Understanding Union Types in Python

Union types are used to inform the type checker:

“This variable/parameter can be any one of several different types at the same time.”

• Before Python 3.10: typing.Union

from typing import Union

def print_id(uid: Union[int, str]) -> None:
    print(uid)

# Valid calls
print_id(123)      # int
print_id("abc")    # str

Union[int, str] indicates “either int or str is acceptable.”

• From Python 3.10 onwards: using the | operator (PEP 604)

def print_id(uid: int | str) -> None:
    print(uid)

The functionality is completely equivalent, but shorter and more readable.