Extract list subtypes from python list

July 8, 2023

i want to create a decorator to do input type checking on python functions, but i cannot get list subtypes in annotations

im trying to get the subtypes contained in a list annotation but list[str] is of type <class ‘types.GenericAlias’> so i cannot extract the str part, is there a way to do this?

>Solution :

You can get the list[str] subtypes with the __args__ attribute:

class MyClass:
    my_list: list[str]

import inspect
for attribute, annotation in inspect.get_annotations(MyClass).items():
    print(f'{attribute}: {annotation} with subtypes {annotation.__args__}')

# my_list: list[str] of type (<class 'str'>,)

Note that it’s a tuple, as list[str, int] is not forbidden.