This may seem like a good idea, but it gets problematic since you need to always use isinstance checks. Its actually discouraged to return union types by the mypy team: https://github.com/python/mypy/issues/1693
I've read the issue, and it seems that it's more about the lack of overloading/dependent types, e.g. their example with pow function. In c++ you'd have two overloads:
float pow(float, float)
int pow(int, int)
def pow(base: Union[int, float], power: Union[int, float]) -> Union[int, float]
In my case I do want to return a variant type unconditionally, so this warning doesn't really apply. It's more similar to an Optional type (just instead of None I am using a more meaningful Exception): while it's not great if your code is riddled with optional return types, sometimes you don't have other choice but using it.
from typing import TypeVar
T = TypeVar('T', int, float)
def pow(base: T, power: T) -> T: ...As evidenced by your comment, most people forget about negative exponents. The vast majority of usage is positive exponents; people expect an int back when they do `pow(2, 2)` and returning a Union or float would result in annoying false positives.