8000 Union of a class with Any base and None is sometimes None · Issue #2712 · python/mypy · GitHub
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Union of a class with Any base and None is sometimes None #2712

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JukkaL opened this issue Jan 19, 2017 · 1 comment · Fixed by #2718
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Union of a class with Any base and None is sometimes None #2712

JukkaL opened this issue Jan 19, 2017 · 1 comment · Fixed by #2718
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bug mypy got something wrong priority-0-high topic-strict-optional topic-union-types

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@JukkaL
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JukkaL commented Jan 19, 2017

Without --strict-optional, the second reveal_type produces an incorrect result:

from typing import Any, Union, TypeVar

class C(Any):
    pass

T = TypeVar('T')
S = TypeVar('S')

def f(x: T, y: S) -> Union[T, S]:
    pass

reveal_type(f(C(), None))  # t.C
reveal_type(f(None, C()))  # builtins.None   <--- wrong

With --strict-optional, both of them are wrong:

...   # as above

reveal_type(f(C(), None))  # builtins.None   <--- wrong
reveal_type(f(None, C()))  # builtins.None   <--- wrong

This seems to cause problems in real-world code together with python/typeshed@6008b9d, so this is high priority.
@JukkaL JukkaL self-assigned this Jan 19, 2017
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JukkaL commented Jan 19, 2017

This looks very similar to the issue fixed by #2197. Maybe I'll just finish up that PR.

@JukkaL JukkaL mentioned this issue Jan 19, 2017
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JukkaL added a commit that referenced this issue Jan 19, 2017
@JukkaL
Fix simplification of some unions
f43fd8f 
In particular, this helps with classes that have Any base classes.

This is enough to fix #2712. This is almost a subset of #2714
and should land before that, as this should cause less disruption.
@JukkaL JukkaL mentioned this issue Jan 19, 2017
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gvanrossum pushed a commit that referenced this issue Jan 20, 2017
@JukkaL @gvanrossum
Fix simplification of some unions (#2718)
9496d6f 
In particular, this helps with classes that have Any base classes.

This is enough to fix #2712. This is almost a subset of #2714
and should land before that, as this should cause less disruption.
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bug mypy got something wrong priority-0-high topic-strict-optional topic-union-types
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