Types

Union and Optional Types

The | operator describes values that may have more than one static type.

Use A | B when a value may have either type. The function body should use operations that make sense for every member of the union.

Source

def label(value: int | str) -> str:
    return f"item-{value}"

print(label(3))
print(label("A"))

Output

item-3
item-A

str | None means the function accepts either a string or explicit absence. Check for None before calling string methods.

Source

def greeting(name: str | None) -> str:
    if name is None:
        return "hello guest"
    return f"hello {name.upper()}"

print(greeting(None))
print(greeting("Ada"))

Output

hello guest
hello ADA

Union annotations are visible at runtime, but Python does not enforce them when the function is called.

Source

print(greeting.__annotations__)

Output

{'name': str | None, 'return': <class 'str'>}

Notes

Use A | B when a value may have either type.
T | None means absence is an expected case, not an error by itself.
Narrow unions before using behavior that belongs to only one member type.

Run the complete example

Expected output

item-3
item-A
hello guest
hello ADA
{'name': str | None, 'return': <class 'str'>}

Execution time appears here after you run the example.

Union and Optional Types

See also

Run the complete example

Example code

Expected output