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Ability to weakly reference an instance of a class lost due to mutliple inheritance #95710

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markshannon opened this issue Aug 5, 2022 · 2 comments
Open

Ability to weakly reference an instance of a class lost due to mutliple inheritance #95710

markshannon opened this issue Aug 5, 2022 · 2 comments
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3.10 only security fixes 3.11 only security fixes 3.12 only security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error

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@markshannon
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This is related to #95589 due to underlying mechanism, but the behavior is distinct.

class C: 
    pass

class D(int, C):
    pass
>>> c = C()
>>> weakref.ref(c)
<weakref at 0x7fa5b2af9c60; to 'C' at 0x7fa5b2e501c0>
>>> d = D()
>>> weakref.ref(d)
TypeError: cannot create weak reference to 'D' object
>>> isinstance(d, C)
True

If I can weakly reference an instance of C, then I should be able to weakly reference an instance of D. Instances of D are instances of C
@markshannon markshannon added type-bug An unexpected behavior, bug, or error 3.11 only security fixes 3.10 only security fixes 3.12 only security fixes labels Aug 5, 2022
@markshannon markshannon changed the title Ability to weakly reference a class lost due to mutliple inheritance Ability to weakly reference an instance of a class lost due to mutliple inheritance Aug 5, 2022
@Mandera
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Mandera commented Aug 5, 2022

I don't think this has much to do with inheritance. It's caused by int not supporting weakref.

See https://stackoverflow.com/a/52011601/3936044

int has a __weakrefoffset__ of 0 meaning it has no weakref pointer, remove the int baseclass and it works fine

@markshannon
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C supports weakrefs, D is a subclass of C, but D does not support weakrefs.
This breaks the Liskov substitution principle.

Whether or not int supports weakrefs is irrelevant.

list does not support weakrefs, but class D(list, C) does.

@iritkatriel iritkatriel added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Nov 25, 2023
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3.10 only security fixes 3.11 only security fixes 3.12 only security fixes interpreter-core (Objects, Python, Grammar, and Parser dirs) type-bug An unexpected behavior, bug, or error
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@iritkatriel @Mandera @markshannon