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[ty] Detect invalid attempts to subclass `Protocol[]` and `Generic[]` simultaneously by AlexWaygood · Pull Request #22948 · astral-sh/ruff · GitHub
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[ty] Detect invalid attempts to subclass Protocol[] and Generic[] simultaneously - #22948

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alex/generic-protocol-contexts
Jan 29, 2026
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[ty] Detect invalid attempts to subclass Protocol[] and Generic[] simultaneously#22948
AlexWaygood merged 1 commit into
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alex/generic-protocol-contexts

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@AlexWaygood AlexWaygood commented Jan 29, 2026 •
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Summary

The typing spec states that a class like this is invalid:

from typing import *

T = TypeVar("T")

class Foo(Protocol[T], Generic[T]): ...

This PR adds the necessary logic so that we detect (and, if possible, offer an unsafe autofix for) this error.

If you want to make a generic protocol, you have three alternative spellings of the above class at your disposal:

from typing import *

T = TypeVar("T")

class Foo[T](Protocol): ...
class Foo(Protocol, Generic[T]): ...
class Foo(Protocol[T]): ...

Test Plan

snapshots and mdtests

@AlexWaygood AlexWaygood added the ty The ty type checker label Jan 29, 2026
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astral-sh-bot Bot commented Jan 29, 2026

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Typing conformance results improved 🎉

The percentage of diagnostics emitted that were expected errors increased from 80.00% to 80.02%. The percentage of expected errors that received a diagnostic increased from 71.42% to 71.51%.

Summary

Metric Old New Diff Outcome
True Positives 772 773 +1 ⏫ (✅)
False Positives 193 193 +0
False Negatives 309 308 -1 ⏬ (✅)
Total Diagnostics 965 966 +1 ⏫
Precision 80.00% 80.02% +0.02% ⏫ (✅)
Recall 71.42% 71.51% +0.09% ⏫ (✅)

True positives added

Details
Location Name Message
protocols_generic.py:44:14 invalid-generic-class Cannot both inherit from subscripted Protocol and subscripted Generic

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mypy_primer results

Changes were detected when running on open source projects 
prefect (https://github.com/PrefectHQ/prefect)
- src/integrations/prefect-dbt/prefect_dbt/core/settings.py:94:28: error[invalid-assignment] Object of type `dict[Any, Any] | int | dict[str, Any] | ... omitted 4 union elements` is not assignable to `dict[str, Any]`
- src/integrations/prefect-dbt/prefect_dbt/core/settings.py:99:28: error[invalid-assignment] Object of type `int | dict[Any, Any] | float | ... omitted 3 union elements` is not assignable to `dict[str, Any]`
- src/prefect/cli/deploy/_core.py:86:21: error[invalid-assignment] Object of type `dict[Any, Any] | int | dict[str, Any] | ... omitted 4 union elements` is not assignable to `dict[str, Any]`
- src/prefect/cli/deploy/_core.py:87:21: error[invalid-assignment] Object of type `int | dict[Any, Any] | float | ... omitted 3 union elements` is not assignable to `dict[str, Any]`
- src/prefect/deployments/runner.py:997:70: warning[possibly-missing-attribute] Attribute `__name__` may be missing on object of type `Unknown | (((...) -> Any) & ((*args: object, **kwargs: object) -> object))`
+ src/prefect/deployments/runner.py:997:70: warning[possibly-missing-attribute] Attribute `__name__` may be missing on object of type `Unknown | ((...) -> Any)`
- src/prefect/deployments/steps/core.py:137:38: error[invalid-argument-type] Argument is incorrect: Argument type `dict[Any, Any] | int | dict[str, Any] | ... omitted 4 union elements` does not satisfy constraints (`str`, `int`, `int | float`, `bool`, `dict[Any, Any]`, `list[Any]`, `None`) of type variable `T`
+ src/prefect/flow_engine.py:989:32: error[invalid-await] `Unknown | R@FlowRunEngine | Coroutine[Any, Any, R@FlowRunEngine]` is not awaitable
+ src/prefect/flow_engine.py:1580:24: error[invalid-await] `Unknown | R@AsyncFlowRunEngine | Coroutine[Any, Any, R@AsyncFlowRunEngine]` is not awaitable
+ src/prefect/flow_engine.py:1661:43: error[invalid-argument-type] Argument to function `next` is incorrect: Expected `SupportsNext[Unknown]`, found `Unknown | R@run_generator_flow_sync`
+ src/prefect/flow_engine.py:1669:21: warning[possibly-missing-attribute] Attribute `throw` may be missing on object of type `Unknown | R@run_generator_flow_sync`
+ src/prefect/flow_engine.py:1703:44: warning[possibly-missing-attribute] Attribute `__anext__` may be missing on object of type `Unknown | R@run_generator_flow_async`
+ src/prefect/flow_engine.py:1710:25: warning[possibly-missing-attribute] Attribute `throw` may be missing on object of type `Unknown | R@run_generator_flow_async`
- src/prefect/flows.py:285:34: error[unresolved-attribute] Object of type `((**P@Flow) -> R@Flow) & ((*args: object, **kwargs: object) -> object)` has no attribute `__name__`
+ src/prefect/flows.py:285:34: error[unresolved-attribute] Object of type `(**P@Flow) -> R@Flow` has no attribute `__name__`
- src/prefect/flows.py:403:68: error[unresolved-attribute] Object of type `((**P@Flow) -> R@Flow) & ((*args: object, **kwargs: object) -> object)` has no attribute `__name__`
+ src/prefect/flows.py:403:68: error[unresolved-attribute] Object of type `(**P@Flow) -> R@Flow` has no attribute `__name__`
- src/prefect/flows.py:1877:53: warning[unused-type-ignore-comment] Unused blanket `type: ignore` directive
+ src/prefect/flows.py:1937:21: error[no-matching-overload] No overload of function `run_coro_as_sync` matches arguments
- src/prefect/utilities/templating.py:320:13: error[invalid-assignment] Invalid subscript assignment with key of type `object` and value of type `Unknown | int | dict[str, Any] | ... omitted 4 union elements` on object of type `dict[str, Any]`
+ src/prefect/utilities/templating.py:320:13: error[invalid-assignment] Invalid subscript assignment with key of type `object` and value of type `Unknown | dict[str, Any]` on object of type `dict[str, Any]`
- src/prefect/utilities/templating.py:323:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_block_document_references | dict[str, Any]`, found `list[Unknown | int | dict[str, Any] | ... omitted 4 union elements]`
+ src/prefect/utilities/templating.py:323:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_block_document_references | dict[str, Any]`, found `list[Unknown | dict[str, Any]]`
- src/prefect/utilities/templating.py:437:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_variables`, found `dict[object, Unknown | int | float | ... omitted 4 union elements]`
+ src/prefect/utilities/templating.py:437:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_variables`, found `dict[object, Unknown]`
- src/prefect/utilities/templating.py:442:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_variables`, found `list[Unknown | int | float | ... omitted 4 union elements]`
+ src/prefect/utilities/templating.py:442:16: error[invalid-return-type] Return type does not match returned value: expected `T@resolve_variables`, found `list[Unknown]`
+ src/prefect/workers/base.py:232:13: error[invalid-argument-type] Argument is incorrect: Argument type `str | dict[str, Any]` does not satisfy constraints (`str`, `int`, `int | float`, `bool`, `dict[Any, Any]`, `list[Any]`, `None`) of type variable `T`
- src/prefect/workers/base.py:232:13: error[invalid-argument-type] Argument is incorrect: Argument type `str | int | dict[str, Any] | ... omitted 3 union elements` does not satisfy constraints (`str`, `int`, `int | float`, `bool`, `dict[Any, Any]`, `list[Any]`, `None`) of type variable `T`
- src/prefect/workers/base.py:234:20: error[invalid-argument-type] Argument expression after ** must be a mapping type: Found `int | Unknown | float | ... omitted 4 union elements`

scikit-build-core (https://github.com/scikit-build/scikit-build-core)
- src/scikit_build_core/build/wheel.py:99:20: error[no-matching-overload] No overload of bound method `__init__` matches arguments
- Found 47 diagnostics
+ Found 46 diagnostics

core (https://github.com/home-assistant/core)
+ homeassistant/util/variance.py:47:12: error[invalid-return-type] Return type does not match returned value: expected `(**_P@ignore_variance) -> _R@ignore_variance`, found `_Wrapped[_P@ignore_variance, int | _R@ignore_variance | float | datetime, _P@ignore_variance, _R@ignore_variance | int | float | datetime]`
- Found 14512 diagnostics
+ Found 14513 diagnostics

No memory usage changes detected ✅

@AlexWaygood
AlexWaygood marked this pull request as ready for review January 29, 2026 17:31
@AlexWaygood
AlexWaygood merged commit 340aae9 into main Jan 29, 2026
50 checks passed
@AlexWaygood
AlexWaygood deleted the alex/generic-protocol-contexts branch January 29, 2026 22:27
carljm added a commit that referenced this pull request Jan 30, 2026
@carljm
Merge branch 'main' into claude/fix-issue-1858-UjARA
a43f1ce 
* main: (76 commits)
  [ty] Improve the check for `NewType`s with generic bases (#22961)
  [ty] Ban legacy `TypeVar` bounds or constraints from containing type variables (#22949)
  Bump the typing conformance suite pin (#22960)
  [ty] Emit an error if a TypeVarTuple is used to subscript `Generic` or `Protocol` without being unpacked (#22952)
  [ty] Reduce false positives when subscripting classes generic over `TypeVarTuple`s (#22950)
  [ty] Detect invalid attempts to subclass `Protocol[]` and `Generic[]` simultaneously (#22948)
  Fix suppression indentation matching (#22903)
  Remove hidden `--output-format` warning (#22944)
  [ty] Validate signatures of dataclass `__post_init__` methods (#22730)
  [ty] extend special-cased `numbers` diagnostic to `invalid-argument-type` errors (#22938)
  [ty] Avoid false positive for `not-iterable` with no-positive intersection types (#22089)
  [ty] Preserve pure negation types in descriptor protocol (#22907)
  [ty] add special-case diagnostic for `numbers` module (#22931)
  [ty] Move the location of more `invalid-overload` diagnostics (#22933)
  [ty] Fix unary and comparison operators for TypeVars with union bounds (#22925)
  [ty] Rule Selection: ignore/warn/select all rules (unless subsequently overriden) (#22832)
  [ty] Fix TypedDict construction from existing TypedDict values (#22904)
  [ty] fix bug in string annotations and clean up diagnostics (#22913)
  [ty] Improve support for goto-type, goto-declaration, hover, and highlighting of string annotations (#22878)
  [ty] Rename old typing imports to new on `unresolved-reference`. (#22827)
  ...
@MeGaGiGaGon MeGaGiGaGon mentioned this pull request Aug 26, 2026
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@charliermarsh charliermarsh charliermarsh approved these changes
@carljm carljm Awaiting requested review from carljm
@sharkdp sharkdp Awaiting requested review from sharkdp
@dcreager dcreager Awaiting requested review from dcreager

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@AlexWaygood @charliermarsh