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 Completed
Python now gets the absolute path of the script filename specified on the command line (ex: python3 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:

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, 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  
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  
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  
Exposed the Linux-specific os.pidfd_open() and os.P_PIDFD  
The os.unsetenv() function is now also available on Windows Completed
The os.putenv() and os.unsetenv() functions are now always available Completed
Added os.waitstatus_to_exitcode() function: convert a wait status to an exit code  
Added a new random.Random.randbytes method: generate random bytes  
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