New Features in Python 3.9

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

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

You should check for DeprecationWarning in your code

When Python 2.7 was still supported, many functions were kept for backward compatibility with Python 2.7. With the end of Python 2.7 support, these backward compatibility layers have been removed, or will be removed soon. Most of them emitted a DeprecationWarning warning for several years. For example, using collections.Mapping instead of collections.abc.Mapping emits a DeprecationWarning since Python 3.3, released in 2012.

Test your application with the -W default command-line option to see DeprecationWarning and PendingDeprecationWarning, or even with -W error to treat them as errors. Warnings Filter can be used to ignore warnings from third-party code.

It has been decided to keep a few backward compatibility layers for one last release, to give more time to Python projects maintainers to organize the removal of the Python 2 support and add support for Python 3.9.

Aliases to Abstract Base Classes in the collections module, like collections.Mapping alias to collections.abc.Mapping, are kept for one last release for backward compatibility. They will be removed from Python 3.10.

More generally, try to run your tests in the Python Development Mode which helps to prepare your code to make it compatible with the next Python version.

New Features

Dictionary Merge & Update Operators

Merge (|) and update (|=) operators have been added to the built-in dict class. See PEP 584 for a full description. (Contributed by Brandt Bucher in bpo-36144.)

PEP 616: New removeprefix() and removesuffix() string methods

str.removeprefix(prefix) and str.removesuffix(suffix) have been added to easily remove an unneeded prefix or a suffix from a string. Corresponding bytes, bytearray, and collections.UserString methods have also been added. See PEP 616 for a full description. (Contributed by Dennis Sweeney in bpo-39939.)

PEP 585: Builtin Generic Types

In type annotations you can now use built-in collection types such as list and dict as generic types instead of importing the corresponding capitalized types (e.g. List or Dict) from typing. Some other types in the standard library are also now generic, for example queue.Queue.

Example:

def greet_all(names: list[str]) -> None:
    for name in names:
        print("Hello", name)

See PEP 585 for more details. (Contributed by Guido van Rossum, Ethan Smith, and Batuhan Taşkaya in bpo-39481.)

PEP 617: New Parser

Python 3.9 uses a new parser, based on PEG instead of LL(1). The new parser’s performance is roughly comparable to that of the old parser, but the PEG formalism is more flexible than LL(1) when it comes to designing new language features. We’ll start using this flexibility in Python 3.10 and later.

The ast module uses the new parser and produces the same AST as the old parser.

In Python 3.10, the old parser will be deleted and so will all functionality that depends on it (primarily the parser module, which has long been deprecated). In Python 3.9 only, you can switch back to the LL(1) parser using a command line switch (-X oldparser) or an environment variable (PYTHONOLDPARSER=1).

See PEP 617 for more details. (Contributed by Guido van Rossum, Pablo Galindo and Lysandros Nikolaou in bpo-40334.)

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 became 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, the 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.)
Any valid expression can now be used as a decorator. Previously, the grammar was much more restrictive. See PEP 614 for details. (Contributed by Brandt Bucher in bpo-39702.)
Improved help for the typing module. Docstrings are now shown for all special forms and special generic aliases (like Union and List). Using help() with generic alias like List[int] will show the help for the correspondent concrete type (list in this case). (Contributed by Serhiy Storchaka in bpo-40257.)

New Modules

zoneinfo

The zoneinfo module brings support for the IANA time zone database to the standard library. It adds zoneinfo.ZoneInfo, a concrete datetime.tzinfo implementation backed by the system’s time zone data.

Example:

>>> from zoneinfo import ZoneInfo
>>> from datetime import datetime, timedelta

>>> ## Daylight saving time
>>> dt = datetime(2020, 10, 31, 12, tzinfo=ZoneInfo("America/Los_Angeles"))
>>> print(dt)
2020-10-31 12:00:00-07:00
>>> dt.tzname()
'PDT'

>>> ## Standard time
>>> dt += timedelta(days=7)
>>> print(dt)
2020-11-07 12:00:00-08:00
>>> print(dt.tzname())
PST

As a fall-back source of data for platforms that don’t ship the IANA database, the tzdata module was released as a first-party package – distributed via PyPI and maintained by the CPython core team.

graphlib

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

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.)

Added docstrings to AST nodes that contains the ASDL signature used to construct that node. (Contributed by Batuhan Taskaya in bpo-39638.)

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)

Added a new coroutine asyncio.to_thread(). It is mainly used for running IO-bound functions in a separate thread to avoid blocking the event loop, and essentially works as a high-level version of run_in_executor() that can directly take keyword arguments. (Contributed by Kyle Stanley and Yury Selivanov in bpo-32309.)

compileall

Added new possibility to use hardlinks for duplicated .pyc files: hardlink_dupes parameter and –hardlink-dupes command line option. (Contributed by Lumír ‘Frenzy’ Balhar in bpo-40495.)

Added new options for path manipulation in resulting .pyc files: stripdir, prependdir, limit_sl_dest parameters and -s, -p, -e command line options. Added the possibility to specify the option for an optimization level multiple times. (Contributed by Lumír ‘Frenzy’ Balhar in bpo-38112.)

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.)

Removed daemon threads from ThreadPoolExecutor and ProcessPoolExecutor. This improves compatibility with subinterpreters and predictability in their shutdown processes. (Contributed by Kyle Stanley in bpo-39812.)

Workers in ProcessPoolExecutor are now spawned on demand, only when there are no available idle workers to reuse. This optimizes startup overhead and reduces the amount of lost CPU time to idle workers. (Contributed by Kyle Stanley in bpo-39207.)

curses

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

datetime

The isocalendar() of datetime.date and isocalendar() of datetime.datetime methods now returns a namedtuple() instead of a tuple. (Contributed by Dong-hee Na in bpo-24416.)

distutils

The upload command now creates SHA2-256 and Blake2b-256 hash digests. It skips MD5 on platforms that block MD5 digest. (Contributed by Christian Heimes in bpo-40698.)

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.)

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.)

hashlib

Builtin hash modules can now be disabled with ./configure --without-builtin-hashlib-hashes or selectively enabled with e.g. ./configure --with-builtin-hashlib-hashes=sha3,blake2 to force use of OpenSSL based implementation. (Contributed by Christian Heimes in bpo-40479)

http

HTTP status codes 103 EARLY_HINTS, 418 IM_A_TEAPOT and 425 TOO_EARLY are added to http.HTTPStatus. (Contributed by Dong-hee Na in bpo-39509 and Ross Rhodes in bpo-39507.)

IDLE and idlelib

Add option to toggle cursor blink off. (Contributed by Zackery Spytz in bpo-4603.)

Escape key now closes IDLE completion windows. (Contributed by Johnny Najera in bpo-38944.)

Add keywords to module name completion list. (Contributed by Terry J. Reedy in bpo-37765.)

The changes above have been backported to 3.8 maintenance releases.

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.)

imaplib.IMAP4.unselect() is added. imaplib.IMAP4.unselect() frees server’s resources associated with the selected mailbox and returns the server to the authenticated state. This command performs the same actions as imaplib.IMAP4.close(), except that no messages are permanently removed from the currently selected mailbox. (Contributed by Dong-hee Na in bpo-40375.)

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.)

inspect

inspect.BoundArguments.arguments is changed from OrderedDict to regular dict. (Contributed by Inada Naoki in bpo-36350 and bpo-39775.)

ipaddress

ipaddress now supports IPv6 Scoped Addresses (IPv6 address with suffix %<scope_id>).

Scoped IPv6 addresses can be parsed using ipaddress.IPv6Address. If present, scope zone ID is available through the scope_id attribute. (Contributed by Oleksandr Pavliuk in bpo-34788.)

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.)

multiprocessing

The multiprocessing.SimpleQueue class has a new close() method to explicitly close the queue. (Contributed by Victor Stinner in bpo-30966.)

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.)

Add os.waitstatus_to_exitcode() function: convert a wait status to an exit code. (Contributed by Victor Stinner in bpo-40094.)

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.)

pydoc

The documentation string is now shown not only for class, function, method etc, but for any object that has its own __doc__ attribute. (Contributed by Serhiy Storchaka in bpo-40257.)

random

Add a new random.Random.randbytes method: generate random bytes. (Contributed by Victor Stinner in bpo-40286.)

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.)

socket

The socket module now exports the CAN_RAW_JOIN_FILTERS constant on Linux 4.1 and greater. (Contributed by Stefan Tatschner and Zackery Spytz in bpo-25780.)

The socket module now supports the CAN_J1939 protocol on platforms that support it. (Contributed by Karl Ding in bpo-40291.)

time

On AIX, thread_time() is now implemented with thread_cputime() which has nanosecond resolution, rather than clock_gettime(CLOCK_THREAD_CPUTIME_ID) which has a resolution of 10 ms. (Contributed by Batuhan Taskaya in bpo-40192)

sys

Add a new sys.platlibdir attribute: name of the platform-specific library directory. It is used to build the path of standard library and the paths of installed extension modules. It is equal to "lib" on most platforms. On Fedora and SuSE, it is equal to "lib64" on 64-bit platforms. (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in bpo-1294959.)

Previously, sys.stderr was block-buffered when non-interactive. Now stderr defaults to always being line-buffered. (Contributed by Jendrik Seipp in bpo-13601.)

tracemalloc

Added tracemalloc.reset_peak() to set the peak size of traced memory blocks to the current size, to measure the peak of specific pieces of code. (Contributed by Huon Wilson in bpo-40630.)

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.)

unicodedata

The Unicode database has been updated to version 13.0.0. (bpo-39926).

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.)

xml

White space characters within attributes are now preserved when serializing xml.etree.ElementTree to XML file. EOLNs are no longer normalized to “n”. This is the result of discussion about how to interpret section 2.11 of XML spec. (Contributed by Mefistotelis in bpo-39011.)

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 the := operator this idiom does not leak a variable to the outer scope.
(Contributed by Serhiy Storchaka in bpo-32856.)
Optimize signal handling in multithreaded applications. If a thread different than the main thread gets a signal, the bytecode evaluation loop is no longer interrupted at each bytecode instruction to check for pending signals which cannot be handled. Only the main thread of the main interpreter can handle signals.
Previously, the bytecode evaluation loop was interrupted at each instruction until the main thread handles signals. (Contributed by Victor Stinner in bpo-40010.)
Optimize the subprocess module on FreeBSD using closefrom(). (Contributed by Ed Maste, Conrad Meyer, Kyle Evans, Kubilay Kocak and Victor Stinner in bpo-38061.)

Here’s a summary of performance improvements from Python 3.4 through Python 3.9:

Python version                       3.4     3.5     3.6     3.7     3.8    3.9
--------------                       ---     ---     ---     ---     ---    ---

Variable and attribute read access:
    read_local                       7.1     7.1     5.4     5.1     3.9    4.0
    read_nonlocal                    7.1     8.1     5.8     5.4     4.4    4.8
    read_global                     15.5    19.0    14.3    13.6     7.6    7.7
    read_builtin                    21.1    21.6    18.5    19.0     7.5    7.7
    read_classvar_from_class        25.6    26.5    20.7    19.5    18.4   18.6
    read_classvar_from_instance     22.8    23.5    18.8    17.1    16.4   20.1
    read_instancevar                32.4    33.1    28.0    26.3    25.4   27.7
    read_instancevar_slots          27.8    31.3    20.8    20.8    20.2   24.5
    read_namedtuple                 73.8    57.5    45.0    46.8    18.4   23.2
    read_boundmethod                37.6    37.9    29.6    26.9    27.7   45.9

Variable and attribute write access:
    write_local                      8.7     9.3     5.5     5.3     4.3    4.2
    write_nonlocal                  10.5    11.1     5.6     5.5     4.7    4.9
    write_global                    19.7    21.2    18.0    18.0    15.8   17.2
    write_classvar                  92.9    96.0   104.6   102.1    39.2   43.2
    write_instancevar               44.6    45.8    40.0    38.9    35.5   40.7
    write_instancevar_slots         35.6    36.1    27.3    26.6    25.7   27.7

Data structure read access:
    read_list                       24.2    24.5    20.8    20.8    19.0   21.1
    read_deque                      24.7    25.5    20.2    20.6    19.8   21.6
    read_dict                       24.3    25.7    22.3    23.0    21.0   22.5
    read_strdict                    22.6    24.3    19.5    21.2    18.9   21.6

Data structure write access:
    write_list                      27.1    28.5    22.5    21.6    20.0   21.6
    write_deque                     28.7    30.1    22.7    21.8    23.5   23.2
    write_dict                      31.4    33.3    29.3    29.2    24.7   27.8
    write_strdict                   28.4    29.9    27.5    25.2    23.1   29.8

Stack (or queue) operations:
    list_append_pop                 93.4   112.7    75.4    74.2    50.8   53.9
    deque_append_pop                43.5    57.0    49.4    49.2    42.5   45.5
    deque_append_popleft            43.7    57.3    49.7    49.7    42.8   45.5

Timing loop:
    loop_overhead                    0.5     0.6     0.4     0.3     0.3    0.3

These results were generated from the variable access benchmark script at: Tools/scripts/var_access_benchmark.py. The benchmark script displays timings in nanoseconds. The benchmarks were measured on an Intel® Core™ i7-4960HQ processor running the macOS 64-bit builds found at python.org.

A number of Python builtins (range, tuple, set, frozenset, list, dict) are now sped up by using PEP 590 vectorcall protocol. (Contributed by Dong-hee Na, Mark Shannon, Jeroen Demeyer and Petr Viktorin in bpo-37207.)

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 and symbol modules are 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 Public C API functions PyParser_SimpleParseStringFlags(), PyParser_SimpleParseStringFlagsFilename(), PyParser_SimpleParseFileFlags() and PyNode_Compile() are deprecated and will be removed in Python 3.10 together with the old parser.
Using NotImplemented in a boolean context has been deprecated, as it is almost exclusively the result of incorrect rich comparator implementations. It will be made a TypeError in a future version of Python. (Contributed by Josh Rosenberg in bpo-35712.)
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:
b2a_hqx(), a2b_hqx()
rlecode_hqx(), rledecode_hqx()
(Contributed by Victor Stinner in bpo-39353.)
ast classes slice, Index and ExtSlice are considered deprecated and will be removed in future Python versions. value itself should be used instead of Index(value). Tuple(slices, Load()) should be used instead of ExtSlice(slices). (Contributed by Serhiy Storchaka in bpo-32892.)
ast classes Suite, Param, AugLoad and AugStore are considered deprecated and will be removed in future Python versions. They were not generated by the parser and not accepted by the code generator in Python 3. (Contributed by Batuhan Taskaya in bpo-39639 and bpo-39969 and Serhiy Storchaka in bpo-39988.)
The PyEval_InitThreads() and PyEval_ThreadsInitialized() functions are now deprecated and will be removed in Python 3.11. Calling PyEval_InitThreads() now does nothing. The GIL is initialized by Py_Initialize() since Python 3.7. (Contributed by Victor Stinner in bpo-39877.)
Passing None as the first argument to the shlex.split() function has been deprecated. (Contributed by Zackery Spytz in bpo-33262.)
The lib2to3 module now emits a PendingDeprecationWarning. Python 3.9 switched to a PEG parser (see PEP 617), and Python 3.10 may include new language syntax that is not parsable by lib2to3’s LL(1) parser. The lib2to3 module may be removed from the standard library in a future Python version. Consider third-party alternatives such as LibCST or parso. (Contributed by Carl Meyer in bpo-40360.)
The random parameter of random.shuffle() has been deprecated. (Contributed by Raymond Hettinger in bpo-40465)

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() of classes ElementTree and Element in the ElementTree module have been removed. They were deprecated in Python 3.2. Use iter(x) or list(x) instead of x.getchildren() and x.iter() or list(x.iter()) instead of x.getiterator(). (Contributed by Serhiy Storchaka in bpo-36543.)
The old [plistlib](https://docs.python.org/3.9/library/plistlib.html#module-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 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.)
The _field_types attribute of the typing.NamedTuple class has been removed. It was deprecated since Python 3.8. Use the __annotations__ attribute instead. (Contributed by Serhiy Storchaka in bpo-40182.)
The symtable.SymbolTable.has_exec() method has been removed. It was deprecated since 2006, and only returning False when it’s called. (Contributed by Batuhan Taskaya in bpo-40208)
The asyncio.Task.current_task() and asyncio.Task.all_tasks() have been removed. They were deprecated since Python 3.7 and you can use asyncio.current_task() and asyncio.all_tasks() instead. (Contributed by Rémi Lapeyre in bpo-40967)
The unescape() method in the html.parser.HTMLParser class has been removed (it was deprecated since Python 3.4). html.unescape() should be used for converting character references to the corresponding unicode characters.

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

__import__() and importlib.util.resolve_name() now raise ImportError where it previously raised 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 venv activation scripts no longer special-case when __VENV_PROMPT__ is set to "".
The select.epoll.unregister() method no longer ignores the EBADF error. (Contributed by Victor Stinner in bpo-39239.)
The compresslevel parameter of bz2.BZ2File became keyword-only, since the buffering parameter has been removed. (Contributed by Victor Stinner in bpo-39357.)
Simplified AST for subscription. Simple indices will be represented by their value, extended slices will be represented as tuples. Index(value) will return a value itself, ExtSlice(slices) will return Tuple(slices, Load()). (Contributed by Serhiy Storchaka in bpo-34822.)
The importlib module now ignores the PYTHONCASEOK environment variable when the -E or -I command line options are being used.
The encoding parameter has been added to the classes ftplib.FTP and ftplib.FTP_TLS as a keyword-only parameter, and the default encoding is changed from Latin-1 to UTF-8 to follow RFC 2640.
asyncio.loop.shutdown_default_executor() has been added to AbstractEventLoop, meaning alternative event loops that inherit from it should have this method defined. (Contributed by Kyle Stanley in bpo-34037.)
The constant values of future flags in the __future__ module is updated in order to prevent collision with compiler flags. Previously PyCF_ALLOW_TOP_LEVEL_AWAIT was clashing with CO_FUTURE_DIVISION. (Contributed by Batuhan Taskaya in bpo-39562)
array('u') now uses wchar_t as C type instead of Py_UNICODE. This change doesn’t affect to its behavior because Py_UNICODE is alias of wchar_t since Python 3.3. (Contributed by Inada Naoki in bpo-34538.)

Changes in the C API

Instances of heap-allocated types (such as those created with PyType_FromSpec() and similar APIs) hold a reference to their type object since Python 3.8. As indicated in the “Changes in the C API” of Python 3.8, for the vast majority of cases, there should be no side effect but for types that have a custom tp_traverse function, ensure that all custom tp_traverse functions of heap-allocated types visit the object’s type.

Example:

int
foo_traverse(foo_struct *self, visitproc visit, void *arg) {
// Rest of the traverse function
#if PY_VERSION_HEX >= 0x03090000
    // This was not needed before Python 3.9 (Python issue 35810 and 40217)
    Py_VISIT(Py_TYPE(self));
#endif
}

If your traverse function delegates to tp_traverse of its base class (or another type), ensure that Py_TYPE(self) is visited only once. Note that only heap types are expected to visit the type in tp_traverse.

For example, if your tp_traverse function includes:

base->tp_traverse(self, visit, arg)

then add:

#if PY_VERSION_HEX >= 0x03090000
    // This was not needed before Python 3.9 (Python issue 35810 and 40217)
    if (base->tp_flags & Py_TPFLAGS_HEAPTYPE) {
        // a heap type's tp_traverse already visited Py_TYPE(self)
    } else {
        Py_VISIT(Py_TYPE(self));
    }
#else

(See bpo-35810 and bpo-40217 for more information.)

CPython bytecode changes

The LOAD_ASSERTION_ERROR opcode was added for handling the assert statement. Previously, the assert statement would not work correctly if the AssertionError exception was being shadowed. (Contributed by Zackery Spytz in bpo-34880.)

Build Changes

Add --with-platlibdir option to the configure script: name of the platform-specific library directory, stored in the new sys.platlibdir attribute. See sys.platlibdir attribute for more information. (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in bpo-1294959.)
The COUNT_ALLOCS special build macro has been removed. (Contributed by Victor Stinner in bpo-39489.)
On non-Windows platforms, the setenv() and unsetenv() functions are now required to build Python. (Contributed by Victor Stinner in bpo-39395.)

C API Changes

New Features

PEP 573: Add PyType_FromModuleAndSpec() to associate a module with a class; PyType_GetModule() and PyType_GetModuleState() to retrieve the module and its state; and PyCMethod and METH_METHOD to allow a method to access the class it was defined in. (Contributed by Marcel Plch and Petr Viktorin in bpo-38787.)
Add PyFrame_GetCode() function: get a frame code. Add PyFrame_GetBack() function: get the frame next outer frame. (Contributed by Victor Stinner in bpo-40421.)
Add PyFrame_GetLineNumber() to the limited C API. (Contributed by Victor Stinner in bpo-40421.)
Add PyThreadState_GetInterpreter() and PyInterpreterState_Get() functions to get the interpreter. Add PyThreadState_GetFrame() function to get the current frame of a Python thread state. Add PyThreadState_GetID() function: get the unique identifier of a Python thread state. (Contributed by Victor Stinner in bpo-39947.)
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.)
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).
PyObject_INIT() and PyObject_INIT_VAR() become regular “opaque” function to hide implementation details.
(Contributed by Victor Stinner in bpo-38644 and bpo-39542.)
The PyModule_AddType() function is added to help adding a type to a module. (Contributed by Dong-hee Na in bpo-40024.)
Add the functions PyObject_GC_IsTracked() and PyObject_GC_IsFinalized() to the public API to allow to query if Python objects are being currently tracked or have been already finalized by the garbage collector respectively. (Contributed by Pablo Galindo in bpo-40241.)

Porting to Python 3.9

PyInterpreterState.eval_frame (PEP 523) now requires a new mandatory tstate parameter (PyThreadState*). (Contributed by Victor Stinner in bpo-38500.)
Extension modules: m_traverse, m_clear and m_free functions of PyModuleDef are no longer called if the module state was requested but is not allocated yet. This is the case immediately after the module is created and before the module is executed (Py_mod_exec function). More precisely, these functions are not called if m_size is greater than 0 and the module state (as returned by PyModule_GetState()) is NULL.
Extension modules without module state (m_size <= 0) are not affected.
If Py_AddPendingCall() is called in a subinterpreter, the function is now scheduled to be called from the subinterpreter, rather than being called from the main interpreter. Each subinterpreter now has its own list of scheduled calls. (Contributed by Victor Stinner in bpo-39984.)
The Windows registry is no longer used to initialize [sys.path](https://docs.python.org/3.9/library/sys.html#sys.path) when the -E option is used (if PyConfig.use_environment is set to 0). This is significant when embedding Python on Windows. (Contributed by Zackery Spytz in bpo-8901.)
The global variable PyStructSequence_UnnamedField is now a constant and refers to a constant string. (Contributed by Serhiy Storchaka in bpo-38650.)
The PyGC_Head structure is now opaque. It is only defined in the internal C API (pycore_gc.h). (Contributed by Victor Stinner in bpo-40241.)
The Py_UNICODE_COPY, Py_UNICODE_FILL, PyUnicode_WSTR_LENGTH, PyUnicode_FromUnicode(), PyUnicode_AsUnicode(), _PyUnicode_AsUnicode, and PyUnicode_AsUnicodeAndSize() are marked as deprecated in C. They have been deprecated by PEP 393 since Python 3.3. (Contributed by Inada Naoki in bpo-36346.)

Removed

Exclude PyFPE_START_PROTECT() and PyFPE_END_PROTECT() macros of pyfpe.h from the limited C API. (Contributed by Victor Stinner in bpo-38835.)
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.)
Changes in the limited C API (if Py_LIMITED_API macro is defined):
Exclude the following functions from the limited C API:
PyThreadState_DeleteCurrent() (Contributed by Joannah Nanjekye in bpo-37878.)
_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
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.)
Remove _PyRuntime.getframe hook and remove _PyThreadState_GetFrame macro which was an alias to _PyRuntime.getframe. They were only exposed by the internal C API. Remove also PyThreadFrameGetter type. (Contributed by Victor Stinner in bpo-39946.)
Remove the following functions from the C API. Call PyGC_Collect() explicitly to clear all free lists. (Contributed by Inada Naoki and Victor Stinner in bpo-37340, bpo-38896 and bpo-40428.)
PyAsyncGen_ClearFreeLists()
PyContext_ClearFreeList()
PyDict_ClearFreeList()
PyFloat_ClearFreeList()
PyFrame_ClearFreeList()
PyList_ClearFreeList()
PyMethod_ClearFreeList() and PyCFunction_ClearFreeList(): the free lists of bound method objects have been removed.
PySet_ClearFreeList(): the set free list has been removed in Python 3.4.
PyTuple_ClearFreeList()
PyUnicode_ClearFreeList(): the Unicode free list has been removed in Python 3.3.
Remove _PyUnicode_ClearStaticStrings() function. (Contributed by Victor Stinner in bpo-39465.)
Remove Py_UNICODE_MATCH. It has been deprecated by PEP 393, and broken since Python 3.3. The [PyUnicode_Tailmatch()](https://docs.python.org/3.9/c-api/unicode.html#c.PyUnicode_Tailmatch) function can be used instead. (Contributed by Inada Naoki in bpo-36346.)

#python #web-development #machine-learning #programming #developer

Summary – Release highlights

You should check for DeprecationWarning in your code

New Features

Dictionary Merge & Update Operators

PEP 616: New removeprefix() and removesuffix() string methods

PEP 585: Builtin Generic Types

PEP 617: New Parser

Other Language Changes

New Modules

zoneinfo

graphlib

Improved Modules

ast

asyncio

compileall

concurrent.futures

curses

datetime

distutils

fcntl

ftplib

gc

hashlib

http

IDLE and idlelib

imaplib

importlib

inspect

ipaddress

math

multiprocessing

nntplib

os

pathlib

poplib

pprint

pydoc

random

signal

smtplib

socket

time

sys

tracemalloc

typing

unicodedata

venv

xml

Optimizations

Deprecated

Removed

Porting to Python 3.9

Changes in the Python API

Changes in the C API

CPython bytecode changes

Build Changes

C API Changes

New Features

Porting to Python 3.9

Removed

docs.python.org

New Features in Python 3.9