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

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

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.

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

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.

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

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.

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 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.

• 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`.)

• None yet.

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`.)

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`.)

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`.)

: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`.)

:func:`contextlib.asynccontextmanager` has been added. (Contributed by Jelle Zijlstra in :issue:`29679`.)

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

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`.)

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`.)

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`.)

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

: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`.)

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`.)

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

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`.)

: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`.)

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`.)

: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`.)

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

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`.)

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.

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`.)

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

The internal :mod:`unicodedata` database has been upgraded to use Unicode 10. (Contributed by Benjamin Peterson.)

: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`.)

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`.)

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`)

: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`.)

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.

• 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`)

• 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`.).

• 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`.)

• 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.

• 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`.)

• 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`.)

• 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.

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

• 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`.)

• 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 between any pair of non-whitespace characters. 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*?$' will not only match empty strings at positions 2 and 3, but also the string '\n' at positions 2--3. To match only blank lines, the pattern should be rewritten as r'(?m)^[^\S\n]*$'.

  (Contributed by Serhiy Storchaka in :issue:`25054`.)

• :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`.)

• 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`.)

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

• 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.

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`.)