Standardizing Union of Literal and None

[Avasam]

Literal["foo", None] is equivalent to Literal["foo"] | None and Optional[Literal["foo"]]. This came up a handful of times in my PR reviews on typeshed. I can still find usages of either form. Flake8-PYI should be able to detect simple Literal | None unions (ie: as long as it's not aliased) and could help standardize its usage, if this feature is wanted.

Ref. https://peps.python.org/pep-0586/#legal-parameters-for-literal-at-type-check-time

[srittau]

If we do this, I'd prefer to standardize on Literal["foo"] | None. To me that's the most obvious form, especially since Literal[None] == None.

[AlexWaygood]

I don't mind much either way, but I agree it would be nice for flake8-pyi to have an opinion on this, so we can standardise this across typeshed. Literal["foo"] | None is more verbose than Literal["foo", None], but it is true that None is very special, so I can definitely see the argument for the former version.

[Avasam]

Some of my thoughts:

• It's more verbose by only one character
• The | union is more consistent with other Optional/None-able type unions. (Making it easier to search for and visually parse)
• Whilst it's cool that the type system allows for Literal[None], it is surprising and I managed to forget about it more than once. (this point kinda becomes moot if a linter checks for it)

But I don't feel strongly either way.

[AlexWaygood]

Let's standardise on Literal["foo"] | None, then

[srittau]

Also, it would be kind of strange (to me) to have an Union like Literal["", None] | int instead of Literal[""] | int | None.

[tomasr8]

Hey again! I should have the time this week so I thought I'd take a stab at this (in case none of you is already working on it?)

[AlexWaygood]

Hey again! I should have the time this week so I thought I'd take a stab at this (in case none of you is already working on it?)

You're very welcome to it! :D