pythonpython-typingmypypython-3.10

Python 3.10+: Optional[Type] or Type | None

Now that Python 3.10 has been released, is there any preference when indicating that a parameter or returned value might be optional, i.e., can be None. So what is preferred:

Option 1:

def f(parameter: Optional[int]) -> Optional[str]:

Option 2:

def f(parameter: int | None) -> str | None:

Also, is there any preference between Type | None and None | Type?

Solution

  • PEP 604 covers these topics in the specification section.

    The existing typing.Union and | syntax should be equivalent.

    int | str == typing.Union[int, str]

    The order of the items in the Union should not matter for equality.

    (int | str) == (str | int)
(int | str | float) == typing.Union[str, float, int]

    Optional values should be equivalent to the new union syntax

    None | t == typing.Optional[t]

    As @jonrsharpe comments, Union and Optional are not deprecated, so the Union and | syntax are acceptable.

    Łukasz Langa, a Python core developer, replied on a YouTube live related to the Python 3.10 release that Type | None is preferred over Optional[Type] for Python 3.10+.

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