Update the behaviour of the type function for metaclasses

Calling type on a class A yields Type[A], even if A has a metaclass other than type. This leads to incorrectly typechecking interactions with type(A) when when attempting to use metaclass attributes.

Somewhat related to #2655

Can you give a concrete example?
The interaction with other features might be interesting. Maybe it should work something like this

class M(Generic(T), Type[T]): pass  # maybe M(type) is a shorthand for this

T1 = TypeVar('T1', bound='A')

class A(metaclass=M):
    @classmethod
    def foo(cls: M[T1]) -> T1:
        return cls()

reveal_type(A)  # E: Revealed type is M[A]

class B(A): pass

reveal_type(B)  # E: Revealed type is M[B]
reveal_type(B.foo()) # E: Revealed type is B

All of this should be how things work right now, except that M[B] is written Type[B]. Where does this fail?

I agree it should be possible to write M[T] for "a type whose metaclass is M".

class Meta(type):
    x = 1

class A(metaclass=Meta): pass

print(type(A).x) # test2826.py:6: error: Type[] has no attribute "x"

type(A) is Meta which definitely has an attribute x. Do I need to make Meta subclass Type[T] for it to work?

Thanks.

The culprit in my example seems to be that the type variable T is quantified over type constructors (TypeInfo) and not over types. Perhaps there is a workaround.
(Just thinking out loud).

I have a fix for this particular issue, but the WiFi here is not working.

..., but the WiFi here is not working.

Sounds like an updated version of Fermat's claim about his last theorem. :-)

:D

Strip type objects when constructing TypeType (#2832)

Navigation Menu

Repository navigation

Update the behaviour of the `type` function for metaclasses #2826

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Update the behaviour of the type function for metaclasses #2826

Description

Activity

TRManderson commented on Feb 8, 2017

elazarg commented on Feb 8, 2017

elazarg commented on Feb 8, 2017

TRManderson commented on Feb 8, 2017

elazarg commented on Feb 8, 2017

elazarg commented on Feb 8, 2017

gvanrossum commented on Mar 29, 2017

elazarg commented on Mar 29, 2017

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions

Update the behaviour of the `type` function for metaclasses #2826