**************************** What's New In Python 3.8 **************************** .. 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 Git log when researching a change. :Editor: Raymond Hettinger This article explains the new features in Python 3.8, compared to 3.7. For full details, see the :ref:`changelog `. Prerelease users should be aware that this document is currently in draft form. It will be updated as Python 3.8 moves towards release, so it's worth checking back even after reading earlier versions. Some notable items not yet covered are: * :pep:`578` - Runtime audit hooks for potentially sensitive operations * ``python -m asyncio`` runs a natively async REPL .. testsetup:: from datetime import date from math import cos, radians from unicodedata import normalize import re import math Summary -- Release highlights ============================= .. This section singles out the most important changes in Python 3.8. Brevity is key. .. PEP-sized items next. New Features ============ Assignment expressions ---------------------- There is new syntax ``:=`` that assigns values to variables as part of a larger expression. It is affectionately known as "the walrus operator" due to its resemblance to `the eyes and tusks of a walrus `_. In this example, the assignment expression helps avoid calling :func:`len` twice:: if (n := len(a)) > 10: print(f"List is too long ({n} elements, expected <= 10)") A similar benefit arises during regular expression matching where match objects are needed twice, once to test whether a match occurred and another to extract a subgroup:: discount = 0.0 if (mo := re.search(r'(\d+)% discount', advertisement)): discount = float(mo.group(1)) / 100.0 The operator is also useful with while-loops that compute a value to test loop termination and then need that same value again in the body of the loop:: # Loop over fixed length blocks while (block := f.read(256)) != '': process(block) Another motivating use case arises in list comprehensions where a value computed in a filtering condition is also needed in the expression body:: [clean_name.title() for name in names if (clean_name := normalize('NFC', name)) in allowed_names] Try to limit use of the walrus operator to clean cases that reduce complexity and improve readability. See :pep:`572` for a full description. (Contributed by Emily Morehouse in :issue:`35224`.) Positional-only parameters -------------------------- There is a new function parameter syntax ``/`` to indicate that some function parameters must be specified positionally and cannot be used as keyword arguments. This is the same notation shown by ``help()`` for C functions annotated with Larry Hastings' `Argument Clinic `_ tool. In the following example, parameters *a* and *b* are positional-only, while *c* or *d* can be positional or keyword, and *e* or *f* are required to be keywords:: def f(a, b, /, c, d, *, e, f): print(a, b, c, d, e, f) The following is a valid call:: f(10, 20, 30, d=40, e=50, f=60) However, these are invalid calls:: f(10, b=20, c=30, d=40, e=50, f=60) # b cannot be a keyword argument f(10, 20, 30, 40, 50, f=60) # e must be a keyword argument One use case for this notation is that it allows pure Python functions to fully emulate behaviors of existing C coded functions. For example, the built-in :func:`pow` function does not accept keyword arguments:: def pow(x, y, z=None, /): "Emulate the built in pow() function" r = x ** y return r if z is None else r%z Another use case is to preclude keyword arguments when the parameter name is not helpful. For example, the builtin :func:`len` function has the signature ``len(obj, /)``. This precludes awkward calls such as:: len(obj='hello') # The "obj" keyword argument impairs readability A further benefit of marking a parameter as positional-only is that it allows the parameter name to be changed in the future without risk of breaking client code. For example, in the :mod:`statistics` module, the parameter name *dist* may be changed in the future. This was made possible with the following function specification:: def quantiles(dist, /, *, n=4, method='exclusive') ... Since the parameters to the left of ``/`` are not exposed as possible keywords, the parameters names remain available for use in ``**kwargs``:: >>> def f(a, b, /, **kwargs): ... print(a, b, kwargs) ... >>> f(10, 20, a=1, b=2, c=3) # a and b are used in two ways 10 20 {'a': 1, 'b': 2, 'c': 3} This greatly simplifies the implementation of functions and methods that need to accept arbitrary keyword arguments. For example, here is an excerpt from code in the :mod:`collections` module:: class Counter(dict): def __init__(self, iterable=None, /, **kwds): # Note "iterable" is a possible keyword argument See :pep:`570` for a full description. (Contributed by Pablo Galindo in :issue:`36540`.) .. TODO: Pablo will sprint on docs at PyCon US 2019. Parallel filesystem cache for compiled bytecode files ----------------------------------------------------- The new :envvar:`PYTHONPYCACHEPREFIX` setting (also available as :option:`-X` ``pycache_prefix``) configures the implicit bytecode cache to use a separate parallel filesystem tree, rather than the default ``__pycache__`` subdirectories within each source directory. The location of the cache is reported in :data:`sys.pycache_prefix` (:const:`None` indicates the default location in ``__pycache__`` subdirectories). (Contributed by Carl Meyer in :issue:`33499`.) Debug build uses the same ABI as release build ----------------------------------------------- Python now uses the same ABI whether it's built in release or debug mode. On Unix, when Python is built in debug mode, it is now possible to load C extensions built in release mode and C extensions built using the stable ABI. Release builds and debug builds are now ABI compatible: defining the ``Py_DEBUG`` macro no longer implies the ``Py_TRACE_REFS`` macro, which introduces the only ABI incompatibility. The ``Py_TRACE_REFS`` macro, which adds the :func:`sys.getobjects` function and the :envvar:`PYTHONDUMPREFS` environment variable, can be set using the new ``./configure --with-trace-refs`` build option. (Contributed by Victor Stinner in :issue:`36465`.) On Unix, C extensions are no longer linked to libpython except on Android and Cygwin. It is now possible for a statically linked Python to load a C extension built using a shared library Python. (Contributed by Victor Stinner in :issue:`21536`.) On Unix, when Python is built in debug mode, import now also looks for C extensions compiled in release mode and for C extensions compiled with the stable ABI. (Contributed by Victor Stinner in :issue:`36722`.) To embed Python into an application, a new ``--embed`` option must be passed to ``python3-config --libs --embed`` to get ``-lpython3.8`` (link the application to libpython). To support both 3.8 and older, try ``python3-config --libs --embed`` first and fallback to ``python3-config --libs`` (without ``--embed``) if the previous command fails. Add a pkg-config ``python-3.8-embed`` module to embed Python into an application: ``pkg-config python-3.8-embed --libs`` includes ``-lpython3.8``. To support both 3.8 and older, try ``pkg-config python-X.Y-embed --libs`` first and fallback to ``pkg-config python-X.Y --libs`` (without ``--embed``) if the previous command fails (replace ``X.Y`` with the Python version). On the other hand, ``pkg-config python3.8 --libs`` no longer contains ``-lpython3.8``. C extensions must not be linked to libpython (except on Android and Cygwin, whose cases are handled by the script); this change is backward incompatible on purpose. (Contributed by Victor Stinner in :issue:`36721`.) f-strings support ``=`` for self-documenting expressions and debugging ---------------------------------------------------------------------- Added an ``=`` specifier to :term:`f-string`\s. An f-string such as ``f'{expr=}'`` will expand to the text of the expression, an equal sign, then the representation of the evaluated expression. For example: >>> user = 'eric_idle' >>> member_since = date(1975, 7, 31) >>> f'{user=} {member_since=}' "user='eric_idle' member_since=datetime.date(1975, 7, 31)" The usual :ref:`f-string format specifiers ` allow more control over how the result of the expression is displayed:: >>> delta = date.today() - member_since >>> f'{user=!s} {delta.days=:,d}' 'user=eric_idle delta.days=16,075' The ``=`` specifier will display the whole expression so that calculations can be shown:: >>> print(f'{theta=} {cos(radians(theta))=:.3f}') theta=30 cos(radians(theta))=0.866 (Contributed by Eric V. Smith and Larry Hastings in :issue:`36817`.) PEP 587: Python Initialization Configuration -------------------------------------------- The :pep:`587` adds a new C API to configure the Python Initialization providing finer control on the whole configuration and better error reporting. New structures: * :c:type:`PyConfig` * :c:type:`PyPreConfig` * :c:type:`PyStatus` * :c:type:`PyWideStringList` New functions: * :c:func:`PyConfig_Clear` * :c:func:`PyConfig_InitIsolatedConfig` * :c:func:`PyConfig_InitPythonConfig` * :c:func:`PyConfig_Read` * :c:func:`PyConfig_SetArgv` * :c:func:`PyConfig_SetBytesArgv` * :c:func:`PyConfig_SetBytesString` * :c:func:`PyConfig_SetString` * :c:func:`PyPreConfig_InitIsolatedConfig` * :c:func:`PyPreConfig_InitPythonConfig` * :c:func:`PyStatus_Error` * :c:func:`PyStatus_Exception` * :c:func:`PyStatus_Exit` * :c:func:`PyStatus_IsError` * :c:func:`PyStatus_IsExit` * :c:func:`PyStatus_NoMemory` * :c:func:`PyStatus_Ok` * :c:func:`PyWideStringList_Append` * :c:func:`PyWideStringList_Insert` * :c:func:`Py_BytesMain` * :c:func:`Py_ExitStatusException` * :c:func:`Py_InitializeFromConfig` * :c:func:`Py_PreInitialize` * :c:func:`Py_PreInitializeFromArgs` * :c:func:`Py_PreInitializeFromBytesArgs` * :c:func:`Py_RunMain` This PEP also adds ``_PyRuntimeState.preconfig`` (:c:type:`PyPreConfig` type) and ``PyInterpreterState.config`` (:c:type:`PyConfig` type) fields to these internal structures. ``PyInterpreterState.config`` becomes the new reference configuration, replacing global configuration variables and other private variables. See :ref:`Python Initialization Configuration ` for the documentation. See :pep:`587` for a full description. (Contributed by Victor Stinner in :issue:`36763`.) Vectorcall: a fast calling protocol for CPython ----------------------------------------------- The "vectorcall" protocol is added to the Python/C API. It is meant to formalize existing optimizations which were already done for various classes. Any extension type implementing a callable can use this protocol. This is currently provisional, the aim is to make it fully public in Python 3.9. See :pep:`590` for a full description. (Contributed by Jeroen Demeyer and Mark Shannon in :issue:`36974`.) Pickle protocol 5 with out-of-band data buffers ----------------------------------------------- When :mod:`pickle` is used to transfer large data between Python processes in order to take advantage of multi-core or multi-machine processing, it is important to optimize the transfer by reducing memory copies, and possibly by applying custom techniques such as data-dependent compression. The :mod:`pickle` protocol 5 introduces support for out-of-band buffers where :pep:`3118`-compatible data can be transmitted separately from the main pickle stream, at the discretion of the communication layer. See :pep:`574` for a full description. (Contributed by Antoine Pitrou in :issue:`36785`.) Other Language Changes ====================== * A :keyword:`continue` statement was illegal in the :keyword:`finally` clause due to a problem with the implementation. In Python 3.8 this restriction was lifted. (Contributed by Serhiy Storchaka in :issue:`32489`.) * The :class:`bool`, :class:`int`, and :class:`fractions.Fraction` types now have an :meth:`~int.as_integer_ratio` method like that found in :class:`float` and :class:`decimal.Decimal`. This minor API extension makes it possible to write ``numerator, denominator = x.as_integer_ratio()`` and have it work across multiple numeric types. (Contributed by Lisa Roach in :issue:`33073` and Raymond Hettinger in :issue:`37819`.) * Constructors of :class:`int`, :class:`float` and :class:`complex` will now use the :meth:`~object.__index__` special method, if available and the corresponding method :meth:`~object.__int__`, :meth:`~object.__float__` or :meth:`~object.__complex__` is not available. (Contributed by Serhiy Storchaka in :issue:`20092`.) * Added support of ``\N{name}`` escapes in :mod:`regular expressions `:: >>> notice = 'Copyright © 2019' >>> copyright_year_pattern = re.compile(r'\N{copyright sign}\s*(\d{4})') >>> int(copyright_year_pattern.search(notice).group(1)) 2019 (Contributed by Jonathan Eunice and Serhiy Storchaka in :issue:`30688`.) * Dict and dictviews are now iterable in reversed insertion order using :func:`reversed`. (Contributed by Rémi Lapeyre in :issue:`33462`.) * The syntax allowed for keyword names in function calls was further restricted. In particular, ``f((keyword)=arg)`` is no longer allowed. It was never intended to permit more than a bare name on the left-hand side of a keyword argument assignment term. See :issue:`34641`. * Generalized iterable unpacking in :keyword:`yield` and :keyword:`return` statements no longer requires enclosing parentheses. This brings the *yield* and *return* syntax into better agreement with normal assignment syntax:: >>> def parse(family): lastname, *members = family.split() return lastname.upper(), *members >>> parse('simpsons homer marge bart lisa sally') ('SIMPSONS', 'homer', 'marge', 'bart', 'lisa', 'sally') (Contributed by David Cuthbert and Jordan Chapman in :issue:`32117`.) * When a comma is missed in code such as ``[(10, 20) (30, 40)]``, the compiler displays a :exc:`SyntaxWarning` with a helpful suggestion. This improves on just having a :exc:`TypeError` indicating that the first tuple was not callable. (Contributed by Serhiy Storchaka in :issue:`15248`.) * Arithmetic operations between subclasses of :class:`datetime.date` or :class:`datetime.datetime` and :class:`datetime.timedelta` objects now return an instance of the subclass, rather than the base class. This also affects the return type of operations whose implementation (directly or indirectly) uses :class:`datetime.timedelta` arithmetic, such as :meth:`datetime.datetime.astimezone`. (Contributed by Paul Ganssle in :issue:`32417`.) * When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the resulting :exc:`KeyboardInterrupt` exception is not caught, the Python process now exits via a SIGINT signal or with the correct exit code such that the calling process can detect that it died due to a Ctrl-C. Shells on POSIX and Windows use this to properly terminate scripts in interactive sessions. (Contributed by Google via Gregory P. Smith in :issue:`1054041`.) * Some advanced styles of programming require updating the :class:`types.CodeType` object for an existing function. Since code objects are immutable, a new code object needs to be created, one that is modeled on the existing code object. With 19 parameters, this was somewhat tedious. Now, the new ``replace()`` method makes it possible to create a clone with a few altered parameters. Here's an example that alters the :func:`statistics.mean` function to prevent the *data* parameter from being used as a keyword argument:: >>> from statistics import mean >>> mean(data=[10, 20, 90]) 40 >>> mean.__code__ = mean.__code__.replace(co_posonlyargcount=1) >>> mean(data=[10, 20, 90]) Traceback (most recent call last): ... TypeError: mean() got some positional-only arguments passed as keyword arguments: 'data' (Contributed by Victor Stinner in :issue:`37032`.) * For integers, the three-argument form of the :func:`pow` function now permits the exponent to be negative in the case where the base is relatively prime to the modulus. It then computes a modular inverse to the base when the exponent is ``-1``, and a suitable power of that inverse for other negative exponents. For example, to compute the `modular multiplicative inverse `_ of 38 modulo 137, write:: >>> pow(38, -1, 137) 119 >>> 119 * 38 % 137 1 Modular inverses arise in the solution of `linear Diophantine equations `_. For example, to find integer solutions for ``4258𝑥 + 147𝑦 = 369``, first rewrite as ``4258𝑥 ≡ 369 (mod 147)`` then solve: >>> x = 369 * pow(4258, -1, 147) % 147 >>> y = (4258 * x - 369) // -147 >>> 4258 * x + 147 * y 369 (Contributed by Mark Dickinson in :issue:`36027`.) * Dict comprehensions have been synced-up with dict literals so that the key is computed first and the value second:: >>> # Dict comprehension >>> cast = {input('role? '): input('actor? ') for i in range(2)} role? King Arthur actor? Chapman role? Black Knight actor? Cleese >>> # Dict literal >>> cast = {input('role? '): input('actor? ')} role? Sir Robin actor? Eric Idle The guaranteed execution order is helpful with assignment expressions because variables assigned in the key expression will be available in the value expression:: >>> names = ['Martin von Löwis', 'Łukasz Langa', 'Walter Dörwald'] >>> {(n := normalize('NFC', name)).casefold() : n for name in names} {'martin von löwis': 'Martin von Löwis', 'łukasz langa': 'Łukasz Langa', 'walter dörwald': 'Walter Dörwald'} New Modules =========== * The new :mod:`importlib.metadata` module provides (provisional) support for reading metadata from third-party packages. For example, it can extract an installed package's version number, list of entry points, and more:: >>> # Note following example requires that the popular "requests" >>> # package has been installed. >>> >>> from importlib.metadata import version, requires, files >>> version('requests') '2.22.0' >>> list(requires('requests')) ['chardet (<3.1.0,>=3.0.2)'] >>> list(files('requests'))[:5] [PackagePath('requests-2.22.0.dist-info/INSTALLER'), PackagePath('requests-2.22.0.dist-info/LICENSE'), PackagePath('requests-2.22.0.dist-info/METADATA'), PackagePath('requests-2.22.0.dist-info/RECORD'), PackagePath('requests-2.22.0.dist-info/WHEEL')] (Contributed in :issue:`34632` by Barry Warsaw and Jason R. Coombs.) Improved Modules ================ ast --- AST nodes now have ``end_lineno`` and ``end_col_offset`` attributes, which give the precise location of the end of the node. (This only applies to nodes that have ``lineno`` and ``col_offset`` attributes.) The :func:`ast.parse` function has some new flags: * ``type_comments=True`` causes it to return the text of :pep:`484` and :pep:`526` type comments associated with certain AST nodes; * ``mode='func_type'`` can be used to parse :pep:`484` "signature type comments" (returned for function definition AST nodes); * ``feature_version=(3, N)`` allows specifying an earlier Python 3 version. (For example, ``feature_version=(3, 4)`` will treat ``async`` and ``await`` as non-reserved words.) New function :func:`ast.get_source_segment` returns the source code for a specific AST node. asyncio ------- On Windows, the default event loop is now :class:`~asyncio.ProactorEventLoop`. (Contributed by Victor Stinner in :issue:`34687`.) :class:`~asyncio.ProactorEventLoop` now also supports UDP. (Contributed by Adam Meily and Andrew Svetlov in :issue:`29883`.) :class:`~asyncio.ProactorEventLoop` can now be interrupted by :exc:`KeyboardInterrupt` ("CTRL+C"). (Contributed by Vladimir Matveev in :issue:`23057`.) builtins -------- The :func:`compile` built-in has been improved to accept the ``ast.PyCF_ALLOW_TOP_LEVEL_AWAIT`` flag. With this new flag passed, :func:`compile` will allow top-level ``await``, ``async for`` and ``async with`` constructs that are usually considered invalid syntax. Asynchronous code object marked with the ``CO_COROUTINE`` flag may then be returned. (Contributed by Matthias Bussonnier in :issue:`34616`) collections ----------- The :meth:`_asdict()` method for :func:`collections.namedtuple` now returns a :class:`dict` instead of a :class:`collections.OrderedDict`. This works because regular dicts have guaranteed ordering since Python 3.7. If the extra features of :class:`OrderedDict` are required, the suggested remediation is to cast the result to the desired type: ``OrderedDict(nt._asdict())``. (Contributed by Raymond Hettinger in :issue:`35864`.) ctypes ------ On Windows, :class:`~ctypes.CDLL` and subclasses now accept a *winmode* parameter to specify flags for the underlying ``LoadLibraryEx`` call. The default flags are set to only load DLL dependencies from trusted locations, including the path where the DLL is stored (if a full or partial path is used to load the initial DLL) and paths added by :func:`~os.add_dll_directory`. datetime -------- Added new alternate constructors :meth:`datetime.date.fromisocalendar` and :meth:`datetime.datetime.fromisocalendar`, which construct :class:`date` and :class:`datetime` objects respectively from ISO year, week number, and weekday; these are the inverse of each class's ``isocalendar`` method. (Contributed by Paul Ganssle in :issue:`36004`.) functools --------- :func:`functools.lru_cache` can now be used as a straight decorator rather than as a function returning a decorator. So both of these are now supported:: @lru_cache def f(x): ... @lru_cache(maxsize=256) def f(x): ... (Contributed by Raymond Hettinger in :issue:`36772`.) gc -- :func:`~gc.get_objects` can now receive an optional *generation* parameter indicating a generation to get objects from. Contributed in :issue:`36016` by Pablo Galindo. gettext ------- Added :func:`~gettext.pgettext` and its variants. (Contributed by Franz Glasner, Éric Araujo, and Cheryl Sabella in :issue:`2504`.) gzip ---- Added the *mtime* parameter to :func:`gzip.compress` for reproducible output. (Contributed by Guo Ci Teo in :issue:`34898`.) A :exc:`~gzip.BadGzipFile` exception is now raised instead of :exc:`OSError` for certain types of invalid or corrupt gzip files. (Contributed by Filip Gruszczyński, Michele Orrù, and Zackery Spytz in :issue:`6584`.) idlelib and IDLE ---------------- Output over N lines (50 by default) is squeezed down to a button. N can be changed in the PyShell section of the General page of the Settings dialog. Fewer, but possibly extra long, lines can be squeezed by right clicking on the output. Squeezed output can be expanded in place by double-clicking the button or into the clipboard or a separate window by right-clicking the button. (Contributed by Tal Einat in :issue:`1529353`.) Add "Run Customized" to the Run menu to run a module with customized settings. Any command line arguments entered are added to sys.argv. They also re-appear in the box for the next customized run. One can also suppress the normal Shell main module restart. (Contributed by Cheryl Sabella, Terry Jan Reedy, and others in :issue:`5680` and :issue:`37627`.) Add optional line numbers for IDLE editor windows. Windows open without line numbers unless set otherwise in the General tab of the configuration dialog. Line numbers for an existing window are shown and hidden in the Options menu. (Contributed by Tal Einat and Saimadhav Heblikar in :issue:`17535`.) The changes above have been backported to 3.7 maintenance releases. inspect ------- The :func:`inspect.getdoc` function can now find docstrings for ``__slots__`` if that attribute is a :class:`dict` where the values are docstrings. This provides documentation options similar to what we already have for :func:`property`, :func:`classmethod`, and :func:`staticmethod`:: class AudioClip: __slots__ = {'bit_rate': 'expressed in kilohertz to one decimal place', 'duration': 'in seconds, rounded up to an integer'} def __init__(self, bit_rate, duration): self.bit_rate = round(bit_rate / 1000.0, 1) self.duration = ceil(duration) io -- In development mode (:option:`-X` ``env``) and in debug build, the :class:`io.IOBase` finalizer now logs the exception if the ``close()`` method fails. The exception is ignored silently by default in release build. (Contributed by Victor Stinner in :issue:`18748`.) json.tool --------- Add option ``--json-lines`` to parse every input line as separate JSON object. (Contributed by Weipeng Hong in :issue:`31553`.) math ---- Added new function :func:`math.dist` for computing Euclidean distance between two points. (Contributed by Raymond Hettinger in :issue:`33089`.) Expanded the :func:`math.hypot` function to handle multiple dimensions. Formerly, it only supported the 2-D case. (Contributed by Raymond Hettinger in :issue:`33089`.) Added new function, :func:`math.prod`, as analogous function to :func:`sum` that returns the product of a 'start' value (default: 1) times an iterable of numbers:: >>> prior = 0.8 >>> likelihoods = [0.625, 0.84, 0.30] >>> math.prod(likelihoods, start=prior) 0.126 (Contributed by Pablo Galindo in :issue:`35606`) Added new function :func:`math.isqrt` for computing integer square roots. (Contributed by Mark Dickinson in :issue:`36887`.) The function :func:`math.factorial` no longer accepts arguments that are not int-like. (Contributed by Pablo Galindo in :issue:`33083`.) mmap ---- The :class:`mmap.mmap` class now has an :meth:`~mmap.mmap.madvise` method to access the ``madvise()`` system call. (Contributed by Zackery Spytz in :issue:`32941`.) multiprocessing --------------- Added new :mod:`multiprocessing.shared_memory` module. (Contributed Davin Potts in :issue:`35813`.) On macOS, the *spawn* start method is now used by default. (Contributed by Victor Stinner in :issue:`33725`.) os -- Added new function :func:`~os.add_dll_directory` on Windows for providing additional search paths for native dependencies when importing extension modules or loading DLLs using :mod:`ctypes`. A new :func:`os.memfd_create` function was added to wrap the ``memfd_create()`` syscall. (Contributed by Zackery Spytz and Christian Heimes in :issue:`26836`.) On Windows, much of the manual logic for handling reparse points (including symlinks and directory junctions) has been delegated to the operating system. Specifically, :func:`os.stat` will now traverse anything supported by the operating system, while :func:`os.lstat` will only open reparse points that identify as "name surrogates" while others are opened as for :func:`os.stat`. In all cases, :attr:`stat_result.st_mode` will only have ``S_IFLNK`` set for symbolic links and not other kinds of reparse points. To identify other kinds of reparse point, check the new :attr:`stat_result.st_reparse_tag` attribute. On Windows, :func:`os.readlink` is now able to read directory junctions. Note that :func:`~os.path.islink` will return ``False`` for directory junctions, and so code that checks ``islink`` first will continue to treat junctions as directories, while code that handles errors from :func:`os.readlink` may now treat junctions as links. os.path ------- :mod:`os.path` functions that return a boolean result like :func:`~os.path.exists`, :func:`~os.path.lexists`, :func:`~os.path.isdir`, :func:`~os.path.isfile`, :func:`~os.path.islink`, and :func:`~os.path.ismount` now return ``False`` instead of raising :exc:`ValueError` or its subclasses :exc:`UnicodeEncodeError` and :exc:`UnicodeDecodeError` for paths that contain characters or bytes unrepresentable at the OS level. (Contributed by Serhiy Storchaka in :issue:`33721`.) :func:`~os.path.expanduser` on Windows now prefers the :envvar:`USERPROFILE` environment variable and does not use :envvar:`HOME`, which is not normally set for regular user accounts. :func:`~os.path.isdir` on Windows no longer returns true for a link to a non-existent directory. :func:`~os.path.realpath` on Windows now resolves reparse points, including symlinks and directory junctions. ncurses ------- Added a new variable holding structured version information for the underlying ncurses library: :data:`~curses.ncurses_version`. (Contributed by Serhiy Storchaka in :issue:`31680`.) pathlib ------- :mod:`pathlib.Path` methods that return a boolean result like :meth:`~pathlib.Path.exists()`, :meth:`~pathlib.Path.is_dir()`, :meth:`~pathlib.Path.is_file()`, :meth:`~pathlib.Path.is_mount()`, :meth:`~pathlib.Path.is_symlink()`, :meth:`~pathlib.Path.is_block_device()`, :meth:`~pathlib.Path.is_char_device()`, :meth:`~pathlib.Path.is_fifo()`, :meth:`~pathlib.Path.is_socket()` now return ``False`` instead of raising :exc:`ValueError` or its subclass :exc:`UnicodeEncodeError` for paths that contain characters unrepresentable at the OS level. (Contributed by Serhiy Storchaka in :issue:`33721`.) Added :meth:`pathlib.Path.link_to()` which creates a hard link pointing to a path. (Contributed by Joannah Nanjekye in :issue:`26978`) pickle ------ Reduction methods can now include a 6th item in the tuple they return. This item should specify a custom state-setting method that's called instead of the regular ``__setstate__`` method. (Contributed by Pierre Glaser and Olivier Grisel in :issue:`35900`) :mod:`pickle` extensions subclassing the C-optimized :class:`~pickle.Pickler` can now override the pickling logic of functions and classes by defining the special :meth:`~pickle.Pickler.reducer_override` method. (Contributed by Pierre Glaser and Olivier Grisel in :issue:`35900`) plistlib -------- Added new :class:`plistlib.UID` and enabled support for reading and writing NSKeyedArchiver-encoded binary plists. (Contributed by Jon Janzen in :issue:`26707`.) py_compile ---------- :func:`py_compile.compile` now supports silent mode. (Contributed by Joannah Nanjekye in :issue:`22640`.) shlex ---------- The new :func:`shlex.join` function acts as the inverse of :func:`shlex.split`. (Contributed by Bo Bayles in :issue:`32102`.) shutil ------ :func:`shutil.copytree` now accepts a new ``dirs_exist_ok`` keyword argument. (Contributed by Josh Bronson in :issue:`20849`.) :func:`shutil.make_archive` now defaults to the modern pax (POSIX.1-2001) format for new archives to improve portability and standards conformance, inherited from the corresponding change to the :mod:`tarfile` module. (Contributed by C.A.M. Gerlach in :issue:`30661`.) :func:`shutil.rmtree` on Windows now removes directory junctions without recursively removing their contents first. socket ------ Added :meth:`~socket.create_server()` and :meth:`~socket.has_dualstack_ipv6()` convenience functions to automate the necessary tasks usually involved when creating a server socket, including accepting both IPv4 and IPv6 connections on the same socket. (Contributed by Giampaolo Rodolà in :issue:`17561`.) The :func:`socket.if_nameindex()`, :func:`socket.if_nametoindex()`, and :func:`socket.if_indextoname()` functions have been implemented on Windows. (Contributed by Zackery Spytz in :issue:`37007`.) ssl --- Added :attr:`SSLContext.post_handshake_auth` to enable and :meth:`ssl.SSLSocket.verify_client_post_handshake` to initiate TLS 1.3 post-handshake authentication. (Contributed by Christian Heimes in :issue:`34670`.) statistics ---------- Added :func:`statistics.fmean` as a faster, floating point variant of :func:`statistics.mean()`. (Contributed by Raymond Hettinger and Steven D'Aprano in :issue:`35904`.) Added :func:`statistics.geometric_mean()` (Contributed by Raymond Hettinger in :issue:`27181`.) Added :func:`statistics.multimode` that returns a list of the most common values. (Contributed by Raymond Hettinger in :issue:`35892`.) Added :func:`statistics.quantiles` that divides data or a distribution in to equiprobable intervals (e.g. quartiles, deciles, or percentiles). (Contributed by Raymond Hettinger in :issue:`36546`.) Added :class:`statistics.NormalDist`, a tool for creating and manipulating normal distributions of a random variable. (Contributed by Raymond Hettinger in :issue:`36018`.) :: >>> temperature_feb = NormalDist.from_samples([4, 12, -3, 2, 7, 14]) >>> temperature_feb.mean 6.0 >>> temperature_feb.stdev 6.356099432828281 >>> temperature_feb.cdf(3) # Chance of being under 3 degrees 0.3184678262814532 >>> # Relative chance of being 7 degrees versus 10 degrees >>> temperature_feb.pdf(7) / temperature_feb.pdf(10) 1.2039930378537762 >>> el_niño = NormalDist(4, 2.5) >>> temperature_feb += el_niño # Add in a climate effect >>> temperature_feb NormalDist(mu=10.0, sigma=6.830080526611674) >>> temperature_feb * (9/5) + 32 # Convert to Fahrenheit NormalDist(mu=50.0, sigma=12.294144947901014) >>> temperature_feb.samples(3) # Generate random samples [7.672102882379219, 12.000027119750287, 4.647488369766392] sys --- Add new :func:`sys.unraisablehook` function which can be overridden to control how "unraisable exceptions" are handled. It is called when an exception has occurred but there is no way for Python to handle it. For example, when a destructor raises an exception or during garbage collection (:func:`gc.collect`). (Contributed by Victor Stinner in :issue:`36829`.) tarfile ------- The :mod:`tarfile` module now defaults to the modern pax (POSIX.1-2001) format for new archives, instead of the previous GNU-specific one. This improves cross-platform portability with a consistent encoding (UTF-8) in a standardized and extensible format, and offers several other benefits. (Contributed by C.A.M. Gerlach in :issue:`36268`.) threading --------- * Add a new :func:`threading.excepthook` function which handles uncaught :meth:`threading.Thread.run` exception. It can be overridden to control how uncaught :meth:`threading.Thread.run` exceptions are handled. (Contributed by Victor Stinner in :issue:`1230540`.) * Add a new :func:`threading.get_native_id` function and a :data:`~threading.Thread.native_id` attribute to the :class:`threading.Thread` class. These return the native integral Thread ID of the current thread assigned by the kernel. This feature is only available on certain platforms, see :func:`get_native_id ` for more information. (Contributed by Jake Tesler in :issue:`36084`.) tokenize -------- The :mod:`tokenize` module now implicitly emits a ``NEWLINE`` token when provided with input that does not have a trailing new line. This behavior now matches what the C tokenizer does internally. (Contributed by Ammar Askar in :issue:`33899`.) tkinter ------- Added methods :meth:`~tkinter.Spinbox.selection_from`, :meth:`~tkinter.Spinbox.selection_present`, :meth:`~tkinter.Spinbox.selection_range` and :meth:`~tkinter.Spinbox.selection_to` in the :class:`tkinter.Spinbox` class. (Contributed by Juliette Monsel in :issue:`34829`.) Added method :meth:`~tkinter.Canvas.moveto` in the :class:`tkinter.Canvas` class. (Contributed by Juliette Monsel in :issue:`23831`.) The :class:`tkinter.PhotoImage` class now has :meth:`~tkinter.PhotoImage.transparency_get` and :meth:`~tkinter.PhotoImage.transparency_set` methods. (Contributed by Zackery Spytz in :issue:`25451`.) time ---- Added new clock :data:`~time.CLOCK_UPTIME_RAW` for macOS 10.12. (Contributed by Joannah Nanjekye in :issue:`35702`.) typing ------ The :mod:`typing` module incorporates several new features: * Protocol definitions. See :pep:`544`, :class:`typing.Protocol` and :func:`typing.runtime_checkable`. Simple ABCs like :class:`typing.SupportsInt` are now ``Protocol`` subclasses. * A dictionary type with per-key types. See :pep:`589` and :class:`typing.TypedDict`. * Literal types. See :pep:`586` and :class:`typing.Literal`. * "Final" variables, functions, methods and classes. See :pep:`591`, :class:`typing.Final` and :func:`typing.final`. * New protocol class :class:`typing.SupportsIndex`. * New functions :func:`typing.get_origin` and :func:`typing.get_args`. unicodedata ----------- * The :mod:`unicodedata` module has been upgraded to use the `Unicode 12.1.0 `_ release. * New function :func:`~unicodedata.is_normalized` can be used to verify a string is in a specific normal form, often much faster than by actually normalizing the string. (Contributed by Max Belanger, David Euresti, and Greg Price in :issue:`32285` and :issue:`37966`). unittest -------- * Added :class:`AsyncMock` to support an asynchronous version of :class:`Mock`. Appropriate new assert functions for testing have been added as well. (Contributed by Lisa Roach in :issue:`26467`). * Added :func:`~unittest.addModuleCleanup()` and :meth:`~unittest.TestCase.addClassCleanup()` to unittest to support cleanups for :func:`~unittest.setUpModule()` and :meth:`~unittest.TestCase.setUpClass()`. (Contributed by Lisa Roach in :issue:`24412`.) * Several mock assert functions now also print a list of actual calls upon failure. (Contributed by Petter Strandmark in :issue:`35047`.) * :mod:`unittest` module gained support for coroutines to be used as test cases with :class:`unittest.IsolatedAsyncioTestCase`. (Contributed by Andrew Svetlov in :issue:`32972`.) Example:: import unittest class TestRequest(unittest.IsolatedAsyncioTestCase): async def asyncSetUp(self): self.connection = await AsyncConnection() async def test_get(self): response = await self.connection.get("https://example.com") self.assertEqual(response.status_code, 200) async def asyncTearDown(self): await self.connection.close() if __name__ == "__main__": unittest.main() venv ---- * :mod:`venv` now includes an ``Activate.ps1`` script on all platforms for activating virtual environments under PowerShell Core 6.1. (Contributed by Brett Cannon in :issue:`32718`.) weakref ------- * The proxy objects returned by :func:`weakref.proxy` now support the matrix multiplication operators ``@`` and ``@=`` in addition to the other numeric operators. (Contributed by Mark Dickinson in :issue:`36669`.) xml --- * As mitigation against DTD and external entity retrieval, the :mod:`xml.dom.minidom` and :mod:`xml.sax` modules no longer process external entities by default. (Contributed by Christian Heimes in :issue:`17239`.) * The ``.find*()`` methods in the :mod:`xml.etree.ElementTree` module support wildcard searches like ``{*}tag`` which ignores the namespace and ``{namespace}*`` which returns all tags in the given namespace. (Contributed by Stefan Behnel in :issue:`28238`.) * The :mod:`xml.etree.ElementTree` module provides a new function :func:`–xml.etree.ElementTree.canonicalize()` that implements C14N 2.0. (Contributed by Stefan Behnel in :issue:`13611`.) * The target object of :class:`xml.etree.ElementTree.XMLParser` can receive namespace declaration events through the new callback methods ``start_ns()`` and ``end_ns()``. Additionally, the :class:`xml.etree.ElementTree.TreeBuilder` target can be configured to process events about comments and processing instructions to include them in the generated tree. (Contributed by Stefan Behnel in :issue:`36676` and :issue:`36673`.) Optimizations ============= * The :mod:`subprocess` module can now use the :func:`os.posix_spawn` function in some cases for better performance. Currently, it is only used on macOS and Linux (using glibc 2.24 or newer) if all these conditions are met: * *close_fds* is false; * *preexec_fn*, *pass_fds*, *cwd* and *start_new_session* parameters are not set; * the *executable* path contains a directory. (Contributed by Joannah Nanjekye and Victor Stinner in :issue:`35537`.) * :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`, :func:`shutil.copytree` and :func:`shutil.move` use platform-specific "fast-copy" syscalls on Linux and macOS in order to copy the file more efficiently. "fast-copy" means that the copying operation occurs within the kernel, avoiding the use of userspace buffers in Python as in "``outfd.write(infd.read())``". On Windows :func:`shutil.copyfile` uses a bigger default buffer size (1 MiB instead of 16 KiB) and a :func:`memoryview`-based variant of :func:`shutil.copyfileobj` is used. The speedup for copying a 512 MiB file within the same partition is about +26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles are consumed. See :ref:`shutil-platform-dependent-efficient-copy-operations` section. (Contributed by Giampaolo Rodolà in :issue:`33671`.) * :func:`shutil.copytree` uses :func:`os.scandir` function and all copy functions depending from it use cached :func:`os.stat` values. The speedup for copying a directory with 8000 files is around +9% on Linux, +20% on Windows and +30% on a Windows SMB share. Also the number of :func:`os.stat` syscalls is reduced by 38% making :func:`shutil.copytree` especially faster on network filesystems. (Contributed by Giampaolo Rodolà in :issue:`33695`.) * The default protocol in the :mod:`pickle` module is now Protocol 4, first introduced in Python 3.4. It offers better performance and smaller size compared to Protocol 3 available since Python 3.0. * Removed one ``Py_ssize_t`` member from ``PyGC_Head``. All GC tracked objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes. (Contributed by Inada Naoki in :issue:`33597`) * :class:`uuid.UUID` now uses ``__slots__`` to reduce its memory footprint. * Improved performance of :func:`operator.itemgetter` by 33%. Optimized argument handling and added a fast path for the common case of a single non-negative integer index into a tuple (which is the typical use case in the standard library). (Contributed by Raymond Hettinger in :issue:`35664`.) * Sped-up field lookups in :func:`collections.namedtuple`. They are now more than two times faster, making them the fastest form of instance variable lookup in Python. (Contributed by Raymond Hettinger, Pablo Galindo, and Joe Jevnik, Serhiy Storchaka in :issue:`32492`.) * The :class:`list` constructor does not overallocate the internal item buffer if the input iterable has a known length (the input implements ``__len__``). This makes the created list 12% smaller on average. (Contributed by Raymond Hettinger and Pablo Galindo in :issue:`33234`.) * Doubled the speed of class variable writes. When a non-dunder attribute was updated, there was an unnecessary call to update slots. (Contributed by Stefan Behnel, Pablo Galindo Salgado, Raymond Hettinger, Neil Schemenauer, and Serhiy Storchaka in :issue:`36012`.) * Reduced an overhead of converting arguments passed to many builtin functions and methods. This sped up calling some simple builtin functions and methods up to 20--50%. (Contributed by Serhiy Storchaka in :issue:`23867`, :issue:`35582` and :issue:`36127`.) * ``LOAD_GLOBAL`` instruction now uses new "per opcode cache" mechanism. It is about 40% faster now. (Contributed by Yury Selivanov and Inada Naoki in :issue:`26219`.) Build and C API Changes ======================= * Default :data:`sys.abiflags` became an empty string: the ``m`` flag for pymalloc became useless (builds with and without pymalloc are ABI compatible) and so has been removed. (Contributed by Victor Stinner in :issue:`36707`.) Example of changes: * Only ``python3.8`` program is installed, ``python3.8m`` program is gone. * Only ``python3.8-config`` script is installed, ``python3.8m-config`` script is gone. * The ``m`` flag has been removed from the suffix of dynamic library filenames: extension modules in the standard library as well as those produced and installed by third-party packages, like those downloaded from PyPI. On Linux, for example, the Python 3.7 suffix ``.cpython-37m-x86_64-linux-gnu.so`` became ``.cpython-38-x86_64-linux-gnu.so`` in Python 3.8. * The header files have been reorganized to better separate the different kinds of APIs: * ``Include/*.h`` should be the portable public stable C API. * ``Include/cpython/*.h`` should be the unstable C API specific to CPython; public API, with some private API prefixed by ``_Py`` or ``_PY``. * ``Include/internal/*.h`` is the private internal C API very specific to CPython. This API comes with no backward compatibility warranty and should not be used outside CPython. It is only exposed for very specific needs like debuggers and profiles which has to access to CPython internals without calling functions. This API is now installed by ``make install``. (Contributed by Victor Stinner in :issue:`35134` and :issue:`35081`, work initiated by Eric Snow in Python 3.7.) * Some macros have been converted to static inline functions: parameter types and return type are well defined, they don't have issues specific to macros, variables have a local scopes. Examples: * :c:func:`Py_INCREF`, :c:func:`Py_DECREF` * :c:func:`Py_XINCREF`, :c:func:`Py_XDECREF` * :c:func:`PyObject_INIT`, :c:func:`PyObject_INIT_VAR` * Private functions: :c:func:`_PyObject_GC_TRACK`, :c:func:`_PyObject_GC_UNTRACK`, :c:func:`_Py_Dealloc` (Contributed by Victor Stinner in :issue:`35059`.) * The :c:func:`PyByteArray_Init` and :c:func:`PyByteArray_Fini` functions have been removed. They did nothing since Python 2.7.4 and Python 3.2.0, were excluded from the limited API (stable ABI), and were not documented. (Contributed by Victor Stinner in :issue:`35713`.) * The result of :c:func:`PyExceptionClass_Name` is now of type ``const char *`` rather of ``char *``. (Contributed by Serhiy Storchaka in :issue:`33818`.) * The duality of ``Modules/Setup.dist`` and ``Modules/Setup`` has been removed. Previously, when updating the CPython source tree, one had to manually copy ``Modules/Setup.dist`` (inside the source tree) to ``Modules/Setup`` (inside the build tree) in order to reflect any changes upstream. This was of a small benefit to packagers at the expense of a frequent annoyance to developers following CPython development, as forgetting to copy the file could produce build failures. Now the build system always reads from ``Modules/Setup`` inside the source tree. People who want to customize that file are encouraged to maintain their changes in a git fork of CPython or as patch files, as they would do for any other change to the source tree. (Contributed by Antoine Pitrou in :issue:`32430`.) * Functions that convert Python number to C integer like :c:func:`PyLong_AsLong` and argument parsing functions like :c:func:`PyArg_ParseTuple` with integer converting format units like ``'i'`` will now use the :meth:`~object.__index__` special method instead of :meth:`~object.__int__`, if available. The deprecation warning will be emitted for objects with the ``__int__()`` method but without the ``__index__()`` method (like :class:`~decimal.Decimal` and :class:`~fractions.Fraction`). :c:func:`PyNumber_Check` will now return ``1`` for objects implementing ``__index__()``. :c:func:`PyNumber_Long`, :c:func:`PyNumber_Float` and :c:func:`PyFloat_AsDouble` also now use the ``__index__()`` method if available. (Contributed by Serhiy Storchaka in :issue:`36048` and :issue:`20092`.) * Heap-allocated type objects will now increase their reference count in :c:func:`PyObject_Init` (and its parallel macro ``PyObject_INIT``) instead of in :c:func:`PyType_GenericAlloc`. Types that modify instance allocation or deallocation may need to be adjusted. (Contributed by Eddie Elizondo in :issue:`35810`.) * The new function :c:func:`PyCode_NewWithPosOnlyArgs` allows to create code objects like :c:func:`PyCode_New`, but with an extra *posonlyargcount* parameter for indicating the number of positional-only arguments. (Contributed by Pablo Galindo in :issue:`37221`.) * :c:func:`Py_SetPath` now sets :data:`sys.executable` to the program full path (:c:func:`Py_GetProgramFullPath`) rather than to the program name (:c:func:`Py_GetProgramName`). (Contributed by Victor Stinner in :issue:`38234`.) Deprecated ========== * The distutils ``bdist_wininst`` command is now deprecated, use ``bdist_wheel`` (wheel packages) instead. (Contributed by Victor Stinner in :issue:`37481`.) * Deprecated methods ``getchildren()`` and ``getiterator()`` in the :mod:`~xml.etree.ElementTree` module now emit a :exc:`DeprecationWarning` instead of :exc:`PendingDeprecationWarning`. They will be removed in Python 3.9. (Contributed by Serhiy Storchaka in :issue:`29209`.) * Passing an object that is not an instance of :class:`concurrent.futures.ThreadPoolExecutor` to :meth:`asyncio.loop.set_default_executor()` is deprecated and will be prohibited in Python 3.9. (Contributed by Elvis Pranskevichus in :issue:`34075`.) * The :meth:`__getitem__` methods of :class:`xml.dom.pulldom.DOMEventStream`, :class:`wsgiref.util.FileWrapper` and :class:`fileinput.FileInput` have been deprecated. Implementations of these methods have been ignoring their *index* parameter, and returning the next item instead. (Contributed by Berker Peksag in :issue:`9372`.) * The :class:`typing.NamedTuple` class has deprecated the ``_field_types`` attribute in favor of the ``__annotations__`` attribute which has the same information. (Contributed by Raymond Hettinger in :issue:`36320`.) * :mod:`ast` classes ``Num``, ``Str``, ``Bytes``, ``NameConstant`` and ``Ellipsis`` are considered deprecated and will be removed in future Python versions. :class:`~ast.Constant` should be used instead. (Contributed by Serhiy Storchaka in :issue:`32892`.) * :class:`ast.NodeVisitor` methods ``visit_Num()``, ``visit_Str()``, ``visit_Bytes()``, ``visit_NameConstant()`` and ``visit_Ellipsis()`` are deprecated now and will not be called in future Python versions. Add the :meth:`~ast.NodeVisitor.visit_Constant` method to handle all constant nodes. (Contributed by Serhiy Storchaka in :issue:`36917`.) * The following functions and methods are deprecated in the :mod:`gettext` module: :func:`~gettext.lgettext`, :func:`~gettext.ldgettext`, :func:`~gettext.lngettext` and :func:`~gettext.ldngettext`. They return encoded bytes, and it's possible that you will get unexpected Unicode-related exceptions if there are encoding problems with the translated strings. It's much better to use alternatives which return Unicode strings in Python 3. These functions have been broken for a long time. Function :func:`~gettext.bind_textdomain_codeset`, methods :meth:`~gettext.NullTranslations.output_charset` and :meth:`~gettext.NullTranslations.set_output_charset`, and the *codeset* parameter of functions :func:`~gettext.translation` and :func:`~gettext.install` are also deprecated, since they are only used for for the ``l*gettext()`` functions. (Contributed by Serhiy Storchaka in :issue:`33710`.) * The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread` has been deprecated. (Contributed by Dong-hee Na in :issue:`35283`.) * Many builtin and extension functions that take integer arguments will now emit a deprecation warning for :class:`~decimal.Decimal`\ s, :class:`~fractions.Fraction`\ s and any other objects that can be converted to integers only with a loss (e.g. that have the :meth:`~object.__int__` method but do not have the :meth:`~object.__index__` method). In future version they will be errors. (Contributed by Serhiy Storchaka in :issue:`36048`.) * Deprecated passing the following arguments as keyword arguments: - *func* in :func:`functools.partialmethod`, :func:`weakref.finalize`, :meth:`profile.Profile.runcall`, :meth:`cProfile.Profile.runcall`, :meth:`bdb.Bdb.runcall`, :meth:`trace.Trace.runfunc` and :func:`curses.wrapper`. - *function* in :meth:`unittest.TestCase.addCleanup`. - *fn* in the :meth:`~concurrent.futures.Executor.submit` method of :class:`concurrent.futures.ThreadPoolExecutor` and :class:`concurrent.futures.ProcessPoolExecutor`. - *callback* in :meth:`contextlib.ExitStack.callback`, :meth:`contextlib.AsyncExitStack.callback` and :meth:`contextlib.AsyncExitStack.push_async_callback`. - *c* and *typeid* in the :meth:`~multiprocessing.managers.Server.create` method of :class:`multiprocessing.managers.Server` and :class:`multiprocessing.managers.SharedMemoryServer`. - *obj* in :func:`weakref.finalize`. In future releases of Python they will be :ref:`positional-only `. (Contributed by Serhiy Storchaka in :issue:`36492`.) API and Feature Removals ======================== The following features and APIs have been removed from Python 3.8: * The :mod:`macpath` module, deprecated in Python 3.7, has been removed. (Contributed by Victor Stinner in :issue:`35471`.) * The function :func:`platform.popen` has been removed, after having been deprecated since Python 3.3: use :func:`os.popen` instead. (Contributed by Victor Stinner in :issue:`35345`.) * The function :func:`time.clock` has been removed, after having been deprecated since Python 3.3: use :func:`time.perf_counter` or :func:`time.process_time` instead, depending on your requirements, to have well-defined behavior. (Contributed by Matthias Bussonnier in :issue:`36895`.) * The ``pyvenv`` script has been removed in favor of ``python3.8 -m venv`` to help eliminate confusion as to what Python interpreter the ``pyvenv`` script is tied to. (Contributed by Brett Cannon in :issue:`25427`.) * ``parse_qs``, ``parse_qsl``, and ``escape`` are removed from the :mod:`cgi` module. They are deprecated in Python 3.2 or older. They should be imported from the ``urllib.parse`` and ``html`` modules instead. * ``filemode`` function is removed from the :mod:`tarfile` module. It is not documented and deprecated since Python 3.3. * The :class:`~xml.etree.ElementTree.XMLParser` constructor no longer accepts the *html* argument. It never had an effect and was deprecated in Python 3.4. All other parameters are now :ref:`keyword-only `. (Contributed by Serhiy Storchaka in :issue:`29209`.) * Removed the ``doctype()`` method of :class:`~xml.etree.ElementTree.XMLParser`. (Contributed by Serhiy Storchaka in :issue:`29209`.) * "unicode_internal" codec is removed. (Contributed by Inada Naoki in :issue:`36297`.) * The ``Cache`` and ``Statement`` objects of the :mod:`sqlite3` module are not exposed to the user. (Contributed by Aviv Palivoda in :issue:`30262`.) * The ``bufsize`` keyword argument of :func:`fileinput.input` and :func:`fileinput.FileInput` which was ignored and deprecated since Python 3.6 has been removed. :issue:`36952` (Contributed by Matthias Bussonnier.) * The functions :func:`sys.set_coroutine_wrapper` and :func:`sys.get_coroutine_wrapper` deprecated in Python 3.7 have been removed; :issue:`36933` (Contributed by Matthias Bussonnier.) Porting to Python 3.8 ===================== This section lists previously described changes and other bugfixes that may require changes to your code. Changes in Python behavior -------------------------- * Yield expressions (both ``yield`` and ``yield from`` clauses) are now disallowed in comprehensions and generator expressions (aside from the iterable expression in the leftmost :keyword:`!for` clause). (Contributed by Serhiy Storchaka in :issue:`10544`.) * The compiler now produces a :exc:`SyntaxWarning` when identity checks (``is`` and ``is not``) are used with certain types of literals (e.g. strings, ints). These can often work by accident in CPython, but are not guaranteed by the language spec. The warning advises users to use equality tests (``==`` and ``!=``) instead. (Contributed by Serhiy Storchaka in :issue:`34850`.) * The CPython interpreter can swallow exceptions in some circumstances. In Python 3.8 this happens in fewer cases. In particular, exceptions raised when getting the attribute from the type dictionary are no longer ignored. (Contributed by Serhiy Storchaka in :issue:`35459`.) * Removed ``__str__`` implementations from builtin types :class:`bool`, :class:`int`, :class:`float`, :class:`complex` and few classes from the standard library. They now inherit ``__str__()`` from :class:`object`. As result, defining the ``__repr__()`` method in the subclass of these classes will affect their string representation. (Contributed by Serhiy Storchaka in :issue:`36793`.) * On AIX, :attr:`sys.platform` doesn't contain the major version anymore. It is always ``'aix'``, instead of ``'aix3'`` .. ``'aix7'``. Since older Python versions include the version number, so it is recommended to always use ``sys.platform.startswith('aix')``. (Contributed by M. Felt in :issue:`36588`.) * :c:func:`PyEval_AcquireLock` and :c:func:`PyEval_AcquireThread` now terminate the current thread if called while the interpreter is finalizing, making them consistent with :c:func:`PyEval_RestoreThread`, :c:func:`Py_END_ALLOW_THREADS`, and :c:func:`PyGILState_Ensure`. If this behaviour is not desired, guard the call by checking :c:func:`_Py_IsFinalizing` or :c:func:`sys.is_finalizing`. Changes in the Python API ------------------------- * The :func:`os.getcwdb` function now uses the UTF-8 encoding on Windows, rather than the ANSI code page: see :pep:`529` for the rationale. The function is no longer deprecated on Windows. (Contributed by Victor Stinner in :issue:`37412`.) * :class:`subprocess.Popen` can now use :func:`os.posix_spawn` in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, the :class:`Popen` constructor using :func:`os.posix_spawn` no longer raises an exception on errors like "missing program". Instead the child process fails with a non-zero :attr:`~Popen.returncode`. (Contributed by Joannah Nanjekye and Victor Stinner in :issue:`35537`.) * The *preexec_fn* argument of * :class:`subprocess.Popen` is no longer compatible with subinterpreters. The use of the parameter in a subinterpreter now raises :exc:`RuntimeError`. (Contributed by Eric Snow in :issue:`34651`, modified by Christian Heimes in :issue:`37951`.) * The :meth:`imap.IMAP4.logout` method no longer ignores silently arbitrary exceptions. * The function :func:`platform.popen` has been removed, after having been deprecated since Python 3.3: use :func:`os.popen` instead. (Contributed by Victor Stinner in :issue:`35345`.) * The :func:`statistics.mode` function no longer raises an exception when given multimodal data. Instead, it returns the first mode encountered in the input data. (Contributed by Raymond Hettinger in :issue:`35892`.) * The :meth:`~tkinter.ttk.Treeview.selection` method of the :class:`tkinter.ttk.Treeview` class no longer takes arguments. Using it with arguments for changing the selection was deprecated in Python 3.6. Use specialized methods like :meth:`~tkinter.ttk.Treeview.selection_set` for changing the selection. (Contributed by Serhiy Storchaka in :issue:`31508`.) * The :meth:`writexml`, :meth:`toxml` and :meth:`toprettyxml` methods of :mod:`xml.dom.minidom`, and the :meth:`write` method of :mod:`xml.etree`, now preserve the attribute order specified by the user. (Contributed by Diego Rojas and Raymond Hettinger in :issue:`34160`.) * A :mod:`dbm.dumb` database opened with flags ``'r'`` is now read-only. :func:`dbm.dumb.open` with flags ``'r'`` and ``'w'`` no longer creates a database if it does not exist. (Contributed by Serhiy Storchaka in :issue:`32749`.) * The ``doctype()`` method defined in a subclass of :class:`~xml.etree.ElementTree.XMLParser` will no longer be called and will emit a :exc:`RuntimeWarning` instead of a :exc:`DeprecationWarning`. Define the :meth:`doctype() ` method on a target for handling an XML doctype declaration. (Contributed by Serhiy Storchaka in :issue:`29209`.) * A :exc:`RuntimeError` is now raised when the custom metaclass doesn't provide the ``__classcell__`` entry in the namespace passed to ``type.__new__``. A :exc:`DeprecationWarning` was emitted in Python 3.6--3.7. (Contributed by Serhiy Storchaka in :issue:`23722`.) * The :class:`cProfile.Profile` class can now be used as a context manager. (Contributed by Scott Sanderson in :issue:`29235`.) * :func:`shutil.copyfile`, :func:`shutil.copy`, :func:`shutil.copy2`, :func:`shutil.copytree` and :func:`shutil.move` use platform-specific "fast-copy" syscalls (see :ref:`shutil-platform-dependent-efficient-copy-operations` section). * :func:`shutil.copyfile` default buffer size on Windows was changed from 16 KiB to 1 MiB. * The ``PyGC_Head`` struct has changed completely. All code that touched the struct member should be rewritten. (See :issue:`33597`.) * The ``PyInterpreterState`` struct has been moved into the "internal" header files (specifically Include/internal/pycore_pystate.h). An opaque ``PyInterpreterState`` is still available as part of the public API (and stable ABI). The docs indicate that none of the struct's fields are public, so we hope no one has been using them. However, if you do rely on one or more of those private fields and have no alternative then please open a BPO issue. We'll work on helping you adjust (possibly including adding accessor functions to the public API). (See :issue:`35886`.) * Asyncio tasks can now be named, either by passing the ``name`` keyword argument to :func:`asyncio.create_task` or the :meth:`~asyncio.loop.create_task` event loop method, or by calling the :meth:`~asyncio.Task.set_name` method on the task object. The task name is visible in the ``repr()`` output of :class:`asyncio.Task` and can also be retrieved using the :meth:`~asyncio.Task.get_name` method. * The :meth:`mmap.flush() ` method now returns ``None`` on success and raises an exception on error under all platforms. Previously, its behavior was platform-dependent: a nonzero value was returned on success; zero was returned on error under Windows. A zero value was returned on success; an exception was raised on error under Unix. (Contributed by Berker Peksag in :issue:`2122`.) * :mod:`xml.dom.minidom` and :mod:`xml.sax` modules no longer process external entities by default. (Contributed by Christian Heimes in :issue:`17239`.) * Deleting a key from a read-only :mod:`dbm` database (:mod:`dbm.dumb`, :mod:`dbm.gnu` or :mod:`dbm.ndbm`) raises :attr:`error` (:exc:`dbm.dumb.error`, :exc:`dbm.gnu.error` or :exc:`dbm.ndbm.error`) instead of :exc:`KeyError`. (Contributed by Xiang Zhang in :issue:`33106`.) * :func:`~os.path.expanduser` on Windows now prefers the :envvar:`USERPROFILE` environment variable and does not use :envvar:`HOME`, which is not normally set for regular user accounts. * The Exception :class:`asyncio.CancelledError` now inherits from :class:`BaseException` rather than a :class:`Exception`. (Contributed by Yury Selivanov in :issue:`13528`.) .. _bpo-36085-whatsnew: * DLL dependencies for extension modules and DLLs loaded with :mod:`ctypes` on Windows are now resolved more securely. Only the system paths, the directory containing the DLL or PYD file, and directories added with :func:`~os.add_dll_directory` are searched for load-time dependencies. Specifically, :envvar:`PATH` and the current working directory are no longer used, and modifications to these will no longer have any effect on normal DLL resolution. If your application relies on these mechanisms, you should check for :func:`~os.add_dll_directory` and if it exists, use it to add your DLLs directory while loading your library. Note that Windows 7 users will need to ensure that Windows Update KB2533625 has been installed (this is also verified by the installer). (See :issue:`36085`.) * The header files and functions related to pgen have been removed after its replacement by a pure Python implementation. (Contributed by Pablo Galindo in :issue:`36623`.) * :class:`types.CodeType` has a new parameter in the second position of the constructor (*posonlyargcount*) to support positional-only arguments defined in :pep:`570`. The first argument (*argcount*) now represents the total number of positional arguments (including positional-only arguments). The new ``replace()`` method of :class:`types.CodeType` can be used to make the code future-proof. Changes in the C API -------------------- * The :c:type:`PyCompilerFlags` structure got a new *cf_feature_version* field. It should be initialized to ``PY_MINOR_VERSION``. The field is ignored by default, and is used if and only if ``PyCF_ONLY_AST`` flag is set in *cf_flags*. * The :c:func:`PyEval_ReInitThreads` function has been removed from the C API. It should not be called explicitly: use :c:func:`PyOS_AfterFork_Child` instead. (Contributed by Victor Stinner in :issue:`36728`.) * On Unix, C extensions are no longer linked to libpython except on Android and Cygwin. When Python is embedded, ``libpython`` must not be loaded with ``RTLD_LOCAL``, but ``RTLD_GLOBAL`` instead. Previously, using ``RTLD_LOCAL``, it was already not possible to load C extensions which were not linked to ``libpython``, like C extensions of the standard library built by the ``*shared*`` section of ``Modules/Setup``. (Contributed by Victor Stinner in :issue:`21536`.) * Use of ``#`` variants of formats in parsing or building value (e.g. :c:func:`PyArg_ParseTuple`, :c:func:`Py_BuildValue`, :c:func:`PyObject_CallFunction`, etc.) without ``PY_SSIZE_T_CLEAN`` defined raises ``DeprecationWarning`` now. It will be removed in 3.10 or 4.0. Read :ref:`arg-parsing` for detail. (Contributed by Inada Naoki in :issue:`36381`.) * Instances of heap-allocated types (such as those created with :c:func:`PyType_FromSpec`) hold a reference to their type object. Increasing the reference count of these type objects has been moved from :c:func:`PyType_GenericAlloc` to the more low-level functions, :c:func:`PyObject_Init` and :c:func:`PyObject_INIT`. This makes types created through :c:func:`PyType_FromSpec` behave like other classes in managed code. Statically allocated types are not affected. For the vast majority of cases, there should be no side effect. However, types that manually increase the reference count after allocating an instance (perhaps to work around the bug) may now become immortal. To avoid this, these classes need to call Py_DECREF on the type object during instance deallocation. To correctly port these types into 3.8, please apply the following changes: * Remove :c:macro:`Py_INCREF` on the type object after allocating an instance - if any. This may happen after calling :c:func:`PyObject_New`, :c:func:`PyObject_NewVar`, :c:func:`PyObject_GC_New`, :c:func:`PyObject_GC_NewVar`, or any other custom allocator that uses :c:func:`PyObject_Init` or :c:func:`PyObject_INIT`. Example:: static foo_struct * foo_new(PyObject *type) { foo_struct *foo = PyObject_GC_New(foo_struct, (PyTypeObject *) type); if (foo == NULL) return NULL; #if PY_VERSION_HEX < 0x03080000 // Workaround for Python issue 35810; no longer necessary in Python 3.8 PY_INCREF(type) #endif return foo; } * Ensure that all custom ``tp_dealloc`` functions of heap-allocated types decrease the type's reference count. Example:: static void foo_dealloc(foo_struct *instance) { PyObject *type = Py_TYPE(instance); PyObject_GC_Del(instance); #if PY_VERSION_HEX >= 0x03080000 // This was not needed before Python 3.8 (Python issue 35810) Py_DECREF(type); #endif } (Contributed by Eddie Elizondo in :issue:`35810`.) * The :c:macro:`Py_DEPRECATED()` macro has been implemented for MSVC. The macro now must be placed before the symbol name. Example:: Py_DEPRECATED(3.8) PyAPI_FUNC(int) Py_OldFunction(void); (Contributed by Zackery Spytz in :issue:`33407`.) * The interpreter does not pretend to support binary compatibility of extension types across feature releases, anymore. A :c:type:`PyTypeObject` exported by a third-party extension module is supposed to have all the slots expected in the current Python version, including :c:member:`~PyTypeObject.tp_finalize` (:const:`Py_TPFLAGS_HAVE_FINALIZE` is not checked anymore before reading :c:member:`~PyTypeObject.tp_finalize`). (Contributed by Antoine Pitrou in :issue:`32388`.) * The :c:func:`PyCode_New` has a new parameter in the second position (*posonlyargcount*) to support :pep:`570`, indicating the number of positional-only arguments. * The functions :c:func:`PyNode_AddChild` and :c:func:`PyParser_AddToken` now accept two additional ``int`` arguments *end_lineno* and *end_col_offset*. .. highlight:: shell * The :file:`libpython38.a` file to allow MinGW tools to link directly against :file:`python38.dll` is no longer included in the regular Windows distribution. If you require this file, it may be generated with the ``gendef`` and ``dlltool`` tools, which are part of the MinGW binutils package:: gendef python38.dll > tmp.def dlltool --dllname python38.dll --def tmp.def --output-lib libpython38.a The location of an installed :file:`pythonXY.dll` will depend on the installation options and the version and language of Windows. See :ref:`using-on-windows` for more information. The resulting library should be placed in the same directory as :file:`pythonXY.lib`, which is generally the :file:`libs` directory under your Python installation. .. highlight:: python3 CPython bytecode changes ------------------------ * The interpreter loop has been simplified by moving the logic of unrolling the stack of blocks into the compiler. The compiler emits now explicit instructions for adjusting the stack of values and calling the cleaning-up code for :keyword:`break`, :keyword:`continue` and :keyword:`return`. Removed opcodes :opcode:`BREAK_LOOP`, :opcode:`CONTINUE_LOOP`, :opcode:`SETUP_LOOP` and :opcode:`SETUP_EXCEPT`. Added new opcodes :opcode:`ROT_FOUR`, :opcode:`BEGIN_FINALLY`, :opcode:`CALL_FINALLY` and :opcode:`POP_FINALLY`. Changed the behavior of :opcode:`END_FINALLY` and :opcode:`WITH_CLEANUP_START`. (Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in :issue:`17611`.) * Added new opcode :opcode:`END_ASYNC_FOR` for handling exceptions raised when awaiting a next item in an :keyword:`async for` loop. (Contributed by Serhiy Storchaka in :issue:`33041`.) * The :opcode:`MAP_ADD` now expects the value as the first element in the stack and the key as the second element. This change was made so the key is always evaluated before the value in dictionary comprehensions, as proposed by :pep:`572`. (Contributed by Jörn Heissler in :issue:`35224`.) Demos and Tools --------------- * Added a benchmark script for timing various ways to access variables: ``Tools/scripts/var_access_benchmark.py``. (Contributed by Raymond Hettinger in :issue:`35884`.)