I'm trying to do something like this:

T = TypeVar('T')

def identity(string):
  # type: (str) -> str
  return string

def f(normalize=identity):
  # type: (Callable[[str], T] -> str)

But mypy says this does not type-check. After adding a cast (normalize=cast(Any, identity), or even normalize=cast(Callable[[str], T], identity), this works fine.