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GeezerVison is showing github.com — fix: Remove None-related special-casing during inference with literal contexts by edgarrmondragon · Pull Request #22351 · python/mypy

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@edgarrmondragon

@edgarrmondragon edgarrmondragon commented Nov 2, 2026 •
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Fixes #22334

Old impl

When the return type is a bare type variable (or an optional one) and the type context is a union mixing literals (or sentinels, or enums) with other types, the context was used to solve the type variable before looking at the arguments. This could produce a spurious error if an argument was wider than the context:

def none_if_one[T](value: T | Literal[1]) -> T | None: ...

x: str | Literal[1] | None = 1
x = none_if_one(x)  # error: expected "str | Literal[1]"

Now, if using the literal context produces errors, retry inference from the arguments only, and use that if it type checks and the result is compatible with the context.

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@ilevkivskyi

ilevkivskyi commented Nov 2, 2026

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Sorry, but I think this is (very) wrong. IMO this issue should be solved by removing None-related special-casing in infer_function_type_arguments_using_context() after #22286 (and its equivalent for second pass) is merged. Only if that doesn't work for some reasons we can consider some hacks.

(FWIW I verified my idea locally and it works, but there is also primer...)
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@edgarrmondragon

edgarrmondragon commented Nov 2, 2026 •
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Sorry, but I think this is (very) wrong [...]

Ok, thanks for taking a look 🙇‍♂️. Removing the is_overlapping_none checks in your branch does make the tests in this PR pass and I couldn't find any obvious regression related to generics and literals/enums/sentinels.
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@edgarrmondragon
edgarrmondragon force-pushed the 22334-generic-and-literal branch 2 times, most recently from 7f62eeb to 0eb0c1d Compare November 6, 2026 07:42
@edgarrmondragon
fix: Remove None-related special-casing during inference with literal…
c353380 
… contexts

Signed-off-by: Edgar Ramírez Mondragón <[email protected]>
@edgarrmondragon
edgarrmondragon force-pushed the 22334-generic-and-literal branch from 0eb0c1d to c353380 Compare November 6, 2026 07:42
@edgarrmondragon edgarrmondragon changed the title Fall back to argument-only inference when literal context is too narrow fix: Remove None-related special-casing during inference with literal contexts Nov 6, 2026
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Diff from mypy_primer, showing the effect of this PR on open source code:

scrapy (https://github.com/scrapy/scrapy)
+ tests/test_crawler.py:230: error: Unused "type: ignore" comment  [unused-ignore]
+ tests/test_crawler.py:230: error: Argument 1 to "get_addon" of "Crawler" has incompatible type "type[DefaultSpider]"; expected "type[TrackingAddon] | type[None]"  [arg-type]
+ tests/test_crawler.py:230: note: Error code "arg-type" not covered by "type: ignore[assignment]" comment
+ tests/test_crawler.py:233: error: Argument 1 to "get_addon" of "Crawler" has incompatible type "type[ParentAddon]"; expected "type[TrackingAddon] | type[None]"  [arg-type]

discord.py (https://github.com/Rapptz/discord.py)
+ discord/state.py:525: error: Item "None" of "Message | None" has no attribute "id"  [union-attr]
+ discord/message.py:576: error: Item "None" of "Message | None" has no attribute "created_at"  [union-attr]
+ discord/message.py:577: error: Item "None" of "Message | None" has no attribute "edited_at"  [union-attr]
+ discord/message.py:578: error: Item "None" of "Message | None" has no attribute "content"  [union-attr]
+ discord/message.py:579: error: Item "None" of "Message | None" has no attribute "embeds"  [union-attr]
+ discord/message.py:580: error: Item "None" of "Message | None" has no attribute "components"  [union-attr]
+ discord/message.py:581: error: Item "None" of "Message | None" has no attribute "stickers"  [union-attr]
+ discord/message.py:582: error: Item "None" of "Message | None" has no attribute "attachments"  [union-attr]
+ discord/message.py:583: error: Item "None" of "Message | None" has no attribute "flags"  [union-attr]
- discord/ext/commands/converter.py:269: error: Incompatible types in assignment (expression has type "Member | User | None", variable has type "Member | None")  [assignment]
+ discord/ext/commands/converter.py:269: error: Argument 1 has incompatible type "list[User | Member]"; expected "Iterable[Member | None]"  [arg-type]
+ discord/ext/commands/converter.py:321: error: Argument 1 has incompatible type "list[User | Member]"; expected "Iterable[User | None]"  [arg-type]
+ discord/ext/commands/converter.py:328: error: Unused "type: ignore" comment  [unused-ignore]
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Sentinel and enum type merge issues with generic

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@edgarrmondragon @ilevkivskyi
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