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:
Asked By: Yahya Uddin



From the documentation – Union Type:

A union object holds the value of the | (bitwise or) operation on multiple type objects. These types are intended primarily for type annotations. The union type expression enables cleaner type hinting syntax compared to typing.Union.

This use of | was added in Python 3.10. Hence the proper way to represent more than one return data type is:

def foo(client_id: str) -> list | bool:

For earlier versions, use typing.Union:

from typing import Union

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

But do note that typing is not enforced. Python continues to remain a dynamically-typed language. The annotation syntax has been developed to help during the development of the code prior to being released into production. As PEP 484 states, "no type checking happens at runtime."

>>> def foo(a: str) -> list:
...     return "Works"
>>> foo(1)

As you can see I am passing an int value and returning a str. However the __annotations__ will be set to the respective values.

>>> foo.__annotations__ 
{'return': <class 'list'>, 'a': <class 'str'>}

Please go through PEP 483 for more about Type hints. Also see What are type hints in Python 3.5??

Kindly note that this is available only for Python 3.5 and upwards. This is mentioned clearly in PEP 484.

Answered By: Bhargav Rao

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]:

Or, starting with Python 3.10 and beyond, using the | operator:

def foo(client_id: str) -> 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]

Answered By: Felk

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

In Python 3.9+:

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

In earlier versions, use typing.Tuple:

from typing import Tuple

def f() -> Tuple[dict, str]:

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

Answered By: Anton Khodak

Python 3.10 or newer: 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].

Answered By: xjcl