Checking if a type is generic and correct type hint

[johhnry]

Hi,

(I am using Python 3.7.9)

I have a function that takes a type and checks if it's generic:

def is_generic(t: type):
    return hasattr(t, "__origin__")

First question: is object the appropriate type hint for t? Or should I use a more specific type?

Then I have a function that checks the non-parameterized generic class for a type:

def is_generic_of(t: object, origin: object) -> bool:
    return is_generic(t) and t.__origin__ == origin  # type: ignore

# Example: is_generic_of(List[str], list) == True

The thing is I don't know how to access the field __origin__ safely with type hints.
I need to do the same with __args__ when checking if a type is an optional:

def is_union(t: object):
    return is_generic_of(t, Union)

def is_optional(t: object) -> bool:
    return is_union(t) and t.__args__[1] == type(None)  # type: ignore

I found that type(List[int]) is <class 'typing._GenericAlias'> but I can't use _GenericAlias as a type hint to constrain the parameters...

Any thoughts on this?

Thanks! ;)

[JelleZijlstra]

I'd suggest you use a library like https://github.com/ilevkivskyi/typing_inspect. You can also typing_extensions.get_origin and get_args (backports of https://docs.python.org/3.10/library/typing.html#typing.get_args).

[johhnry]

Thanks for the answer I'll use the typing_extensions methods for that!