What's New In Python 3.9

Release

3.9.0a3

Date

二月 25, 2020

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

For full details, see the changelog.

注解

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

New Features

Other Language Changes

  • __import__() now raises ImportError instead of ValueError, which used to occur when a relative import went past its top-level package. (Contributed by Ngalim Siregar in bpo-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 __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 os.chdir(). As a side effect, a traceback also displays the absolute path for __main__ module frames in this case. (Contributed by Victor Stinner in bpo-20443.)

  • In the Python Development Mode and in debug build, the encoding and errors arguments are now checked for string encoding and decoding operations. Examples: open(), str.encode() and 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 bpo-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 bytes and bytearray objects. (Contributed by Serhiy Storchaka in bpo-28029.)

New Modules

  • None yet.

Improved Modules

ast

Added the indent option to dump() which allows it to produce a multiline indented output. (Contributed by Serhiy Storchaka in bpo-37995.)

Added ast.unparse() as a function in the ast module that can be used to unparse an ast.AST object and produce a string with code that would produce an equivalent ast.AST object when parsed. (Contributed by Pablo Galindo and Batuhan Taskaya in bpo-38870.)

asyncio

Due to significant security concerns, the reuse_address parameter of 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 bpo-37228.)

Added a new coroutine shutdown_default_executor() that schedules a shutdown for the default executor that waits on the ThreadPoolExecutor to finish closing. Also, asyncio.run() has been updated to use the new coroutine. (Contributed by Kyle Stanley in bpo-34037.)

Added asyncio.PidfdChildWatcher, a Linux-specific child watcher implementation that polls process file descriptors. (bpo-38692)

concurrent.futures

Added a new cancel_futures parameter to concurrent.futures.Executor.shutdown() that cancels all pending futures which have not started running, instead of waiting for them to complete before shutting down the executor. (Contributed by Kyle Stanley in bpo-39349.)

curses

Add curses.get_escdelay(), curses.set_escdelay(), curses.get_tabsize(), and curses.set_tabsize() functions. (Contributed by Anthony Sottile in bpo-38312.)

fcntl

Added constants F_OFD_GETLK, F_OFD_SETLK and F_OFD_SETLKW. (Contributed by Dong-hee Na in bpo-38602.)

ftplib

FTP and FTP_TLS now raise a 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 bpo-39259.)

functools

Add the functools.TopologicalSorter class to offer functionality to perform topological sorting of graphs. (Contributed by Pablo Galindo, Tim Peters and Larry Hastings in bpo-17005.)

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 bpo-38379.)

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

imaplib

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

importlib

To improve consistency with import statements, importlib.util.resolve_name() now raises ImportError instead of ValueError for invalid relative import attempts. (Contributed by Ngalim Siregar in bpo-37444.)

math

Expanded the math.gcd() function to handle multiple arguments. Formerly, it only supported two arguments. (Contributed by Serhiy Storchaka in bpo-39648.)

Add math.lcm(): return the least common multiple of specified arguments. (Contributed by Mark Dickinson, Ananthakrishnan and Serhiy Storchaka in bpo-39479 and bpo-39648.)

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

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

nntplib

NNTP and NNTP_SSL now raise a 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 bpo-39259.)

os

Added CLD_KILLED and CLD_STOPPED for si_code. (Contributed by Dong-hee Na in bpo-38493.)

Exposed the Linux-specific os.pidfd_open() (bpo-38692) and os.P_PIDFD (bpo-38713) for process management with file descriptors.

The os.unsetenv() function is now also available on Windows. (Contributed by Victor Stinner in bpo-39413.)

The os.putenv() and os.unsetenv() functions are now always available. (Contributed by Victor Stinner in bpo-39395.)

pathlib

Added pathlib.Path.readlink() which acts similarly to os.readlink(). (Contributed by Girts Folkmanis in bpo-30618)

poplib

POP3 and POP3_SSL now raise a 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 bpo-39259.)

pprint

pprint can now pretty-print types.SimpleNamespace. (Contributed by Carl Bordum Hansen in bpo-37376.)

signal

Exposed the Linux-specific signal.pidfd_send_signal() for sending to signals to a process using a file descriptor instead of a pid. (bpo-38712)

smtplib

SMTP and SMTP_SSL now raise a 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 bpo-39259.)

LMTP constructor now has an optional timeout parameter. (Contributed by Dong-hee Na in bpo-39329.)

threading

In a subinterpreter, spawning a daemon thread now raises a 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 bpo-37266.)

typing

PEP 593 introduced an typing.Annotated type to decorate existing types with context-specific metadata and new include_extras parameter to typing.get_type_hints() to access the metadata at runtime. (Contributed by Till Varoquaux and Konstantin Kashin.)

venv

The activation scripts provided by 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 bpo-37663.)

Optimizations

  • Optimized the idiom for assignment a temporary variable in comprehensions. Now for y in [expr] in comprehensions is as fast as a simple assignment y = expr. For example:

    sums = [s for s in [0] for x in data for s in [s + x]]

    Unlike to the := operator this idiom does not leak a variable to the outer scope.

    (Contributed by Serhiy Storchaka in bpo-32856.)

Build and C API Changes

  • Add a new public 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 bpo-37194.)

  • The global variable PyStructSequence_UnnamedField is now a constant and refers to a constant string. (Contributed by Serhiy Storchaka in bpo-38650.)

  • Exclude PyFPE_START_PROTECT() and PyFPE_END_PROTECT() macros of pyfpe.h from Py_LIMITED_API (stable API). (Contributed by Victor Stinner in bpo-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 bpo-37340.)

  • Remove PyUnicode_ClearFreeList() function: the Unicode free list has been removed in Python 3.3. (Contributed by Victor Stinner in bpo-38896.)

  • The tp_print slot of PyTypeObject has been removed. It was used for printing objects to files in Python 2.7 and before. Since Python 3.0, it has been ignored and unused. (Contributed by Jeroen Demeyer in bpo-36974.)

  • On non-Windows platforms, the setenv() and unsetenv() functions are now required to build Python. (Contributed by Victor Stinner in bpo-39395.)

  • The COUNT_ALLOCS special build macro has been removed. (Contributed by Victor Stinner in bpo-39489.)

  • Changes in the limited C API (if Py_LIMITED_API macro is defined):

    • Provide Py_EnterRecursiveCall() and Py_LeaveRecursiveCall() as regular functions for the limited API. Previously, there were defined as macros, but these macros didn't compile with the limited C API which cannot access PyThreadState.recursion_depth field (the structure is opaque in the limited C API).

    • Exclude the following functions from the limited C API:

      • _Py_CheckRecursionLimit

      • _Py_NewReference()

      • _Py_ForgetReference()

      • _PyTraceMalloc_NewReference()

      • _Py_GetRefTotal()

      • The trashcan mechanism which never worked in the limited C API.

      • PyTrash_UNWIND_LEVEL

      • Py_TRASHCAN_BEGIN_CONDITION

      • Py_TRASHCAN_BEGIN

      • Py_TRASHCAN_END

      • Py_TRASHCAN_SAFE_BEGIN

      • Py_TRASHCAN_SAFE_END

    • The following static inline functions or macros become regular "opaque" function to hide implementation details:

      • _Py_NewReference()

      • PyObject_INIT() and PyObject_INIT_VAR() become aliases to PyObject_Init() and PyObject_InitVar() in the limited C API, but are overriden with static inline function otherwise. Thanks to that, it was possible to exclude _Py_NewReference() from the limited C API.

    • Move following functions and definitions to the internal C API:

      • _PyDebug_PrintTotalRefs()

      • _Py_PrintReferences()

      • _Py_PrintReferenceAddresses()

      • _Py_tracemalloc_config

      • _Py_AddToAllObjects() (specific to Py_TRACE_REFS build)

    (Contributed by Victor Stinner in bpo-38644 and bpo-39542.)

Deprecated

  • The distutils bdist_msi command is now deprecated, use bdist_wheel (wheel packages) instead. (Contributed by Hugo van Kemenade in bpo-39586.)

  • Currently math.factorial() accepts float instances with non-negative integer values (like 5.0). It raises a ValueError for non-integral and negative floats. It is now deprecated. In future Python versions it will raise a TypeError for all floats. (Contributed by Serhiy Storchaka in bpo-37315.)

  • The 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 ast module.

  • The 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 None, int, float, str, bytes, and bytearray.

  • Opening the 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 bpo-28286.)

  • Deprecated the split() method of _tkinter.TkappType in favour of the splitlist() method which has more consistent and predicable behavior. (Contributed by Serhiy Storchaka in bpo-38371.)

  • The explicit passing of coroutine objects to asyncio.wait() has been deprecated and will be removed in version 3.11. (Contributed by Yury Selivanov and Kyle Stanley in bpo-34790.)

  • binhex4 and hexbin4 standards are now deprecated. The :binhex module and the following binascii functions are now deprecated:

    (Contributed by Victor Stinner in bpo-39353.)

Removed

  • The erroneous version at unittest.mock.__version__ has been removed.

  • 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 nntplib.NNTP.descriptions() or nntplib.NNTP.description() instead. (Contributed by Dong-hee Na in bpo-39366.)

  • 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 bpo-38916.)

  • The undocumented sys.callstats() function has been removed. Since Python 3.7, it was deprecated and always returned None. It required a special build option CALL_PROFILE which was already removed in Python 3.7. (Contributed by Victor Stinner in bpo-37414.)

  • The sys.getcheckinterval() and sys.setcheckinterval() functions have been removed. They were deprecated since Python 3.2. Use sys.getswitchinterval() and sys.setswitchinterval() instead. (Contributed by Victor Stinner in bpo-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 bpo-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 bpo-37312.)

  • aifc.openfp() alias to aifc.open(), sunau.openfp() alias to sunau.open(), and wave.openfp() alias to wave.open() have been removed. They were deprecated since Python 3.7. (Contributed by Victor Stinner in bpo-37320.)

  • The isAlive() method of threading.Thread has been removed. It was deprecated since Python 3.8. Use is_alive() instead. (Contributed by Dong-hee Na in bpo-37804.)

  • Methods getchildren() and getiterator() in the ElementTree module have been removed. They were deprecated in Python 3.2. Use functions list() and iter() instead. The xml.etree.cElementTree module has been removed. (Contributed by Serhiy Storchaka in bpo-36543.)

  • The old plistlib API has been removed, it was deprecated since Python 3.4. Use the load(), loads(), dump(), and dumps() functions. Additionally, the use_builtin_types parameter was removed, standard bytes objects are always used instead. (Contributed by Jon Janzen in bpo-36409.)

  • The C function PyThreadState_DeleteCurrent() has been removed. It was not documented. (Contributed by Joannah Nanjekye in bpo-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 bpo-15088)

  • base64.encodestring() and base64.decodestring(), aliases deprecated since Python 3.1, have been removed: use base64.encodebytes() and base64.decodebytes() instead. (Contributed by Victor Stinner in bpo-39351.)

  • fractions.gcd() function has been removed, it was deprecated since Python 3.5 (bpo-22486): use math.gcd() instead. (Contributed by Victor Stinner in bpo-39350.)

  • The buffering parameter of bz2.BZ2File has been removed. Since Python 3.0, it was ignored and using it emitted a DeprecationWarning. Pass an open file object to control how the file is opened. (Contributed by Victor Stinner in bpo-39357.)

  • The encoding parameter of json.loads() has been removed. As of Python 3.1, it was deprecated and ignored; using it has emitted a DeprecationWarning since Python 3.8. (Contributed by Inada Naoki in bpo-39377)

  • with (await asyncio.lock): and with (yield from asyncio.lock): statements are not longer supported, use async with lock instead. The same is correct for asyncio.Condition and asyncio.Semaphore. (Contributed by Andrew Svetlov in bpo-34793.)

  • The sys.getcounts() function, the -X showalloccount command line option and the show_alloc_count field of the C structure PyConfig have been removed. They required a special Python build by defining COUNT_ALLOCS macro. (Contributed by Victor Stinner in bpo-39489.)

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

CPython bytecode changes