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Metaclasses should inherit from type. #2818

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ilevkivskyi opened this issue Feb 7, 2017 · 3 comments
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Metaclasses should inherit from type. #2818

ilevkivskyi opened this issue Feb 7, 2017 · 3 comments

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@ilevkivskyi
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There is something missing in #2475: this code:

class M:
    ...
class A(metaclass=M):
    ...

Passes without errors, but fails at runtime.

Btw, I have found two open issues that were fixed by #2475, I left comments referencing the #2475 in that issues.
@elazarg
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elazarg commented Feb 7, 2017

I am for fixing it. The reason I have allowed this behavior is the documentation.

Quote:

The appropriate metaclass for a class definition is determined as follows:

  • if no bases and no explicit metaclass are given, then type() is used
  • if an explicit metaclass is given and it is not an instance of type(), then it is used directly as the metaclass
  • if an instance of type() is given as the explicit metaclass, or bases are defined, then the most derived metaclass is used

End quote.

What do you think?

@ilevkivskyi
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@elazarg I think the use case where metaclass is not an instance of type is quite rare, and (more importantly) this case could be difficult to typecheck. I think mypy could give an error message like: Metaclasses not inheriting from "type" are not yet supported.

@elazarg
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elazarg commented Feb 7, 2017 •
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I have a branch ready , but it requires multiple changes to typeshed before it can be merged.

@elazarg elazarg mentioned this issue Feb 7, 2017
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JukkaL pushed a commit that referenced this issue Feb 7, 2017
@elazarg @JukkaL
Do not support metaclasses not inheriting from type (#2820)
452a64f 
Fix #2818.

ABCMeta requires special treatment.

Related: #2475
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@elazarg @ilevkivskyi