This issue is intended to track the status of Python 3.9 core features as implemented by MicroPython.

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.

• PEP 584, union operators added to dict;
• PEP 585, type hinting generics in standard collections;
• PEP 614, relaxed grammar restrictions on decorators.
• PEP 616, string methods to remove prefixes and suffixes.
• PEP 593, flexible function and variable annotations;
• PEP 573, fast access to module state from methods of C extension types;
• PEP 617, CPython now uses a new parser based on PEG;
• PEP 615, the IANA Time Zone Database is now present in the standard library in the zoneinfo module;
• PEP 602, CPython adopts an annual release cycle. Instead of annual, aiming for two month release cycle

Other language changes

• __import__() now raises ImportError instead of ValueError; Done, see 53519e3
• 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 MicroPython 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
  • The os.putenv() and os.unsetenv() functions are now always available
  • 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.

(Changes to non-built-in modules will need to be documented elsewhere.)