc.Traceback` frames are now sorted from oldest to most
  recent to be more consistent with :mod:`traceback`.
  (Contributed by Jesse Bakker in :issue:`32121`.)

* 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_STREAM``
  work as expected on all platforms.
  (Contributed by Yury Selivanov in :issue:`32331`.)

.. _Unicode Technical Standard #18: https://unicode.org/reports/tr18/

* 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=False`` to preserve the
  previous behaviour, or use
  :attr:`STARTUPINFO.lpAttributeList <subprocess.STARTUPINFO.lpAttributeList>`.


Changes in the C API
--------------------

* The function :c:func:`PySlice_GetIndicesEx` is considered not safe 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
------------------------

* Added two new opcodes: :opcode:`LOAD_METHOD` and :opcode:`CALL_METHOD`.
  (Contributed by Yury Selivanov and INADA Naoki in :issue:`26110`.)


Other CPython implementation changes
------------------------------------

* In preparation for potential future changes to the public CPython runtime
  initialization API (see :pep:`432` for details), 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.
  (Contributed by Nick Coghlan and Eric Snow as part of :issue:`22257`.)

* 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.

Documentation
=============

.. _whatsnew37-pep545:

PEP 545: Python Documentation Translations
------------------------------------------

:pep:`545` describes the process to translate Python documentation,
and two translations have been added:

- Japanese: https://docs.python.org/ja/ and associated GitHub
  repository: https://github.com/python/python-docs-ja

- French: https://docs.python.org/fr/ and associated GitHub
  repository: https://github.com/python/python-docs-fr

(Contributed by Julien Palard, Inada Naoki, and Victor Stinner in
:issue:`26546`.)