[python] How to specify multiple return types using type-hints

I have a function in python that can either return a bool or a list. Is there a way to specify the return types using type hints.

For example, Is this the correct way to do it?

def foo(id) -> list or bool:
      ...

Python 3.10 (use |): Example for a function which takes a single argument that is either an int or str and returns either an int or str:

def func(arg: int | str) -> int | str:
              ^^^^^^^^^     ^^^^^^^^^ 
             type of arg   return type

Python 3.5 - 3.9 (use typing.Union):

from typing import Union
def func(arg: Union[int, str]) -> Union[int, str]:
              ^^^^^^^^^^^^^^^     ^^^^^^^^^^^^^^^ 
                type of arg         return type

For the special case of X | None you can use Optional[X].

In case anyone landed here in search of "how to specify types of multiple return values?", use Tuple[type_value1, ..., type_valueN]

from typing import Tuple

def f() -> Tuple[dict, str]:
    a = {1: 2}
    b = "hello"
    return a, b

More info: How to annotate types of multiple return values?

The statement def foo(client_id: str) -> list or bool: when evaluated is equivalent to def foo(client_id: str) -> list: and will therefore not do what you want.

The native way to describe a "either A or B" type hint is Union (thanks to Bhargav Rao):

def foo(client_id: str) -> Union[list, bool]:

I do not want to be the "Why do you want to do this anyway" guy, but maybe having 2 return types isn't what you want:

If you want to return a bool to indicate some type of special error-case, consider using Exceptions instead. If you want to return a bool as some special value, maybe an empty list would be a good representation. You can also indicate that None could be returned with Optional[list]