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Description
Currently, covariant type variables are allowed in __init__ so that you can put create an immutable containers with something inside of it. See #2850.
It would be useful if covariant types are also allowed in class method that serve as alternative constructors. Currently mypy errors on this example, which is an extension of the other issues example:
from typing import Generic, TypeVar
T_co = TypeVar("T_co", covariant=True)
class C(Generic[T_co]):
def __init__(self, x: T_co) -> None:
...
@classmethod
def custom_init(cls, x: T_co) -> "C[T_co]":
# error: Cannot use a covariant type variable as a parameter
return cls(x)My use case is trying to define an immutable container protocol that has a create class method, which will initialize the container with some contents, like this:
T_cov = typing.TypeVar("T_cov", covariant=True)
class ListProtocol(typing_extensions.Protocol[T_cov]):
@classmethod
def create(cls, *items: T_cov) -> "ListProtocol[T_cov]":
...
def getitem(self) -> T_cov:
...
...