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Metaclass is not properly inherited when using multiple inheritance with int #2824

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gvanrossum opened this issue Feb 7, 2017 · 12 comments
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@gvanrossum
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@gvanrossum gvanrossum commented Feb 7, 2017

Consider:

class M(type):
    def foo(self): pass
class C(metaclass=M):
    pass
print(C.__class__)
C.foo()
class D(int, C):
    pass
print(D.__class__)
D.foo()

Running this with Python 3 shows that D's metaclass is M, but when checking this with mypy, we get the error

u.py:10: error: "D" has no attribute "foo"

on the last line, showing that mypy doesn't believe so.

This seems unique to int (or perhaps builtin types?) -- when create a separate class A (without a metaclass) and use that instead of int, there is no error.

FWIW this prevents me from fixing #2305 (iterating on enums) -- I can get it to work for Enum but not for IntEnum, because it multiply inherits from int and from Enum.

@elazarg
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@elazarg elazarg commented Feb 7, 2017

It does not reproduce in unit tests.

@elazarg
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@elazarg elazarg commented Feb 7, 2017

In general, the class/metaclass hierarchy in typeshed does not mirror the hierarchy in Python. I'm not sure, but it could be the reason. Perhaps the special casing of ABCMeta in mypy is also a problem.

@elazarg
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@elazarg elazarg commented Feb 7, 2017

Removing SupportsInt and SupportsFloat from baseclasses makes the error go away. SupportsAbs[int] has nothing to do with the error. Since the formers have ABCMeta as a metaclass, and the latter does not, It points to ABCMeta as the problem.

@elazarg
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@elazarg elazarg commented Feb 7, 2017

Well I guess I'm completely wrong. Perhaps it's a simple case of choosing the most derived common metaclass. In typeshed type(int) == ABCMeta. In Python type(int) == type.

@elazarg
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@elazarg elazarg commented Feb 7, 2017

A possible solution: make EnumMeta inherit ABCMeta, so it will be the most derived common metaclass.

@gvanrossum
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@gvanrossum gvanrossum commented Feb 8, 2017

That mostly works, but I needed to add a # type: ignore. Thoughts? https://github.com/python/typeshed/tree/enum-iter

@elazarg
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@elazarg elazarg commented Feb 9, 2017

Is there some problem with selftype? I have encountered something, but I wasn't sure.

@gvanrossum
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@gvanrossum gvanrossum commented Feb 9, 2017

Without that I get

typeshed/stdlib/3.4/enum.pyi:11: error: The erased type of self 'Type[enum.Enum]' is not a supertype of its class 'enum.EnumMeta'

FWIW the test program is

import enum
class E(enum.Enum):
    a = 1
    b = 2
for i in E:
    reveal_type(i)
class I(enum.IntEnum):
    a = 1
    b = 2
for k in I:
    reveal_type(k)

and its output (less the error) is

t.py:6: error: Revealed type is 't.E*'
t.py:11: error: Revealed type is 't.I*'

which is exactly what it should be.

@elazarg
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@elazarg elazarg commented Feb 9, 2017

There are additional checks to add to the subtype visitor that makes this problem go away, but something bothers me. The variable seems to be overly constrained by a class not defined yet. We should add another check for the metaclass declaration, to verify that we are constrained correctly by the type of cls.

@elazarg
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@elazarg elazarg commented Feb 9, 2017

As I have mentioned in a different discussion here, it might make sense to use the form cls: Meta[T] which means precisely the type whose metaclass is a subclass of Meta (directly inheriting the generic [] operator from Type). Then we don't need to constrain further T in any way.

@elazarg
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@elazarg elazarg commented Sep 8, 2017

Root cause: python/typeshed#1595

@JelleZijlstra
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@JelleZijlstra JelleZijlstra commented Sep 11, 2017

Recap:

  • There was no mypy bug here; the problem is because of incorrect inheritance in typeshed (fix being tracked in python/typeshed#1595)
  • This problem usually appears in practice with enum classes, and we merged a workaround for those in python/typeshed#1598.
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