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[ty] Disallow assignment to Final class attributes #19457

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merged 2 commits into from
Jul 21, 2025
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[ty] Disallow assignment to Final class attributes #19457

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[ty] Disallow assignment to `Final` class attributes
sharkdp Jul 21, 2025
aa4ff1e
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sharkdp Jul 21, 2025
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2 changes: 1 addition & 1 deletion crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md
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Original file line number Diff line number Diff line change
Expand Up @@ -498,7 +498,7 @@ class C:
reveal_type(C.__init__) # revealed: (self: C, instance_variable_no_default: int, instance_variable: int = Literal[1]) -> None

c = C(1)
# TODO: this should be an error
# error: [invalid-assignment] "Cannot assign to final attribute `instance_variable` on type `C`"
c.instance_variable = 2
```

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41 changes: 26 additions & 15 deletions crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md
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Expand Up @@ -170,26 +170,37 @@ Assignments to attributes qualified with `Final` are also not allowed:
```py
from typing import Final

class C:
FINAL_A: Final[int] = 1
FINAL_B: Final = 1
class Meta(type):
META_FINAL_A: Final[int] = 1
META_FINAL_B: Final = 1

class C(metaclass=Meta):
CLASS_FINAL_A: Final[int] = 1
CLASS_FINAL_B: Final = 1

def __init__(self):
self.FINAL_C: Final[int] = 1
self.FINAL_D: Final = 1
self.INSTANCE_FINAL_A: Final[int] = 1
self.INSTANCE_FINAL_B: Final = 1

# error: [invalid-assignment] "Cannot assign to final attribute `META_FINAL_A` on type `<class 'C'>`"
C.META_FINAL_A = 2
# error: [invalid-assignment] "Cannot assign to final attribute `META_FINAL_B` on type `<class 'C'>`"
C.META_FINAL_B = 2

# TODO: these should be errors (that mention `Final`)
C.FINAL_A = 2
# error: [invalid-assignment] "Object of type `Literal[2]` is not assignable to attribute `FINAL_B` of type `Literal[1]`"
C.FINAL_B = 2
# error: [invalid-assignment] "Cannot assign to final attribute `CLASS_FINAL_A` on type `<class 'C'>`"
C.CLASS_FINAL_A = 2
# error: [invalid-assignment] "Cannot assign to final attribute `CLASS_FINAL_B` on type `<class 'C'>`"
C.CLASS_FINAL_B = 2

# TODO: these should be errors (that mention `Final`)
c = C()
c.FINAL_A = 2
# error: [invalid-assignment] "Object of type `Literal[2]` is not assignable to attribute `FINAL_B` of type `Literal[1]`"
c.FINAL_B = 2
c.FINAL_C = 2
c.FINAL_D = 2
# error: [invalid-assignment] "Cannot assign to final attribute `CLASS_FINAL_A` on type `C`"
c.CLASS_FINAL_A = 2
# error: [invalid-assignment] "Cannot assign to final attribute `CLASS_FINAL_B` on type `C`"
c.CLASS_FINAL_B = 2
# TODO: this should be an error
c.INSTANCE_FINAL_A = 2
# TODO: this should be an error
c.INSTANCE_FINAL_B = 2
Comment on lines +200 to +203
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Implementing this for implicit instance attributes is much more involved, so I'm putting this PR up as a first increment.

```

## Mutability
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40 changes: 36 additions & 4 deletions crates/ty_python_semantic/src/types/infer.rs
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Expand Up @@ -3363,6 +3363,24 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
assignable
};

// Return true (and emit a diagnostic) if this is an invalid assignment to a `Final` attribute.
let invalid_assignment_to_final = |qualifiers: TypeQualifiers| -> bool {
if qualifiers.contains(TypeQualifiers::FINAL) {
if emit_diagnostics {
if let Some(builder) = self.context.report_lint(&INVALID_ASSIGNMENT, target) {
builder.into_diagnostic(format_args!(
"Cannot assign to final attribute `{attribute}` \
on type `{}`",
object_ty.display(db)
));
}
}
true
} else {
false
}
};

match object_ty {
Type::Union(union) => {
if union.elements(self.db()).iter().all(|elem| {
Expand Down Expand Up @@ -3558,8 +3576,12 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
}
PlaceAndQualifiers {
place: Place::Type(meta_attr_ty, meta_attr_boundness),
qualifiers: _,
qualifiers,
} => {
if invalid_assignment_to_final(qualifiers) {
return false;
}

let assignable_to_meta_attr =
if let Place::Type(meta_dunder_set, _) =
meta_attr_ty.class_member(db, "__set__".into()).place
Expand Down Expand Up @@ -3669,8 +3691,12 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
match object_ty.class_member(db, attribute.into()) {
PlaceAndQualifiers {
place: Place::Type(meta_attr_ty, meta_attr_boundness),
qualifiers: _,
qualifiers,
} => {
if invalid_assignment_to_final(qualifiers) {
return false;
}

let assignable_to_meta_attr = if let Place::Type(meta_dunder_set, _) =
meta_attr_ty.class_member(db, "__set__".into()).place
{
Expand Down Expand Up @@ -3733,11 +3759,17 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
place: Place::Unbound,
..
} => {
if let Place::Type(class_attr_ty, class_attr_boundness) = object_ty
if let PlaceAndQualifiers {
place: Place::Type(class_attr_ty, class_attr_boundness),
qualifiers,
} = object_ty
.find_name_in_mro(db, attribute)
.expect("called on Type::ClassLiteral or Type::SubclassOf")
.place
{
if invalid_assignment_to_final(qualifiers) {
return false;
}

if class_attr_boundness == Boundness::PossiblyUnbound {
report_possibly_unbound_attribute(
&self.context,
Expand Down
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