This article explains the new features in Python 3.9, compared to 3.8.

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

• :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 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`.)

• None yet.

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`.)

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`)

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`.)

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`.)

:class:`~ftplib.FTP` and :class:`~ftplib.FTP_TLS` now raise a :class:`ValueError` if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Dong-hee Na in :issue:`39259`.)

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`.)

Added a new function :func:`gc.is_finalized` to check if an object has been finalized by the garbage collector. (Contributed by Pablo Galindo in :issue:`39322`.)

:class:`~imaplib.IMAP4` and :class:`~imaplib.IMAP4_SSL` now have an optional timeout parameter for their constructors. Also, the :meth:`~imaplib.IMAP4.open` method now has an optional timeout parameter with this change. The overridden methods of :class:`~imaplib.IMAP4_SSL` and :class:`~imaplib.IMAP4_stream` were applied to this change. (Contributed by Dong-hee Na in :issue:`38615`.)

Add :func:`math.nextafter`: return the next floating-point value after x towards y. (Contributed by Victor Stinner in :issue:`39288`.)

Add :func:`math.ulp`: return the value of the least significant bit of a float. (Contributed by Victor Stinner in :issue:`39310`.)

:class:`~nntplib.NNTP` and :class:`~nntplib.NNTP_SSL` now raise a :class:`ValueError` if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Dong-hee Na in :issue:`39259`.)

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.

:class:`~poplib.POP3` and :class:`~poplib.POP3_SSL` now raise a :class:`ValueError` if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Dong-hee Na in :issue:`39259`.)

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`.)

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`.)

Added :meth:`pathlib.Path.readlink()` which acts similarly to :func:`os.readlink`. (Contributed by Girts Folkmanis in :issue:`30618`)

:mod:`pprint` can now pretty-print :class:`types.SimpleNamespace`. (Contributed by Carl Bordum Hansen in :issue:`37376`.)

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`.)

:class:`~smtplib.SMTP` and :class:`~smtplib.SMTP_SSL` now raise a :class:`ValueError` if the given timeout for their constructor is zero to prevent the creation of a non-blocking socket. (Contributed by Dong-hee Na in :issue:`39259`.)

:class:`~smtplib.LMTP` constructor now has an optional timeout parameter. (Contributed by Dong-hee Na in :issue:`39329`.)

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`)

• 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`.)

• 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`.)

• :class:`nntplib.NNTP`: xpath() and xgtitle() methods have been removed. These methods are deprecated since Python 3.3. Generally, these extensions are not supported or not enabled by NNTP server administrators. For xgtitle(), please use :meth:`nntplib.NNTP.descriptions` or :meth:`nntplib.NNTP.description` instead. (Contributed by Dong-hee Na in :issue:`39366`.)
• :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`)

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

• :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 <open-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 "".
• The :meth:`select.epoll.unregister` method no longer ignores the :data:`~errno.EBADF` error. (Contributed by Victor Stinner in :issue:`39239`.)

• 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`.)