Joshua Bronson <jabron...@gmail.com> added the comment:
Whittled this down to an even more minimal repro:
"""Repro for Python 3.6 slots + weakref + typing.Generic subclass bug."""
from typing import Generic, TypeVar
from weakref import ref
T = TypeVar("T")
class MyGeneric(Generic[T]):
"""MyGeneric works as expected.
>>> example = MyGeneric()
>>> from pickle import dumps, loads
>>> pickled = dumps(example)
>>> roundtripped = loads(pickled)
>>> roundtripped
<__main__.MyGeneric object at ...>
"""
__slots__ = ("_other", "__weakref__")
def __init__(self) -> None:
self._init_other()
def _init_other(self) -> None:
other = self.__class__.__new__(self.__class__)
other._other = self
self._other = ref(other)
def __getstate__(self) -> dict:
"""Needed to enable pickling due to use of __slots__ and weakrefs."""
return {}
def __setstate__(self, _) -> None:
"""Needed because use of __slots__ would prevent unpickling
otherwise."""
self._init_other()
# So far so good, but now let's make a subclass.
# The following class definition works on Python > 3.6, but fails on 3.6 with
# TypeError: __weakref__ slot disallowed: either we already got one, or
__itemsize__ != 0
class FailsInPy36(MyGeneric[T]):
"""Minimal repro.
>>> repro = FailsInPy36()
>>> repro
<__main__.FailsInPy36 object at ...>
"""
if __name__ == "__main__":
import doctest
doctest.testmod(optionflags=doctest.ELLIPSIS)
----------
title: Class with __weakref__ slot cannot inherit from typing.Generic subclass
-> Cannot subclass typing.Generic with __weakref__ slot in Python 3.6
Added file: https://bugs.python.org/file49355/bpo41451-repro-min.py
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue41451>
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