**************************** 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 `. .. 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 ` 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 ` 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 563: Postponed evaluation of annotations -------------------------------------------- The advent of type hints in Python uncovered two glaring usability issues with the functionality of annotations added in :pep:`3107` and refined further in :pep:`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 can be enabled on a per-module basis in Python 3.7 using a ``__future__`` import, like this:: from __future__ import annotations It will become the default in Python 4.0. .. seealso:: :pep:`563` -- Postponed evaluation of annotations PEP written and implemented by Łukasz Langa. PEP 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 .. _whatsnew37-pep557: PEP 557: Data Classes --------------------- Adds a new module ``dataclasses``. It provides a class decorator ``dataclass`` which inspects the class's variable annotations (see :pep:`526`) and using them, adds methods such as ``__init__``, ``__repr__``, and ``__eq__`` to the class. It is similar to ``typing.NamedTuple``, but also works on classes with mutable instances, among other features. For 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)" .. seealso:: :pep:`557` -- Data Classes PEP written and implemented by Eric V. Smith 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 `_ 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`.) * In order to better support dynamic creation of stack traces, :class:`types.TracebackType` can now be instantiated from Python code, and the ``tb_next`` attribute on :ref:`tracebacks ` is now writable. (Contributed by Nathaniel J. Smith in :issue:`30579`.) 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 provide a :meth:`get_resource_reader()` function which returns a :class:`importlib.abc.ResourceReader` instance to support this new API. Built-in file path loaders and zip file loaders both support this. (see the PyPI package `importlib_resources `_ as a compatible back port for older Python versions). 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`.) :class:`contextlib.AsyncExitStack` has been added. (Contributed by Alexander Mohr and Ilya Kulakov in :issue:`29302`.) 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`.) The ``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 :issue:`32304`.) 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`.) hmac ---- The hmac module now has an optimized one-shot :func:`~hmac.digest` function, which is up to three times faster than :func:`~hmac.HMAC`. (Contributed by Christian Heimes in :issue:`32433`.) importlib --------- The :class:`importlib.abc.ResourceReader` ABC was introduced to support the loading of resource from packages. 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 ` 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`.) py_compile ---------- :func:`py_compile.compile` -- and by extension, :mod:`compileall` -- now respects the :envvar:`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 :issue:`29708`.) 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`.) ssl --- The ssl module now uses OpenSSL's builtin API instead of :func:`~ssl.match_hostname` to check host name or IP address. Values are validated during TLS handshake. Any cert validation error including a failing host name match now raises :exc:`~ssl.SSLCertVerificationError` and aborts the handshake with a proper TLS Alert message. The new exception contains additional information. Host name validation can be customized with :attr:`~ssl.SSLContext.host_flags`. (Contributed by Christian Heimes in :issue:`31399`.) .. note:: The improved host name check requires an OpenSSL 1.0.2 or 1.1 compatible libssl. OpenSSL 0.9.8 and 1.0.1 are no longer supported. LibreSSL is temporarily not supported until it gains the necessary OpenSSL 1.0.2 APIs. The ssl module no longer sends IP addresses in SNI TLS extension. (Contributed by Christian Heimes in :issue:`32185`.) :func:`~ssl.match_hostname` no longer supports partial wildcards like ``www*.example.org``. :attr:`~ssl.SSLContext.host_flags` has partial wildcard matching disabled by default. (Contributed by Mandeep Singh in :issue:`23033` and Christian Heimes in :issue:`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 update. Default cipher suite selection can be configured on compile time. (Contributed by Christian Heimes in :issue:`31429`.) 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. Deprecated :func:`sys.set_coroutine_wrapper` and :func:`sys.get_coroutine_wrapper`. tkinter ------- Added :class:`tkinter.ttk.Spinbox`. 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`.) unicodedata ----------- The internal :mod:`unicodedata` database has been upgraded to use `Unicode 10 `_. (Contributed by Benjamin Peterson.) 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 ` or :mod:`pickled `. (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`.) 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 ` 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`.) 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`.) 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 `. 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 ======================= * :mod:`py_compile` and :mod:`compileall` now support the :envvar:`SOURCE_DATE_EPOCH` environment variable by unconditionally building ``.pyc`` files for hash verification instead of potentially timestamp-based ``.pyc`` files. See the notes for the `py_compile`_ improvement notes for more details. * A full copy of libffi is no longer bundled for use when building the :mod:`_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 ========== * In Python 3.8, the abstract base classes in :mod:`collections.abc` will no longer be exposed in the regular :mod:`collections` module. This will help create a clearer distinction between the concrete classes and the abstract base classes. * 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() ` (replaced by :meth:`MetaPathFinder.find_spec() ` ) and :meth:`PathEntryFinder.find_loader() ` (replaced by :meth:`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 :class:`importlib.abc.ResourceLoader` ABC has been deprecated in favour of :class:`importlib.abc.ResourceReader`. 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 asterisk*. Running ``py -0p`` will include the paths. If py is run with a version specifier that cannot be matched it will also print the *short form* list of available specifiers. (Contributed by Steve Barnes in :issue:`30362`.) Removed ======= Platform Support Removals ------------------------- * FreeBSD 9 and older are no longer supported. 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 -------------------------- * :pep:`479` is enabled for all code in Python 3.7, meaning that :exc:`StopIteration` exceptions raised directly or indirectly in coroutines and generators are transformed into :exc:`RuntimeError` exceptions. (Contributed by Yury Selivanov in :issue:`32670`.) * Due to an oversight, earlier Python versions erroneously accepted the following syntax:: f(1 for x in [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 ------------------------- * :meth:`socketserver.ThreadingMixIn.server_close` now waits until all non-daemon threads complete. Use daemonic threads by setting :data:`ThreadingMixIn.daemon_threads` to ``True`` to not wait until threads complete. (Contributed by Victor Stinner in :issue:`31233`.) * :meth:`socketserver.ForkingMixIn.server_close` now waits until all child processes complete. (Contributed by Victor Stinner in :issue:`31151`.) * The :func:`locale.localeconv` function now sets temporarily the ``LC_CTYPE`` locale to the ``LC_NUMERIC`` locale in some cases. * 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 `. 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 ` 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 ` 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 `. 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`.) * Removed the STORE_ANNOTATION opcode. 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. * CPython' :mod:`ssl` module requires OpenSSL 1.0.2 or 1.1 compatible libssl. OpenSSL 1.0.1 has reached end of lifetime on 2016-12-31 and is no longer supported. LibreSSL is temporarily not supported as well. LibreSSL releases up to version 2.6.4 are missing required OpenSSL 1.0.2 APIs. 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`.)