8000 Using generic self in a generic class · Issue #3363 · python/mypy · GitHub
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cyang1 opened this issue May 15, 2017 · 3 comments
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Using generic self in a generic class #3363

cyang1 opened this issue May 15, 2017 · 3 comments

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@cyang1
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cyang1 commented May 15, 2017

The recommended method for writing a factory method is to type the cls argument as Type[T] (http://mypy.readthedocs.io/en/latest/generics.html#generic-methods-and-generic-self). However, this doesn't seem to be able to handle the case where the factory class itself is generic.

In the example below, if we just specify bound=FactoryBase on the _FactoryBase typevar, mypy gives error: Argument 1 to "make" of "FactoryBase" has incompatible type "int"; expected "T".

from typing import (
    Any,
    Generic,
    TypeVar,
    Type,
)

T = TypeVar('T')
_FactoryBase = TypeVar('_FactoryBase', ???)

class FactoryBase(Generic[T]):
    def __init__(self, obj):
        # type: (T) -> None
        pass

    @classmethod
    def make(cls, obj):
        # type: (Type[_FactoryBase], T) -> _FactoryBase
        return cls(obj)

class Factory(FactoryBase[int]):
    pass

Factory.make(2)
@ilevkivskyi
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It looks like this is a duplicate of #2354 (although I am not 100% sure).

@ilevkivskyi
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Maybe it makes to copy this example (i.e. with @classmethod) to the original issue an close this one?

@ilevkivskyi ilevkivskyi mentioned this issue Jun 29, 2017
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@JelleZijlstra JelleZijlstra mentioned this issue Nov 4, 2017
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@ilevkivskyi
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FWIW the original example (with bound=FactoryBase) works correctly on master.

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@cyang1 @ilevkivskyi
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