How to use isinstance on a generic type in Python

Question

I'm trying to check whether an argument is an instance of the generic type specified in the class declaration. However Python does not seem to allow this.

T = TypeVar('T')
class MyTypeChecker(Generic[T]):
    def is_right_type(self, x: Any):
        return isinstance(x, T)

This gives the error 'T' is a type variable and only valid in type context.

Types hints **are not types**. There is no such thing as a "generic type* in the sense of *actual python types*. And furthermore, as is stated in the error message, `T` is a *type variable*. — juanpa.arrivillaga, Apr 03 '21 at 05:36

score 6 · Accepted Answer · answered Apr 04 '21 at 12:33

You could use the __orig_class__ attribute, but keep in mind that this is an implementation detail, in more detail in this answer.

from typing import TypeVar, Generic, Any
T = TypeVar('T')


class MyTypeChecker(Generic[T]):
    def is_right_type(self, x: Any):
        return isinstance(x, self.__orig_class__.__args__[0])  # type: ignore


a = MyTypeChecker[int]()
b = MyTypeChecker[str]()

print(a.is_right_type(1))  # True
print(b.is_right_type(1))  # False
print(a.is_right_type('str'))  # False
print(b.is_right_type('str'))  # True

How to use isinstance on a generic type in Python

1 Answers1

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