[Python] Why are type-checkers fine with covariant method parameters when they are in a union?

Method parameters should be contravariant, hence defining a covariant generic should raise a type error. However when using a covariant generic in a union pyright, mypy and pyre-check all do not report an error on the following code:

from typing import TypeVar, Generic, TypeAlias, Any

T_co = TypeVar("T_co", covariant=True)

class Foo(Generic[T_co]):

def a(self, arg: T_co) -> Any: # Error, like expected as T_co is covariant.
...

def b(self, arg: int | T_co) -> Any: # No error, expected one
...


As all these type-checkers do not raise an error I wonder is this actually fine, shouldn't this also break type-safety? If it is fine can you explain to why, what differs from a pure covariant implementation that is definitely not safe, shouldn't I be able to break it as well? Or if its not safe, is there an explanation why all type-checkers have the same gap here?

Continue reading...