Generic ParamSpec on Python 3.9

The following is correct in Python 3.10, but not 3.9:

from typing import Generic
from typing_extensions import ParamSpec

P = ParamSpec("P")


class Foo(Generic[P]):
    pass


foo: Foo[[int, str]] = Foo()

Running the above triggers a TypeError:

TypeError: Parameters to generic types must be types. Got [<class 'int'>, <class 'str'>].

What must I do to get this piece of code to run in Python 3.9?

I get it that ParamSpec is a 3.10 feature. But typing-extensions is supposed to make the code backward-compatible, right? I’m guessing that I must change the syntax of Foo[[int, str]] to something else, but what?

Answer

Solution is to pass a string as the type definition:

foo: "Foo[[int, str]]" = Foo()

Alternatively, the “unsightly double brackets” can be removed:

foo: "Foo[int, str]" = Foo()

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Answer