I mark the PR as a draft since test_pickle fails on the test that I added:

FAIL: test_recursive_frozendict_and_inst (test.test_pickle.InMemoryPickleTests.test_recursive_frozendict_and_inst) (proto=5)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/test/pickletester.py", line 2852, in _test_recursive_collection_and_inst
    self.assertIs(list(x)[0].attr, x)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
AssertionError: frozendict({<test.picklecommon.Object object at 0x200009adea0>: None}) is not frozendict({<test.picklecommon.Object object at 0x200009adea0>: None})

I mark the PR as a draft since test_pickle fails on the test that I added

Ah it's ok, I found the issue: the C implementation was not correct. It's not fixed.

cc @serhiy-storchaka @avassalotti

You do not need to change the pickle module. Just add the __getnewargs__() method returning (dict(self),) or (list(self.items()),).

Adding a new pickle opcode requires a separate PEP.

You do not need to change the pickle module. Just add the __getnewargs__() method returning (dict(self),) or (list(self.items()),).

Oh, I didn't know that it was so simple :-) I wrote #144967 to implement __getnewargs__().