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.

• 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`.)

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:`~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`.)

:class:`string.Template` now lets you to optionally modify the regular expression pattern for braced placeholders and non-braced placeholders separately. (Contributed by Barry Warsaw in :issue:`1198569`.)

The :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`.)

: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`.)

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.

• 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 to 25 times slower than searching other characters. Now it is slower only by 3 times in worst case. (Contributed by Serhiy Storchaka in :issue:`24821`.)
• Fast implementation from standard C library is now used for functions :func:`~math.tgamma`, :func:`~math.lgamma`, :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`.)
• 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`.)
• :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`.)
• 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`.)
• 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`.)

• 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`.)

• 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.

• :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`.)
• PyInterpreterState no longer has a modules field. Instead use sys.modules.
• 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`.)

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