Python 3 types, custom variadic generic type with arbitrary number of contained types, how?

Question

The class typing.Tuple can be used as with arbitrary number of type arguments, like Tuple[int, str, MyClass] or Tuple[str, float]. How do I implement my own class that can be used like that? I understand how to inherit from typing.Generic. The following code demonstrates this.

from typing import TypeVar, Generic


T = TypeVar("T")


class Thing(Generic[T]):
    def __init__(self, value: T):
        self.value = value


def f(thing: Thing[int]):
    print(thing.value)


if __name__ == '__main__':
    t = Thing("WTF")
    f(t)

The above code would work but the type checker (in my case PyCharm) would catch the fact that t should be of type Thing[int] and not Thing[str]. That's all fine, but how do I make the class Thing support arbitrary number of type arguments, like Tuple does?

Thanks @StephaneBersier. I did realise after a while that it's impossible in Python. — Ray, Feb 24 '20 at 17:54
With [PEP 646](https://peps.python.org/pep-0646/) coming to Python 3.11, variadic generics may become possible through `TypeVarTuple`. — metatoaster, Oct 14 '22 at 02:14

score -4 · Answer 1 · answered May 15 '18 at 07:18

In your example, t is of type Thing, not Thing[str]. So this object accepts anything for T.
You can parametrize it like this : t2 = Thing[str]("WTF")
now, for your question, I think you want to use typing.Union like this: t3=Thing[Union[str,int,float]]("WTF")

By the way, you can check the type of generics by using get_generic_type() from typing_inspect

>>> get_generic_type(t)
__main__.Thing
>>> get_generic_type(t2)
__main__.Thing[str]
>>> get_generic_type(t3)
__main__.Thing[typing.Union[str, int, float]]

Python 3 types, custom variadic generic type with arbitrary number of contained types, how?

1 Answers1

Linked