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[ty] Disallow Final in function parameter/return-type annotations #19480

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Merged
merged 2 commits into from
Jul 22, 2025
Merged

[ty] Disallow Final in function parameter/return-type annotations #19480

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01af039
[ty] Disallow `Final` in function parameter annotations
sharkdp Jul 22, 2025
5a96cfc
Emit errors for return type annotations as well
sharkdp Jul 22, 2025
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17 changes: 15 additions & 2 deletions crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md
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Original file line number Diff line number Diff line change
Expand Up @@ -255,6 +255,11 @@ class Derived(Base):
Final may only be used in assignments or variable annotations. Using it in any other position is an
error.

```toml
[environment]
python-version = "3.12"
```

```py
from typing import Final, ClassVar, Annotated

Expand All @@ -275,13 +280,21 @@ class C:
self.LEGAL_I: Final[int]
self.LEGAL_I = 1

# TODO: This should be an error
# error: [invalid-type-form] "`Final` is not allowed in function parameter annotations"
def f(ILLEGAL: Final[int]) -> None:
pass

# TODO: This should be an error
# error: [invalid-type-form] "`Final` is not allowed in function parameter annotations"
def f[T](ILLEGAL: Final[int]) -> None:
pass

# error: [invalid-type-form] "`Final` is not allowed in function return type annotations"
def f() -> Final[None]: ...

# error: [invalid-type-form] "`Final` is not allowed in function return type annotations"
def f[T](x: T) -> Final[T]:
return x

# TODO: This should be an error
class Foo(Final[tuple[int]]): ...

Expand Down
32 changes: 28 additions & 4 deletions crates/ty_python_semantic/src/types/infer.rs
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Expand Up @@ -2219,7 +2219,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
.as_deref()
.expect("function type params scope without type params");

self.infer_optional_annotation_expression(
self.infer_return_type_annotation(
function.returns.as_deref(),
DeferredExpressionState::None,
);
Expand Down Expand Up @@ -2581,7 +2581,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
if self.defer_annotations() {
self.deferred.insert(definition);
} else {
self.infer_optional_annotation_expression(
self.infer_return_type_annotation(
returns.as_deref(),
DeferredExpressionState::None,
);
Expand Down Expand Up @@ -2638,6 +2638,24 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
);
}

fn infer_return_type_annotation(
&mut self,
returns: Option<&ast::Expr>,
deferred_expression_state: DeferredExpressionState,
) {
if let Some(returns) = returns {
let annotated = self.infer_annotation_expression(returns, deferred_expression_state);

if annotated.qualifiers.contains(TypeQualifiers::FINAL) {
if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, returns) {
builder.into_diagnostic(
"`Final` is not allowed in function return type annotations",
);
}
}
}
}

fn infer_parameters(&mut self, parameters: &ast::Parameters) {
let ast::Parameters {
range: _,
Expand Down Expand Up @@ -2668,10 +2686,16 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
default: _,
} = parameter_with_default;

self.infer_optional_annotation_expression(
let annotated = self.infer_optional_annotation_expression(
parameter.annotation.as_deref(),
DeferredExpressionState::None,
);

if annotated.is_some_and(|annotated| annotated.qualifiers.contains(TypeQualifiers::FINAL)) {
if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, parameter) {
builder.into_diagnostic("`Final` is not allowed in function parameter annotations");
}
}
Comment on lines +2694 to +2698
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@AlexWaygood AlexWaygood Jul 22, 2025

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I believe the same goes for ClassVar FWIW; we could add that rule as well, while we're here?

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@sharkdp sharkdp Jul 22, 2025

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I believe the same goes for ClassVar FWIW; we could add that rule as well, while we're here?

ClassVar should be disallowed in many more positions as well. I'll write up a ticket. Seems like a good beginner task?

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@AlexWaygood AlexWaygood Jul 22, 2025

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Sounds good!

}

fn infer_parameter(&mut self, parameter: &ast::Parameter) {
Expand Down Expand Up @@ -2956,7 +2980,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
}

fn infer_function_deferred(&mut self, function: &ast::StmtFunctionDef) {
self.infer_optional_annotation_expression(
self.infer_return_type_annotation(
function.returns.as_deref(),
DeferredExpressionState::Deferred,
);
Expand Down
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