Starting from Python 3.10, you can use Union type expression in some scenarios.

The built-in functions isinstance() and issubclass() take a type or tuple of types as their second argument. Using tuples requires writing additional parentheses.

Consider the code below:

x = 42

print(isinstance(x, (int, str)))  # output: True

print(issubclass(type(x), (int, str)))  # output: True

Let’s rewrite this example using Union type expression:

x = 42

print(isinstance(x, int | str))  # output: True

print(issubclass(type(x), int | str))  # output: True

This example is equivalent to the previous one, but it looks cleaner. Instead of (X, Y), we write X | Y using the | (bitwise or) operator.

Union type expression is particularly expressive when used in type annotations. Compare the example below without using Union type expression:

from typing import List, Union, Optional

def f(lst: List[Union[int, str]], param: Optional[int]) -> Union[float, str]:
    return ''

f([1, 'abc'], None)

And the same example using Union type expression:

def f(lst: list[int | str], param: int | None) -> float | str:
    return ''

f([1, 'abc'], None)

We didn’t even need to import anything from the typing module!

For more information about Union type expression, refer to PEP 604.