[issue41451] Cannot subclass typing.Generic with __weakref__ slot in Python 3.6

[Joshua Bronson]

Joshua Bronson <jabronson at 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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