Python 3.9

Python 3.9.0 (final) was released on the 5th October 2020. The Features for 3.9 are defined in PEP 596 and a detailed description of the changes can be found in What’s New in Python 3.9

Features

Status

PEP 573

Fast access to module state from methods of C extension types

PEP 584

Union operators added to dict

PEP 585

Type hinting generics in standard collections

PEP 593

Flexible function and variable annotations

PEP 602

CPython adopts an annual release cycle. Instead of annual, aiming for two month release cycle

PEP 614

Relaxed grammar restrictions on decorators

PEP 615

The IANA Time Zone Database is now present in the standard library in the zoneinfo module

PEP 616

String methods to remove prefixes and suffixes

PEP 617

CPython now uses a new parser based on PEG

Other Language Changes:

__import__() now raises ImportError instead of ValueError

Complete

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

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

“”.replace(“”, s, n) now returns s instead of an empty string for all non-zero n. It is now consistent with “”.replace(“”, s)

Any valid expression can now be used as a decorator. Previously, the grammar was much more restrictive

Parallel running of aclose() / asend() / athrow() is now prohibited, and ag_running now reflects the actual running status of the async generator

Unexpected errors in calling the __iter__ method are no longer masked by TypeError in the in operator and functions contains(), indexOf() and countOf() of the operator module

Unparenthesized lambda expressions can no longer be the expression part in an if clause in comprehensions and generator expressions

Changes to built-in modules:

asyncio

Due to significant security concerns, the reuse_address parameter of asyncio.loop.create_datagram_endpoint() is no longer supported

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.

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

added a new coroutine asyncio.to_thread()

When cancelling the task due to a timeout, asyncio.wait_for() will now wait until the cancellation is complete also in the case when timeout is <= 0, like it does with positive timeouts

asyncio now raises TyperError when calling incompatible methods with an ssl.SSLSocket socket

gc

Garbage collection does not block on resurrected objects

Added a new function gc.is_finalized() to check if an object has been finalized by the garbage collector

math

Expanded the math.gcd() function to handle multiple arguments. Formerly, it only supported two arguments

Added math.lcm(): return the least common multiple of specified arguments

Added math.nextafter(): return the next floating-point value after x towards y

Added math.ulp(): return the value of the least significant bit of a float

os

Exposed the Linux-specific os.pidfd_open() and os.P_PIDFD

The os.unsetenv() function is now also available on Windows

Complete

The os.putenv() and os.unsetenv() functions are now always available

Complete

Added os.waitstatus_to_exitcode() function: convert a wait status to an exit code

random

Added a new random.Random.randbytes method: generate random bytes

sys

Added a new sys.platlibdir attribute: name of the platform-specific library directory

Previously, sys.stderr was block-buffered when non-interactive. Now stderr defaults to always being line-buffered