Problems using class type variable in class methods

@ilevkivskyi

Follow up from a discussion on gitter:

from typing import *


T = TypeVar('T')
class Factory(Generic[T]):
    @classmethod
    def instance(cls) -> 'Factory[T]':
        return cls()

    def produce(self) -> T:
        raise NotImplementedError()

    @classmethod
    def get(cls) -> T:
        return cls.instance().produce()


class HelloWorldFactory(Factory[str]):
    def produce(self) -> str:
        return 'Hello World'


# factory.py:25: error: Incompatible return value type (got "T", expected "str")
def error_with_mypy() -> str:
    return HelloWorldFactory.get()

@ilevkivskyi mentioned this should not error and that this code is valid. I don't have the necessary context to understand how to classify this issue.

I would simplify the example to show only the essential part:

class Factory(Generic[T]):
    def produce(self) -> T:
        ...
    @classmethod
    def get(cls) -> T:
        return cls().produce()

class HelloWorldFactory(Factory[str]):
    def produce(self) -> str:
        return 'Hello World'

reveal_type(HelloWorldFactory.get())  # mypy should be able to infer 'str' here

On one hand, type variables are bound on instances, not on classes, but in case of a non-generic subclass I think the type variable can be bound on that class. The problem however is that we need to give a clear error message when this is not possible, something like (using above example):

Factory.get()  # E: cannot bind type variable on class (use an instance or a non-generic subclass)
Facrory[int].get()   # E: cannot bind type variable on class (use an instance or a non-generic subclass)

Similar issues have come up previously, so this would perhaps be a reasonable feature to have eventually -- at least assuming the implementation isn't very complex.

The produce() method is also irrelevant to the example. Shorter yet self-contained:

from typing import Generic, TypeVar
T = TypeVar('T')
class Factory(Generic[T]):
    @classmethod
    def get(cls) -> T: ...
class HelloWorldFactory(Factory[str]): pass
reveal_type(HelloWorldFactory.get())  # mypy should be able to infer 'str' here

@JukkaL

@JukkaL Do you have any thoughts on what it would take to fix this? It's blocking us here at Lyft.

@rowillia

@rowillia My recommendation is to use a regular method instead of a class method. Example:

from typing import Generic, TypeVar

T = TypeVar('T')

class Factory(Generic[T]):
    def get(cls) -> T: ...

class _HelloWorldFactory(Factory[str]): pass

HelloWorldFactory = _HelloWorldFactory()  # This singleton is mostly for convenience

reveal_type(HelloWorldFactory.get())  # this is 'str'

You can avoid the singleton by using HelloWorldFactory().get() instead.

I bumped into this bug today as well, and was surprised to see this comment:

type variables are bound on instances, not on classes

If this is the case, then why is

class G(Generic[T]):
    @classmethod
    def m(cls) -> 'G[T]':
        ...

not itself an error? being able to say G[T] at least implies to me that T is bound.

That method returns an instance of G[T].

According to reveal_type it returns an instance of G[<nothing>].

More specifically: G[int].m() returns G[<nothing>].

Oh, I think in this case it's basically equivalent to a top-level function returning G[T]: there is nothing to constrain the value of the typevar, so mypy infers "". I believe there's a separate issue about that.

@glyph

@glyph

If this is the case, then why is ... not itself an error?

Because it is not, it may be bound in a subclass as said few word later in the comment you quote, there is even a simple example just few comments above #3645 (comment)

Fix member access on generic classes (#6418)

@ilevkivskyi

@ilevkivskyi Thank you for fixing this issue!

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Problems using class type variable in class methods #3645

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Problems using class type variable in class methods #3645

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ilevkivskyi commented on Jul 1, 2017

JukkaL commented on Jul 3, 2017

gvanrossum commented on Jul 5, 2017

rowillia commented on Feb 24, 2018

JukkaL commented on Feb 26, 2018

glyph commented on Apr 18, 2018

JelleZijlstra commented on Apr 18, 2018

glyph commented on Apr 18, 2018

glyph commented on Apr 18, 2018

JelleZijlstra commented on Apr 18, 2018

ilevkivskyi commented on Apr 18, 2018

LiraNuna commented on Feb 24, 2019

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