deepcopy(frozendict[<key type>, <value type>]) returns empty dict instead of type

[JacobHayes]

OS version: macOS Big Sur 11.4
Python3 version (python3 -V -V):

Python 3.9.0 (default, Nov 13 2020, 17:57:54)
[Clang 10.0.1 (clang-1001.0.46.4)]

Steps to reproduce:

>>> from copy import copy, deepcopy
>>>
>>> from frozendict import frozendict
>>>
>>> deepcopy(dict)
<class 'dict'>
>>> deepcopy(dict[str, int])
dict[str, int]
>>>
>>> deepcopy(frozendict)
<class 'frozendict.core.frozendict'>
>>> deepcopy(frozendict[str, int])
{}

I don't exactly know why this is happening, but a bit of debugging info:

• __new__ and __init__ are never called (but did confirm it is called for something like frozendict())
• __deepcopy__ is called, but only for the deepcopy(frozendict[str, int]) case... and then type(self) is dict, not frozendict which seems weird.

[Marco-Sulla]

Can you give me the result of:

import frozendict
frozendict.c_ext

[JacobHayes]

>>> import frozendict
>>> frozendict.c_ext
False

[Marco-Sulla]

And this is a problem because....?

[JacobHayes]

I'm hoping to use frozendict with pydantic (think: dataclasses, but with runtime type checking/parsing - including dict keys and values). The pydantic Models have references to the field types (eg: frozendict[str, int]), but occasionally call deepcopy (eg: when subclassing). After a deepcopy, instead of having model.__fields__['fieldname'] = frozendict[str, int], we end up with model.__fields__['fieldname'] = {}, causing a few other issues.

Small example w/ pydantic if it helps:

from frozendict import frozendict
from pydantic import BaseModel

class Base(BaseModel):
    a: frozendict[str, int]

class Sub(Base):
    pass

print(Sub.__fields__["a"].outer_type_)

[JacobHayes]

I think this is a bug in python's copy module - when using deepcopy with a GenericAlias and a __deepcopy__ method, the GenericAlias is not considered a type and so copy fetches .__deepcopy__ from the class and calls it with the memo dict (hence type(self) is dict). I'll open a bug upstream.

edit: in case you're interested: https://bugs.python.org/issue45167

[Marco-Sulla]

I reopen it, since the type hint does not work at all in the C Extension

[Marco-Sulla]

Fixed ecf771c

[JacobHayes]

Thanks. Btw, the original deepcopy issue was fixed upstream in python/cpython#28324