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[ty] Synthesize __setattr__ and __delattr__ for frozen dataclasses #2

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22620d5
Support relative `--ty-path` in ty-benchmark (#18385)
ibraheemdev May 30, 2025
b838720
[ty] Synthesize __setattr__ and __delattr__ for frozen dataclasses
thejchap Jun 1, 2025
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29 changes: 29 additions & 0 deletions crates/ty_python_semantic/resources/mdtest/dataclasses.md
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Original file line number Diff line number Diff line change
Expand Up @@ -380,6 +380,19 @@ frozen_instance = MyFrozenClass(1)
frozen_instance.x = 2 # error: [invalid-assignment]
```

Deleting fields will also generate a diagnostic.

```py
from dataclasses import dataclass

@dataclass(frozen=True)
class MyFrozenClass:
x: int = 1

frozen_instance = MyFrozenClass()
del frozen_instance.x # TODO: error: [invalid-assignment]
```

If `__setattr__()` or `__delattr__()` is defined in the class, we should emit a diagnostic.

```py
Expand Down Expand Up @@ -427,6 +440,22 @@ frozen = MyFrozenClass()
frozen.x = 2 # error: [unresolved-attribute]
```

A diagnostic is also emitted if a frozen dataclass is inherited, and an attempt is made to mutate an
attribute in the child class:

```py
from dataclasses import dataclass

@dataclass(frozen=True)
class MyFrozenClass:
x: int = 1

class MyFrozenChildClass(MyFrozenClass): ...

frozen = MyFrozenChildClass()
frozen.x = 2 # error: [invalid-assignment]
```

### `match_args`

To do
Expand Down
20 changes: 20 additions & 0 deletions crates/ty_python_semantic/src/types/class.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1400,6 +1400,26 @@ impl<'db> ClassLiteral<'db> {
.with_annotated_type(KnownClass::Type.to_instance(db));
signature_from_fields(vec![cls_parameter])
}
(CodeGeneratorKind::DataclassLike, "__setattr__" | "__delattr__") => {
if !has_dataclass_param(DataclassParams::FROZEN) {
return None;
}

let signature = Signature::new(
Parameters::new([
Parameter::positional_or_keyword(Name::new_static("self"))
.with_annotated_type(Type::instance(
db,
self.apply_optional_specialization(db, specialization),
)),
Parameter::positional_or_keyword(Name::new_static("name")),
Parameter::positional_or_keyword(Name::new_static("value")),
]),
Some(Type::Never),
);

Some(CallableType::function_like(db, signature))
}
(CodeGeneratorKind::DataclassLike, "__lt__" | "__le__" | "__gt__" | "__ge__") => {
if !has_dataclass_param(DataclassParams::ORDER) {
return None;
Expand Down
261 changes: 118 additions & 143 deletions crates/ty_python_semantic/src/types/infer.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3099,101 +3099,147 @@ impl<'db> TypeInferenceBuilder<'db> {
| Type::TypeVar(..)
| Type::AlwaysTruthy
| Type::AlwaysFalsy => {
let is_read_only = || {
let dataclass_params = match object_ty {
Type::NominalInstance(instance) => match instance.class {
ClassType::NonGeneric(cls) => cls.dataclass_params(self.db()),
ClassType::Generic(cls) => {
cls.origin(self.db()).dataclass_params(self.db())
}
},
_ => None,
};

dataclass_params.is_some_and(|params| params.contains(DataclassParams::FROZEN))
};
let setattr_dunder_call_result = object_ty.try_call_dunder_with_policy(
db,
"__setattr__",
&mut CallArgumentTypes::positional([
Type::StringLiteral(StringLiteralType::new(db, Box::from(attribute))),
value_ty,
]),
MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK,
);

match object_ty.class_member(db, attribute.into()) {
meta_attr @ SymbolAndQualifiers { .. } if meta_attr.is_class_var() => {
if emit_diagnostics {
if let Some(builder) =
self.context.report_lint(&INVALID_ATTRIBUTE_ACCESS, target)
{
builder.into_diagnostic(format_args!(
"Cannot assign to ClassVar `{attribute}` \
from an instance of type `{ty}`",
ty = object_ty.display(self.db()),
));
}
}
false
}
SymbolAndQualifiers {
symbol: Symbol::Type(meta_attr_ty, meta_attr_boundness),
qualifiers: _,
} => {
if is_read_only() {
// First, try to call the `__setattr__` dunder method. If this is present/defined, overrides
// assigning the attributed by the normal mechanism.
match setattr_dunder_call_result {
Ok(result) => match result.return_type(db) {
Type::Never => {
if emit_diagnostics {
if let Some(builder) =
self.context.report_lint(&INVALID_ASSIGNMENT, target)
{
builder.into_diagnostic(format_args!(
"Property `{attribute}` defined in `{ty}` is read-only",
ty = object_ty.display(self.db()),
"Attribute `{attribute}` on type `{}` is read-only",
object_ty.display(db)
));
}
}
false
} else {
let assignable_to_meta_attr = if let Symbol::Type(meta_dunder_set, _) =
meta_attr_ty.class_member(db, "__set__".into()).symbol
}
_ => true,
},
Err(CallDunderError::CallError(..)) => {
if emit_diagnostics {
if let Some(builder) =
self.context.report_lint(&UNRESOLVED_ATTRIBUTE, target)
{
let successful_call = meta_dunder_set
.try_call(
db,
&CallArgumentTypes::positional([
meta_attr_ty,
object_ty,
value_ty,
]),
)
.is_ok();

if !successful_call && emit_diagnostics {
builder.into_diagnostic(format_args!(
"Can not assign object of `{}` to attribute \
`{attribute}` on type `{}` with \
custom `__setattr__` method.",
value_ty.display(db),
object_ty.display(db)
));
}
}
false
}
Err(CallDunderError::PossiblyUnbound(_)) => true,
Err(CallDunderError::MethodNotAvailable) => {
match object_ty.class_member(db, attribute.into()) {
meta_attr @ SymbolAndQualifiers { .. } if meta_attr.is_class_var() => {
if emit_diagnostics {
if let Some(builder) =
self.context.report_lint(&INVALID_ASSIGNMENT, target)
self.context.report_lint(&INVALID_ATTRIBUTE_ACCESS, target)
{
// TODO: Here, it would be nice to emit an additional diagnostic that explains why the call failed
builder.into_diagnostic(format_args!(
"Cannot assign to ClassVar `{attribute}` \
from an instance of type `{ty}`",
ty = object_ty.display(self.db()),
));
}
}
false
}
SymbolAndQualifiers {
symbol: Symbol::Type(meta_attr_ty, meta_attr_boundness),
qualifiers: _,
} => {
let assignable_to_meta_attr =
if let Symbol::Type(meta_dunder_set, _) =
meta_attr_ty.class_member(db, "__set__".into()).symbol
{
let successful_call = meta_dunder_set
.try_call(
db,
&CallArgumentTypes::positional([
meta_attr_ty,
object_ty,
value_ty,
]),
)
.is_ok();

if !successful_call && emit_diagnostics {
if let Some(builder) = self
.context
.report_lint(&INVALID_ASSIGNMENT, target)
{
// TODO: Here, it would be nice to emit an additional diagnostic that explains why the call failed
builder.into_diagnostic(format_args!(
"Invalid assignment to data descriptor attribute \
`{attribute}` on type `{}` with custom `__set__` method",
object_ty.display(db)
));
}
}
}
}

successful_call
} else {
ensure_assignable_to(meta_attr_ty)
};
successful_call
} else {
ensure_assignable_to(meta_attr_ty)
};

let assignable_to_instance_attribute =
if meta_attr_boundness == Boundness::PossiblyUnbound {
let (assignable, boundness) = if let Symbol::Type(
instance_attr_ty,
instance_attr_boundness,
) =
object_ty.instance_member(db, attribute).symbol
{
(
ensure_assignable_to(instance_attr_ty),
let assignable_to_instance_attribute =
if meta_attr_boundness == Boundness::PossiblyUnbound {
let (assignable, boundness) = if let Symbol::Type(
instance_attr_ty,
instance_attr_boundness,
)
) =
object_ty.instance_member(db, attribute).symbol
{
(
ensure_assignable_to(instance_attr_ty),
instance_attr_boundness,
)
} else {
(true, Boundness::PossiblyUnbound)
};

if boundness == Boundness::PossiblyUnbound {
report_possibly_unbound_attribute(
&self.context,
target,
attribute,
object_ty,
);
}

assignable
} else {
(true, Boundness::PossiblyUnbound)
true
};

if boundness == Boundness::PossiblyUnbound {
assignable_to_meta_attr && assignable_to_instance_attribute
}

SymbolAndQualifiers {
symbol: Symbol::Unbound,
..
} => {
if let Symbol::Type(instance_attr_ty, instance_attr_boundness) =
object_ty.instance_member(db, attribute).symbol
{
if instance_attr_boundness == Boundness::PossiblyUnbound {
report_possibly_unbound_attribute(
&self.context,
target,
Expand All @@ -3202,79 +3248,8 @@ impl<'db> TypeInferenceBuilder<'db> {
);
}

assignable
ensure_assignable_to(instance_attr_ty)
} else {
true
};

assignable_to_meta_attr && assignable_to_instance_attribute
}
}

SymbolAndQualifiers {
symbol: Symbol::Unbound,
..
} => {
if let Symbol::Type(instance_attr_ty, instance_attr_boundness) =
object_ty.instance_member(db, attribute).symbol
{
if instance_attr_boundness == Boundness::PossiblyUnbound {
report_possibly_unbound_attribute(
&self.context,
target,
attribute,
object_ty,
);
}

if is_read_only() {
if emit_diagnostics {
if let Some(builder) =
self.context.report_lint(&INVALID_ASSIGNMENT, target)
{
builder.into_diagnostic(format_args!(
"Property `{attribute}` defined in `{ty}` is read-only",
ty = object_ty.display(self.db()),
));
}
}
false
} else {
ensure_assignable_to(instance_attr_ty)
}
} else {
let result = object_ty.try_call_dunder_with_policy(
db,
"__setattr__",
&mut CallArgumentTypes::positional([
Type::StringLiteral(StringLiteralType::new(
db,
Box::from(attribute),
)),
value_ty,
]),
MemberLookupPolicy::MRO_NO_OBJECT_FALLBACK,
);

match result {
Ok(_) | Err(CallDunderError::PossiblyUnbound(_)) => true,
Err(CallDunderError::CallError(..)) => {
if emit_diagnostics {
if let Some(builder) =
self.context.report_lint(&UNRESOLVED_ATTRIBUTE, target)
{
builder.into_diagnostic(format_args!(
"Can not assign object of `{}` to attribute \
`{attribute}` on type `{}` with \
custom `__setattr__` method.",
value_ty.display(db),
object_ty.display(db)
));
}
}
false
}
Err(CallDunderError::MethodNotAvailable) => {
if emit_diagnostics {
if let Some(builder) =
self.context.report_lint(&UNRESOLVED_ATTRIBUTE, target)
Expand Down
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