10000 Inconsistency in metaclass/Type interaction · Issue #6416 · python/mypy · GitHub
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msullivan opened this issue Feb 16, 2019 · 1 comment · Fixed by #6419
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Inconsistency in metaclass/Type interaction #6416

msullivan opened this issue Feb 16, 2019 · 1 comment · Fixed by #6419
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bug mypy got something wrong priority-1-normal

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@msullivan
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Since #6370, we reject the following code:

from typing import Type
class Meta(type):
    pass
class Thing(metaclass=Meta):
    pass

def foo(x: Type[Thing]) -> Type[Thing]:
    assert isinstance(x, Meta)
    return x

with the error Incompatible return value type (got "Meta", expected "Type[Thing]").

Previously this worked because we thought that Meta and Type[Thing] had no overlap and so the rest of the function after the assert wasn't checked.

This caused one error in S.

The answer might be that we need to consider Type[Thing] to be a subtype of Meta?
@ilevkivskyi ilevkivskyi added bug mypy got something wrong priority-1-normal labels Feb 16, 2019
@ilevkivskyi
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The answer might be that we need to consider Type[Thing] to be a subtype of Meta?

IIRC this is already true, we need to fix proper subtype (which is still a separate visitor, see also #3297)

Also even with proper subtype fixed, this may not help since isinstance() can widen type, see
#6386. Or maybe better proper subtype check will fix both issues.

@ilevkivskyi ilevkivskyi mentioned this issue Feb 17, 2019
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ilevkivskyi added a commit that referenced this issue Feb 23, 2019
@ilevkivskyi
Fix interaction of isinstance() with Type[...] (#6419)
ed0dcad 
Fixes #4616
Fixes #6416
Fixes #6386

Currently `Type[C]` is considered a subtype of metaclass of `C`, and I think this is right. The idea is to fix `ProperSubtypeVisitor.visit_type_type()` to match `SubtypeVisitor.visit_type_type()` (plus couple small updates to make the former used by `isinstance()` and `issubclass()`).
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Fix interaction of isinstance() with Type[...] ilevkivskyi/mypy
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