[wip] Better handling of generics when narrowing#20673
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hauntsaninja wants to merge 1 commit intopython:masterfrom
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[wip] Better handling of generics when narrowing#20673hauntsaninja wants to merge 1 commit intopython:masterfrom
hauntsaninja wants to merge 1 commit intopython:masterfrom
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hauntsaninja
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Jan 28, 2026
hauntsaninja
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Jan 28, 2026
mypy/checker.py
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| # See testMatchEnumSingleChoice | ||
| target_type = coerce_to_literal(target_type) | ||
| if isinstance(target_type, Instance): | ||
| target_type = erase_type(target_type) |
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We might want to distribute this over unions, but we do want to avoid erasure for several different types. Let's see if there is primer...
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Fixes #20701 / see discussion there Background: - In #20492 I added code to treat bytes, bytearray and memoryview as having custom `__eq__`, since they can compare equal to values that are of other types. I did this based on primer hits on draft versions of that PR - In #20643 I rewrote the handling of custom equality when narrowing. That fixed many issues, but regressed the equivalent of `testNarrowingOptionalBytes` because we're now more principled about custom `__eq__`. Perhaps interestingly the primer on that PR is small and not controversial, so I didn't feel the need to prioritise unsound things However, we only need one of the operands to have custom equality to model these things as reachable, so I think we can probably just remove `builtins.bytes` from here. This behaviour still isn't ideal. For instance, narrowing here shouldn't depend on whether we promote or not (we shouldn't narrow `v_all` to bytes) The real fix is adding more dedicated special casing so that we have an in-between option between "narrow assuming we can only compare equal to values of the same type" and "assume `__eq__` means we can't conclude anything about the positive case" Other relevant PRs: - #20673 attempts to remove the other custom eq logic for list, dict, set I added in #20492 . It should be close to mergeable - #20660 improves reachability and unfortunately to land it I have to do a rethink for how int, float, complex narrowing works. I actually think the current version has pretty good semantics and the primer is valid (but scary). But I want to think about it more
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Diff from mypy_primer, showing the effect of this PR on open source code: graphql-core (https://github.com/graphql-python/graphql-core)
- tests/execution/test_resolve.py:322: error: Value of type "list[SourceLocation] | None" is not indexable [index]
core (https://github.com/home-assistant/core)
+ homeassistant/components/philips_js/media_player.py:203: error: Argument 1 to "get_browse_image_url" of "MediaPlayerEntity" has incompatible type "str | None"; expected "str" [arg-type]
+ homeassistant/components/isy994/sensor.py:206: error: Argument 2 to "temperature" of "UnitSystem" has incompatible type "str | None"; expected "str" [arg-type]
rich (https://github.com/Textualize/rich)
+ rich/console.py:1540: error: Redundant cast to "Literal['left', 'center', 'right']" [redundant-cast]
discord.py (https://github.com/Rapptz/discord.py)
- discord/channel.py:3048: error: Argument "stickers" to "handle_message_parameters" has incompatible type "Sequence[GuildSticker | StickerItem]"; expected "list[str | int] | None" [arg-type]
+ discord/channel.py:3048: error: Argument "stickers" to "handle_message_parameters" has incompatible type "Sequence[Any]"; expected "list[str | int] | None" [arg-type]
- discord/ext/tasks/__init__.py:765: error: Incompatible types in assignment (expression has type "time | Sequence[time]", variable has type "list[time]") [assignment]
+ discord/ext/tasks/__init__.py:765: error: Incompatible types in assignment (expression has type "time | Sequence[Any]", variable has type "list[time]") [assignment]
dulwich (https://github.com/dulwich/dulwich)
+ dulwich/lru_cache.py:164: error: Item "None" of "_LRUNode[K, V] | None" has no attribute "prev" [union-attr]
+ dulwich/lru_cache.py:171: error: Item "None" of "_LRUNode[K, V] | None" has no attribute "prev" [union-attr]
+ dulwich/lru_cache.py:171: error: Item "None" of "_LRUNode[K, V] | None" has no attribute "key" [union-attr]
+ dulwich/lru_cache.py:175: error: Incompatible types in "yield" (actual type "_LRUNode[K, V] | None", expected type "_LRUNode[K, V]") [misc]
rotki (https://github.com/rotki/rotki)
+ rotkehlchen/tests/integration/test_premium.py:820: error: Statement is unreachable [unreachable]
+ rotkehlchen/tests/integration/test_premium.py:834: error: Statement is unreachable [unreachable]
xarray (https://github.com/pydata/xarray)
+ xarray/tests/test_dataarray.py:291: error: Unused "type: ignore" comment [unused-ignore]
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Notably we preserve behaviour on the
testNarrowingCollectionstest I added