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optional type hint python

Bottle Vines answered on September 16, 2022 Popularity 8/10 Helpfulness 2/10

answer optional type hint python

optional type hint python

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Optional[...] is a shorthand notation for Union[..., None], telling the type checker that either an object of the specific type is required, or None is required. ... stands for any valid type hint, including complex compound types or a Union[] of more types. Whenever you have a keyword argument with default value None, you should use Optional. (Note: If you are targeting Python 3.10 or newer, PEP 604 introduced a better syntax, see below).

So for your two examples, you have dict and list container types, but the default value for the a keyword argument shows that None is permitted too so use Optional[...]:

from typing import Optional

def test(a: Optional[dict] = None) -> None:
    #print(a) ==> {'a': 1234}
    #or
    #print(a) ==> None

def test(a: Optional[list] = None) -> None:
    #print(a) ==> [1, 2, 3, 4, 'a', 'b']
    #or
    #print(a) ==> None
There is technically no difference between using Optional[] on a Union[], or just adding None to the Union[]. So Optional[Union[str, int]] and Union[str, int, None] are exactly the same thing.

Personally, I'd stick with always using Optional[] when setting the type for a keyword argument that uses = None to set a default value, this documents the reason why None is allowed better. Moreover, it makes it easier to move the Union[...] part into a separate type alias, or to later remove the Optional[...] part if an argument becomes mandatory.

Popularity 8/10 Helpfulness 2/10 Language python

Source: Grepper

Tags: hint python

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Contributed on Sep 16 2022

Bottle Vines

0 Answers Avg Quality 2/10

optional type hint python

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