Shared via mypy Playground

main.py

from typing import TypeVar
from typing_extensions import TypeIs


_T = TypeVar('_T', str, int)


def is_str(val: str | int) -> TypeIs[str]:
    return isinstance(val, str)

def is_int(val: str | int) -> TypeIs[int]:
    return isinstance(val, int)


def process_str_int(data: _T) -> _T:
    if is_str(data):
        reveal_type(data)  # str
    
    elif is_int(data):
        reveal_type(data)  # int
        
    return data


process_str_int("hello")