**************************** What's New In Python 3.9 **************************** :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 3.9, compared to 3.8. 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 3.9 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 3.9. Brevity is key. .. PEP-sized items next. New Features ============ Other Language Changes ====================== * :func:`__import__` now raises :exc:`ImportError` instead of :exc:`ValueError`, which used to occur when a relative import went past its top-level package. (Contributed by Ngalim Siregar in :issue:`37444`.) * Python now gets the absolute path of the script filename specified on the command line (ex: ``python3 script.py``): the ``__file__`` attribute of the :mod:`__main__` module, ``sys.argv[0]`` and ``sys.path[0]`` become an absolute path, rather than a relative path. These paths now remain valid after the current directory is changed by :func:`os.chdir`. As a side effect, a traceback also displays the absolute path for :mod:`__main__` module frames in this case. (Contributed by Victor Stinner in :issue:`20443`.) * In development mode and in debug build, *encoding* and *errors* arguments are now checked on string encoding and decoding operations. Examples: :func:`open`, :meth:`str.encode` and :meth:`bytes.decode`. By default, for best performance, the *errors* argument is only checked at the first encoding/decoding error and the *encoding* argument is sometimes ignored for empty strings. (Contributed by Victor Stinner in :issue:`37388`.) * ``"".replace("", s, n)`` now returns ``s`` instead of an empty string for all non-zero ``n``. It is now consistent with ``"".replace("", s)``. There are similar changes for :class:`bytes` and :class:`bytearray` objects. (Contributed by Serhiy Storchaka in :issue:`28029`.) New Modules =========== * None yet. Improved Modules ================ ast --- Added the *indent* option to :func:`~ast.dump` which allows it to produce a multiline indented output. (Contributed by Serhiy Storchaka in :issue:`37995`.) Added :func:`ast.unparse` as a function in the :mod:`ast` module that can be used to unparse an :class:`ast.AST` object and produce a string with code that would produce an equivalent :class:`ast.AST` object when parsed. (Contributed by Pablo Galindo and Batuhan Taskaya in :issue:`38870`.) asyncio ------- Due to significant security concerns, the *reuse_address* parameter of :meth:`asyncio.loop.create_datagram_endpoint` is no longer supported. This is because of the behavior of the socket option ``SO_REUSEADDR`` in UDP. For more details, see the documentation for ``loop.create_datagram_endpoint()``. (Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in :issue:`37228`.) Added a new :term:`coroutine` :meth:`~asyncio.loop.shutdown_default_executor` that schedules a shutdown for the default executor that waits on the :class:`~concurrent.futures.ThreadPoolExecutor` to finish closing. Also, :func:`asyncio.run` has been updated to use the new :term:`coroutine`. (Contributed by Kyle Stanley in :issue:`34037`.) Added :class:`asyncio.PidfdChildWatcher`, a Linux-specific child watcher implementation that polls process file descriptors. (:issue:`38692`) curses ------ Add :func:`curses.get_escdelay`, :func:`curses.set_escdelay`, :func:`curses.get_tabsize`, and :func:`curses.set_tabsize` functions. (Contributed by Anthony Sottile in :issue:`38312`.) fcntl ----- Added constants :data:`~fcntl.F_OFD_GETLK`, :data:`~fcntl.F_OFD_SETLK` and :data:`~fcntl.F_OFD_SETLKW`. (Contributed by Dong-hee Na in :issue:`38602`.) gc -- When the garbage collector makes a collection in which some objects resurrect (they are reachable from outside the isolated cycles after the finalizers have been executed), do not block the collection of all objects that are still unreachable. (Contributed by Pablo Galindo and Tim Peters in :issue:`38379`.) os -- Added :data:`~os.CLD_KILLED` and :data:`~os.CLD_STOPPED` for :attr:`si_code`. (Contributed by Dong-hee Na in :issue:`38493`.) Exposed the Linux-specific :func:`os.pidfd_open` (:issue:`38692`) and :data:`os.P_PIDFD` (:issue:`38713`) for process management with file descriptors. threading --------- In a subinterpreter, spawning a daemon thread now raises a :exc:`RuntimeError`. Daemon threads were never supported in subinterpreters. Previously, the subinterpreter finalization crashed with a Python fatal error if a daemon thread was still running. (Contributed by Victor Stinner in :issue:`37266`.) venv ---- The activation scripts provided by :mod:`venv` now all specify their prompt customization consistently by always using the value specified by ``__VENV_PROMPT__``. Previously some scripts unconditionally used ``__VENV_PROMPT__``, others only if it happened to be set (which was the default case), and one used ``__VENV_NAME__`` instead. (Contributed by Brett Cannon in :issue:`37663`.) pathlib ------- Added :meth:`pathlib.Path.readlink()` which acts similarly to :func:`os.readlink`. (Contributed by Girts Folkmanis in :issue:`30618`) pprint ------ :mod:`pprint` can now pretty-print :class:`types.SimpleNamespace`. (Contributed by Carl Bordum Hansen in :issue:`37376`.) importlib --------- To improve consistency with import statements, :func:`importlib.util.resolve_name` now raises :exc:`ImportError` instead of :exc:`ValueError` for invalid relative import attempts. (Contributed by Ngalim Siregar in :issue:`37444`.) signal ------ Exposed the Linux-specific :func:`signal.pidfd_send_signal` for sending to signals to a process using a file descriptor instead of a pid. (:issue:`38712`) Optimizations ============= Build and C API Changes ======================= * Provide :c:func:`Py_EnterRecursiveCall` and :c:func:`Py_LeaveRecursiveCall` as regular functions for the limited API. Previously, there were defined as macros, but these macros didn't work with the limited API which cannot access ``PyThreadState.recursion_depth`` field. Remove ``_Py_CheckRecursionLimit`` from the stable ABI. (Contributed by Victor Stinner in :issue:`38644`.) * Add a new public :c:func:`PyObject_CallNoArgs` function to the C API, which calls a callable Python object without any arguments. It is the most efficient way to call a callable Python object without any argument. (Contributed by Victor Stinner in :issue:`37194`.) * The global variable :c:data:`PyStructSequence_UnnamedField` is now a constant and refers to a constant string. (Contributed by Serhiy Storchaka in :issue:`38650`.) * Exclude ``PyFPE_START_PROTECT()`` and ``PyFPE_END_PROTECT()`` macros of ``pyfpe.h`` from ``Py_LIMITED_API`` (stable API). (Contributed by Victor Stinner in :issue:`38835`.) * Remove ``PyMethod_ClearFreeList()`` and ``PyCFunction_ClearFreeList()`` functions: the free lists of bound method objects have been removed. (Contributed by Inada Naoki and Victor Stinner in :issue:`37340`.) * Remove ``PyUnicode_ClearFreeList()`` function: the Unicode free list has been removed in Python 3.3. (Contributed by Victor Stinner in :issue:`38896`.) Deprecated ========== * Currently :func:`math.factorial` accepts :class:`float` instances with non-negative integer values (like ``5.0``). It raises a :exc:`ValueError` for non-integral and negative floats. It is now deprecated. In future Python versions it will raise a :exc:`TypeError` for all floats. (Contributed by Serhiy Storchaka in :issue:`37315`.) * The :mod:`parser` module is deprecated and will be removed in future versions of Python. For the majority of use cases, users can leverage the Abstract Syntax Tree (AST) generation and compilation stage, using the :mod:`ast` module. * The :mod:`random` module currently accepts any hashable type as a possible seed value. Unfortunately, some of those types are not guaranteed to have a deterministic hash value. After Python 3.9, the module will restrict its seeds to :const:`None`, :class:`int`, :class:`float`, :class:`str`, :class:`bytes`, and :class:`bytearray`. * Opening the :class:`~gzip.GzipFile` file for writing without specifying the *mode* argument is deprecated. In future Python versions it will always be opened for reading by default. Specify the *mode* argument for opening it for writing and silencing a warning. (Contributed by Serhiy Storchaka in :issue:`28286`.) * Deprecated the ``split()`` method of :class:`_tkinter.TkappType` in favour of the ``splitlist()`` method which has more consistent and predicable behavior. (Contributed by Serhiy Storchaka in :issue:`38371`.) * The explicit passing of coroutine objects to :func:`asyncio.wait` has been deprecated and will be removed in version 3.11. (Contributed by Yury Selivanov and Kyle Stanley in :issue:`34790`.) Removed ======= * :class:`array.array`: ``tostring()`` and ``fromstring()`` methods have been removed. They were aliases to ``tobytes()`` and ``frombytes()``, deprecated since Python 3.2. (Contributed by Victor Stinner in :issue:`38916`.) * The abstract base classes in :mod:`collections.abc` no longer are exposed in the regular :mod:`collections` module. This will help create a clearer distinction between the concrete classes and the abstract base classes. * The undocumented ``sys.callstats()`` function has been removed. Since Python 3.7, it was deprecated and always returned :const:`None`. It required a special build option ``CALL_PROFILE`` which was already removed in Python 3.7. (Contributed by Victor Stinner in :issue:`37414`.) * The ``sys.getcheckinterval()`` and ``sys.setcheckinterval()`` functions have been removed. They were deprecated since Python 3.2. Use :func:`sys.getswitchinterval` and :func:`sys.setswitchinterval` instead. (Contributed by Victor Stinner in :issue:`37392`.) * The C function ``PyImport_Cleanup()`` has been removed. It was documented as: "Empty the module table. For internal use only." (Contributed by Victor Stinner in :issue:`36710`.) * ``_dummy_thread`` and ``dummy_threading`` modules have been removed. These modules were deprecated since Python 3.7 which requires threading support. (Contributed by Victor Stinner in :issue:`37312`.) * ``aifc.openfp()`` alias to ``aifc.open()``, ``sunau.openfp()`` alias to ``sunau.open()``, and ``wave.openfp()`` alias to :func:`wave.open()` have been removed. They were deprecated since Python 3.7. (Contributed by Victor Stinner in :issue:`37320`.) * The :meth:`~threading.Thread.isAlive()` method of :class:`threading.Thread` has been removed. It was deprecated since Python 3.8. Use :meth:`~threading.Thread.is_alive()` instead. (Contributed by Dong-hee Na in :issue:`37804`.) * Methods ``getchildren()`` and ``getiterator()`` in the :mod:`~xml.etree.ElementTree` module have been removed. They were deprecated in Python 3.2. Use functions :func:`list` and :func:`iter` instead. The ``xml.etree.cElementTree`` module has been removed. (Contributed by Serhiy Storchaka in :issue:`36543`.) * The old :mod:`plistlib` API has been removed, it was deprecated since Python 3.4. Use the :func:`~plistlib.load`, :func:`~plistlib.loads`, :func:`~plistlib.dump`, and :func:`~plistlib.dumps` functions. Additionally, the *use_builtin_types* parameter was removed, standard :class:`bytes` objects are always used instead. (Contributed by Jon Janzen in :issue:`36409`.) * The C function ``PyThreadState_DeleteCurrent()`` has been removed. It was not documented. (Contributed by Joannah Nanjekye in :issue:`37878`.) * The C function ``PyGen_NeedsFinalizing`` has been removed. It was not documented, tested, or used anywhere within CPython after the implementation of :pep:`442`. Patch by Joannah Nanjekye. (Contributed by Joannah Nanjekye in :issue:`15088`) Porting to Python 3.9 ===================== This section lists previously described changes and other bugfixes that may require changes to your code. Changes in the Python API ------------------------- * :func:`open`, :func:`io.open`, :func:`codecs.open` and :class:`fileinput.FileInput` no longer accept ``'U'`` ("universal newline") in the file mode. This flag was deprecated since Python 3.3. In Python 3, the "universal newline" is used by default when a file is open in text mode. The :ref:`newline parameter ` of :func:`open` controls how universal newlines works. (Contributed by Victor Stinner in :issue:`37330`.) * :func:`__import__` and :func:`importlib.util.resolve_name` now raise :exc:`ImportError` where it previously raised :exc:`ValueError`. Callers catching the specific exception type and supporting both Python 3.9 and earlier versions will need to catch both using ``except (ImportError, ValueError):``. * The :mod:`venv` activation scripts no longer special-case when ``__VENV_PROMPT__`` is set to ``""``. CPython bytecode changes ------------------------ * The :opcode:`LOAD_ASSERTION_ERROR` opcode was added for handling the :keyword:`assert` statement. Previously, the assert statement would not work correctly if the :exc:`AssertionError` exception was being shadowed. (Contributed by Zackery Spytz in :issue:`34880`.)