[ty] Add diagnostic hint for invalid assignments involving invariant generics by sharkdp · Pull Request #24032 · astral-sh/ruff

sharkdp · 2026-03-18T14:38:45Z

Summary

Relates to this FAQ entry. Consider

def modify(xs: list[int]):
    xs.append(42)

answers: list[bool] = [True, False]
modify(answers)

With this change, we now emit some info hints in the invalid-assignment diagnostic:

Test Plan

New snapshot tests

astral-sh-bot · 2026-03-18T14:40:53Z

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 85.29%. The percentage of expected errors that received a diagnostic held steady at 78.13%. The number of fully passing files held steady at 64/132.

...nvalid_assignment_d…_-_Invalid_assignment_d…_-_Invariant_generic_cl…_(4083c269b4d4746f).snap

astral-sh-bot · 2026-03-18T14:47:42Z

Memory usage report

Memory usage unchanged ✅

sharkdp · 2026-03-18T14:48:21Z

...nvalid_assignment_d…_-_Invalid_assignment_d…_-_Invariant_generic_cl…_(4083c269b4d4746f).snap

+11 |
+12 | def _(source: dict[bool, str]):
+   |
+info: `dict` is invariant in its second type parameter


I guess we could also invest a bit more effort and refer to the name of the type parameter (_VT@dict in this case), if that seems helpful.

sharkdp · 2026-03-18T14:49:54Z

...nvalid_assignment_d…_-_Invalid_assignment_d…_-_Invariant_generic_cl…_(4083c269b4d4746f).snap

+   |
+info: `dict` is invariant in its first type parameter
+info: For more information, see https://docs.astral.sh/ty/reference/typing-faq/#invariant-generics
+info: rule `invalid-assignment` is enabled by default


Here, we test that we do not emit the "Consider using … collections.abc.Mapping" hint, because Mapping is still invariant in its key type, so that wouldn't help here.

astral-sh-bot · 2026-03-18T14:50:15Z

`ecosystem-analyzer` results

No diagnostic changes detected ✅

Full report with detailed diff (timing results)

sharkdp · 2026-03-18T14:51:36Z

...nvalid_assignment_d…_-_Invalid_assignment_d…_-_Invariant_generic_cl…_(4083c269b4d4746f).snap

+41 |
+42 | def _(source: MutableSequence[bool]):
+   |
+info: `Counter` is invariant in its type parameter


There is no covariant supertype suggestion for Counter because it's type parameter corresponds to a key-like parameter of dict.

sharkdp · 2026-03-18T14:52:25Z

...nvalid_assignment_d…_-_Invalid_assignment_d…_-_Invariant_generic_cl…_(4083c269b4d4746f).snap

+error[invalid-assignment]: Object of type `MutableSequence[bool]` is not assignable to `MutableSequence[int]`
+  --> src/mdtest_snippet.py:43:13
+   |
+42 | def _(source: MutableSequence[bool]):
+43 |     target: MutableSequence[int] = source  # error: [invalid-assignment]
+   |             --------------------   ^^^^^^ Incompatible value of type `MutableSequence[bool]`
+   |             |
+   |             Declared type
+44 |
+45 | def _(source: MutableSet[bool]):
+   |
+info: `MutableSequence` is invariant in its type parameter
+info: Consider using the covariant supertype `collections.abc.Sequence`
+info: For more information, see https://docs.astral.sh/ty/reference/typing-faq/#invariant-generics
+info: rule `invalid-assignment` is enabled by default
+


I wasn't sure about these. It seems a bit silly. If you explicitly annotated something with MutableSequence, you probably meant it?

sharkdp · 2026-03-18T14:52:53Z

...nvalid_assignment_d…_-_Invalid_assignment_d…_-_Invariant_generic_cl…_(4083c269b4d4746f).snap

+63 |     target: list[str] = source  # error: [invalid-assignment]
+   |
+info: `MyContainer` is invariant in its type parameter
+info: For more information, see https://docs.astral.sh/ty/reference/typing-faq/#invariant-generics


I think it's fine to refer to that FAQ entry even for user types.

sharkdp · 2026-03-18T14:55:28Z

...nvalid_assignment_d…_-_Invalid_assignment_d…_-_Invariant_generic_cl…_(4083c269b4d4746f).snap

+error[invalid-assignment]: Object of type `list[int]` is not assignable to `list[str]`
+  --> src/mdtest_snippet.py:63:13
+   |
+61 |     target: MyContainer[int] = source  # error: [invalid-assignment]
+62 | def _(source: list[int]):
+63 |     target: list[str] = source  # error: [invalid-assignment]
+   |             ---------   ^^^^^^ Incompatible value of type `list[int]`
+   |             |
+   |             Declared type
+64 | from collections.abc import Sequence
+   |
+info: rule `invalid-assignment` is enabled by default


Note that there is no hint at all here. Something is more fundamentally wrong with your code if you're trying to assign a list[str] to a list[int]. And it's unlikely that we can help you by teaching you about invariant generics.

crates/ty_python_semantic/src/types.rs

crates/ty_python_semantic/src/types/diagnostic.rs

* main: [`flake8-bandit`] Check tuple arguments for partial paths in `S607` (#24080) [ty] Update Salsa (#24081) Update Rust toolchain to 1.94 and MSRV to 1.92 (#24076) [ty] Move ruffen-docs formatting config to a `ruff.toml` config file (#24074) [ty] `reveal_type` diagnostics in unreachable code (#24070) [ty] Improve keyword argument narrowing for nested dictionaries (#24010) [ty] Preserve blank lines between comments and imports in add-import action (#24066) [ty] Add diagnostic hint for invalid assignments involving invariant generics (#24032) Clarify `extend-ignore` and `extend-select` settings documentation (#24064) [ty] Batch changes to watched paths (#24045) replace deprecated `std::f64::EPSILON` with `f64::EPSILON` (#24067) [ty] Fix untracked reads in Salsa queries that can lead to backdating panics (#24051) [ty] Unions/intersections of gradual types should be assignable to `Never` (#24056) Fix incorrect path for ty_python_semantic in fuzzer (#24052) Bump 0.15.7 (#24049) [ty] ecosystem-analyzer: Fail on newly panicking projects (#24043) Don't show noqa hover for non-Python documents (#24040)

sharkdp added the ty Multi-file analysis & type inference label Mar 18, 2026

sharkdp force-pushed the david/invariant-generics-hint branch 2 times, most recently from 87e40a0 to fa11478 Compare March 18, 2026 14:43

sharkdp commented Mar 18, 2026

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sharkdp commented Mar 18, 2026

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[ty] Add diagnostic hint for invariant generics

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sharkdp force-pushed the david/invariant-generics-hint branch from fa11478 to 1850d53 Compare March 18, 2026 20:26

sharkdp marked this pull request as ready for review March 18, 2026 20:26

sharkdp requested review from AlexWaygood, carljm, dcreager and ibraheemdev as code owners March 18, 2026 20:26

astral-sh-bot bot assigned oconnor663 Mar 18, 2026

astral-sh-bot bot requested a review from oconnor663 March 18, 2026 20:26

carljm removed their request for review March 18, 2026 20:38

oconnor663 approved these changes Mar 19, 2026

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oconnor663 removed their assignment Mar 19, 2026

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sharkdp merged commit 11ef7f4 into main Mar 20, 2026
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sharkdp deleted the david/invariant-generics-hint branch March 20, 2026 09:16

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[ty] Add diagnostic hint for invalid assignments involving invariant generics#24032

[ty] Add diagnostic hint for invalid assignments involving invariant generics#24032
sharkdp merged 2 commits intomainfrom
david/invariant-generics-hint

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