AnyStr

AnyStr = TypeVar('AnyStr', str, bytes)

def concat(a: AnyStr, b: AnyStr) -> AnyStr:
    return a + b

concat("foo", "bar")     OK, output has type 'str'
concat(b"foo", b"bar")   OK, output has type 'bytes'
concat("foo", b"bar")    Error, cannot mix str and bytes

Invalid use of AnyStr:
 The type variable is used only once in the function signature,
 so cannot be "solved" by the type checker
def greet_bad(cond: bool) -> AnyStr:
    return "hi there!" if cond else b"greetings!"

 The better way of annotating this function:
def greet_proper(cond: bool) -> str | bytes:
    return "hi there!" if cond else b"greetings!"