Python 3.5+Annotations & typingterm

Union[X, Y] / X | Y

union

Union[X, Y] — the value may be of type X or of type Y. Since Python 3.10 the shorthand X | Y is supported.

Syntax

from typing import Union
Union[int, str]   # int или str
# Python 3.10+: int | str

A classic mix-up: Optional[X], which is just X | None, only says the value may be None — it has nothing to do with the argument being optional, that is what a default value is for. The annotation checks nothing by itself; the type checker will simply demand that you narrow, typically with isinstance, before calling a method that exists on only one branch. The X | Y spelling works in annotations and in isinstance from Python 3.10 onward; older versions need Union from typing.

Examples

from typing import Union
def double(x: Union[int, float]) -> Union[int, float]:
    return x * 2
print(double(3))  # → 6
print(double(1.5))  # → 3.0

# Python 3.10+
def parse(val: int | str) -> str:
    return str(val)
print(parse(42))  # → '42'
print(parse('hi'))  # → 'hi'

See also

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