When deepcopying a parametrized types.GenericAlias (eg: a dict subclass) that has a deepcopy method, the copy module doesn't detect the GenericAlias as a type and instead tries to call cls.deepcopy, passing memo inplace of self. This doesn't seem to happen with typing.Generic however.

Example:

from copy import deepcopy


class X(dict):
    def __deepcopy__(self, memo):
        return self


print(deepcopy(X()))
print(deepcopy(X))

print(type(X[str, int]))
print(deepcopy(X[str, int]()))
print(deepcopy(X[str, int]))

shows

{}
<class '__main__.X'>
<class 'types.GenericAlias'>
{}
Traceback (most recent call last):
  File "/tmp/demo.py", line 14, in <module>
    print(deepcopy(X[str, int]))
  File "/Users/jacobhayes/.pyenv/versions/3.9.6/lib/python3.9/copy.py", line 153, in deepcopy
    y = copier(memo)
TypeError: __deepcopy__() missing 1 required positional argument: 'memo'

I don't know if it's better to update copy.deepcopy here or perhaps narrow the __getattr__ for types.GenericAlias (as typing. _BaseGenericAlias seems to).

I think it is better to narrow the __getattr__. It can break protocols using other special methods.

Serhiy, can you elaborate how #72493 can break protocols using other special methods? I can't come up with an example.

I meant that the current __getattr__ breaks deepcopy(). It can break also other protocols. It already has a list of exceptions, we need to add "__copy__" and "__deepcopy__" in this list. Or maybe exclude all dunder names by default and use a white list?

New changeset 5dce51a by Serhiy Storchaka in branch 'main':
bpo-45167: Fix deepcopying of GenericAlias (GH-28324)
5dce51a

New changeset 2746045 by Miss Islington (bot) in branch '3.9':
bpo-45167: Fix deepcopying of GenericAlias (GH-28324) (GH-28368)
2746045

New changeset de4c9c0 by Miss Islington (bot) in branch '3.10':
bpo-45167: Fix deepcopying of GenericAlias (GH-28324) (GH-28367)
de4c9c0

Thanks, Serhiy! ✨ 🍰 ✨