How to check a variable against Union type during runtime?

In Python 3.8 and later, the approach suggested by MSeifert and Richard Xia can be improved by not using the undocumented attributes __origin__ and __args__. This functionality is provided by the new functions typing.get_args(tp) and typing.get_origin(tp):

>> from typing import Union, get_origin, get_args
>> x = Union[int, str]
>> get_origin(x), get_args(x)
(typing.Union, (<class 'int'>, <class 'str'>))
>> get_origin(x) is Union
True
>> isinstance(3, get_args(x))
True
>> isinstance('a', get_args(x))
True
>> isinstance([], get_args(x))
False