[ty] diagnostic on overridden __setattr__ and __delattr__ in frozen dataclasses

astral-sh/ty#111

Author: thejchap

crates/ty/docs/rules.md

@@ -443,7 +443,7 @@ frozen_instance = MyFrozenClass(1)
 frozen_instance.x = 2  # error: [invalid-assignment]
 ```
 
-If `__setattr__()` or `__delattr__()` is defined in the class, we should emit a diagnostic.
+If `__setattr__()` or `__delattr__()` is defined in the class, a diagnostic is emitted.
 
 ```py
 from dataclasses import dataclass
@@ -452,10 +452,10 @@ from dataclasses import dataclass
 class MyFrozenClass:
     x: int
 
-    # TODO: Emit a diagnostic here
+    # error: [cannot-overwrite-attribute] "Cannot overwrite attribute `__setattr__` in class `MyFrozenClass`"
     def __setattr__(self, name: str, value: object) -> None: ...
 
-    # TODO: Emit a diagnostic here
+    # error: [cannot-overwrite-attribute] "Cannot overwrite attribute `__delattr__` in class `MyFrozenClass`"
     def __delattr__(self, name: str) -> None: ...
 ```
 

crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md

@@ -19,7 +19,9 @@ use crate::semantic_index::{
 use crate::types::bound_super::BoundSuperError;
 use crate::types::constraints::{ConstraintSet, IteratorConstraintsExtension};
 use crate::types::context::InferContext;
-use crate::types::diagnostic::{INVALID_TYPE_ALIAS_TYPE, SUPER_CALL_IN_NAMED_TUPLE_METHOD};
+use crate::types::diagnostic::{
+    CANNOT_OVERWRITE_ATTRIBUTE, INVALID_TYPE_ALIAS_TYPE, SUPER_CALL_IN_NAMED_TUPLE_METHOD,
+};
 use crate::types::enums::enum_metadata;
 use crate::types::function::{DataclassTransformerParams, KnownFunction};
 use crate::types::generics::{
@@ -1923,6 +1925,67 @@ impl<'db> ClassLiteral<'db> {
         Some(typed_dict_params_from_class_def(class_stmt))
     }
 
+    fn merged_dataclass_params(
+        self,
+        db: &'db dyn Db,
+        field_policy: CodeGeneratorKind<'db>,
+    ) -> (Option<DataclassParams<'db>>, Option<DataclassParams<'db>>) {
+        let dataclass_params = self.dataclass_params(db);
+
+        let mut transformer_params =
+            if let CodeGeneratorKind::DataclassLike(Some(transformer_params)) = field_policy {
+                Some(DataclassParams::from_transformer_params(
+                    db,
+                    transformer_params,
+                ))
+            } else {
+                None
+            };
+
+        // Dataclass transformer flags can be overwritten using class arguments.
+        if let Some(transformer_params) = transformer_params.as_mut() {
+            if let Some(class_def) = self.definition(db).kind(db).as_class() {
+                let module = parsed_module(db, self.file(db)).load(db);
+
+                if let Some(arguments) = &class_def.node(&module).arguments {
+                    let mut flags = transformer_params.flags(db);
+
+                    for keyword in &arguments.keywords {
+                        if let Some(arg_name) = &keyword.arg {
+                            if let Some(is_set) =
+                                keyword.value.as_boolean_literal_expr().map(|b| b.value)
+                            {
+                                for (flag_name, flag) in DATACLASS_FLAGS {
+                                    if arg_name.as_str() == *flag_name {
+                                        flags.set(*flag, is_set);
+                                    }
+                                }
+                            }
+                        }
+                    }
+
+                    *transformer_params =
+                        DataclassParams::new(db, flags, transformer_params.field_specifiers(db));
+                }
+            }
+        }
+
+        (dataclass_params, transformer_params)
+    }
+
+    /// Checks if the given dataclass parameter flag is set for this class.
+    /// This checks both the `dataclass_params` and `transformer_params`.
+    fn has_dataclass_param(
+        self,
+        db: &'db dyn Db,
+        field_policy: CodeGeneratorKind<'db>,
+        param: DataclassFlags,
+    ) -> bool {
+        let (dataclass_params, transformer_params) = self.merged_dataclass_params(db, field_policy);
+        dataclass_params.is_some_and(|params| params.flags(db).contains(param))
+            || transformer_params.is_some_and(|params| params.flags(db).contains(param))
+    }
+
     /// Return the explicit `metaclass` of this class, if one is defined.
     ///
     /// ## Note
@@ -2332,53 +2395,8 @@ impl<'db> ClassLiteral<'db> {
         inherited_generic_context: Option<GenericContext<'db>>,
         name: &str,
     ) -> Option<Type<'db>> {
-        let dataclass_params = self.dataclass_params(db);
-
         let field_policy = CodeGeneratorKind::from_class(db, self, specialization)?;
 
-        let mut transformer_params =
-            if let CodeGeneratorKind::DataclassLike(Some(transformer_params)) = field_policy {
-                Some(DataclassParams::from_transformer_params(
-                    db,
-                    transformer_params,
-                ))
-            } else {
-                None
-            };
-
-        // Dataclass transformer flags can be overwritten using class arguments.
-        if let Some(transformer_params) = transformer_params.as_mut() {
-            if let Some(class_def) = self.definition(db).kind(db).as_class() {
-                let module = parsed_module(db, self.file(db)).load(db);
-
-                if let Some(arguments) = &class_def.node(&module).arguments {
-                    let mut flags = transformer_params.flags(db);
-
-                    for keyword in &arguments.keywords {
-                        if let Some(arg_name) = &keyword.arg {
-                            if let Some(is_set) =
-                                keyword.value.as_boolean_literal_expr().map(|b| b.value)
-                            {
-                                for (flag_name, flag) in DATACLASS_FLAGS {
-                                    if arg_name.as_str() == *flag_name {
-                                        flags.set(*flag, is_set);
-                                    }
-                                }
-                            }
-                        }
-                    }
-
-                    *transformer_params =
-                        DataclassParams::new(db, flags, transformer_params.field_specifiers(db));
-                }
-            }
-        }
-
-        let has_dataclass_param = |param| {
-            dataclass_params.is_some_and(|params| params.flags(db).contains(param))
-                || transformer_params.is_some_and(|params| params.flags(db).contains(param))
-        };
-
         let instance_ty =
             Type::instance(db, self.apply_optional_specialization(db, specialization));
 
@@ -2456,7 +2474,11 @@ impl<'db> ClassLiteral<'db> {
                 }
 
                 let is_kw_only = name == "__replace__"
-                    || kw_only.unwrap_or(has_dataclass_param(DataclassFlags::KW_ONLY));
+                    || kw_only.unwrap_or(self.has_dataclass_param(
+                        db,
+                        field_policy,
+                        DataclassFlags::KW_ONLY,
+                    ));
 
                 // Use the alias name if provided, otherwise use the field name
                 let parameter_name =
@@ -2498,7 +2520,7 @@ impl<'db> ClassLiteral<'db> {
 
         match (field_policy, name) {
             (CodeGeneratorKind::DataclassLike(_), "__init__") => {
-                if !has_dataclass_param(DataclassFlags::INIT) {
+                if !self.has_dataclass_param(db, field_policy, DataclassFlags::INIT) {
                     return None;
                 }
 
@@ -2513,7 +2535,7 @@ impl<'db> ClassLiteral<'db> {
                 signature_from_fields(vec![cls_parameter], Some(Type::none(db)))
             }
             (CodeGeneratorKind::DataclassLike(_), "__lt__" | "__le__" | "__gt__" | "__ge__") => {
-                if !has_dataclass_param(DataclassFlags::ORDER) {
+                if !self.has_dataclass_param(db, field_policy, DataclassFlags::ORDER) {
                     return None;
                 }
 
@@ -2535,9 +2557,10 @@ impl<'db> ClassLiteral<'db> {
                 Some(Type::function_like_callable(db, signature))
             }
             (CodeGeneratorKind::DataclassLike(_), "__hash__") => {
-                let unsafe_hash = has_dataclass_param(DataclassFlags::UNSAFE_HASH);
-                let frozen = has_dataclass_param(DataclassFlags::FROZEN);
-                let eq = has_dataclass_param(DataclassFlags::EQ);
+                let unsafe_hash =
+                    self.has_dataclass_param(db, field_policy, DataclassFlags::UNSAFE_HASH);
+                let frozen = self.has_dataclass_param(db, field_policy, DataclassFlags::FROZEN);
+                let eq = self.has_dataclass_param(db, field_policy, DataclassFlags::EQ);
 
                 if unsafe_hash || (frozen && eq) {
                     let signature = Signature::new(
@@ -2560,11 +2583,12 @@ impl<'db> ClassLiteral<'db> {
             (CodeGeneratorKind::DataclassLike(_), "__match_args__")
                 if Program::get(db).python_version(db) >= PythonVersion::PY310 =>
             {
-                if !has_dataclass_param(DataclassFlags::MATCH_ARGS) {
+                if !self.has_dataclass_param(db, field_policy, DataclassFlags::MATCH_ARGS) {
                     return None;
                 }
 
-                let kw_only_default = has_dataclass_param(DataclassFlags::KW_ONLY);
+                let kw_only_default =
+                    self.has_dataclass_param(db, field_policy, DataclassFlags::KW_ONLY);
 
                 let fields = self.fields(db, specialization, field_policy);
                 let match_args = fields
@@ -2582,8 +2606,8 @@ impl<'db> ClassLiteral<'db> {
             (CodeGeneratorKind::DataclassLike(_), "__weakref__")
                 if Program::get(db).python_version(db) >= PythonVersion::PY311 =>
             {
-                if !has_dataclass_param(DataclassFlags::WEAKREF_SLOT)
-                    || !has_dataclass_param(DataclassFlags::SLOTS)
+                if !self.has_dataclass_param(db, field_policy, DataclassFlags::WEAKREF_SLOT)
+                    || !self.has_dataclass_param(db, field_policy, DataclassFlags::SLOTS)
                 {
                     return None;
                 }
@@ -2625,7 +2649,7 @@ impl<'db> ClassLiteral<'db> {
                 signature_from_fields(vec![self_parameter], Some(instance_ty))
             }
             (CodeGeneratorKind::DataclassLike(_), "__setattr__") => {
-                if has_dataclass_param(DataclassFlags::FROZEN) {
+                if self.has_dataclass_param(db, field_policy, DataclassFlags::FROZEN) {
                     let signature = Signature::new(
                         Parameters::new(
                             db,
@@ -2646,11 +2670,12 @@ impl<'db> ClassLiteral<'db> {
             (CodeGeneratorKind::DataclassLike(_), "__slots__")
                 if Program::get(db).python_version(db) >= PythonVersion::PY310 =>
             {
-                has_dataclass_param(DataclassFlags::SLOTS).then(|| {
-                    let fields = self.fields(db, specialization, field_policy);
-                    let slots = fields.keys().map(|name| Type::string_literal(db, name));
-                    Type::heterogeneous_tuple(db, slots)
-                })
+                self.has_dataclass_param(db, field_policy, DataclassFlags::SLOTS)
+                    .then(|| {
+                        let fields = self.fields(db, specialization, field_policy);
+                        let slots = fields.keys().map(|name| Type::string_literal(db, name));
+                        Type::heterogeneous_tuple(db, slots)
+                    })
             }
             (CodeGeneratorKind::TypedDict, "__setitem__") => {
                 let fields = self.fields(db, specialization, field_policy);
@@ -3036,6 +3061,42 @@ impl<'db> ClassLiteral<'db> {
             .collect()
     }
 
+    pub(crate) fn validate_members(self, context: &InferContext<'db, '_>) {
+        let db = context.db();
+        let Some(field_policy) = CodeGeneratorKind::from_class(db, self, None) else {
+            return;
+        };
+        let class_body_scope = self.body_scope(db);
+        let table = place_table(db, class_body_scope);
+        let use_def = use_def_map(db, class_body_scope);
+        for (symbol_id, declarations) in use_def.all_end_of_scope_symbol_declarations() {
+            let result = place_from_declarations(db, declarations.clone());
+            let attr = result.ignore_conflicting_declarations();
+            let symbol = table.symbol(symbol_id);
+            let name = symbol.name();
+            if let Some(Type::FunctionLiteral(literal)) = attr.place.ignore_possibly_undefined()
+                && matches!(name.as_str(), "__setattr__" | "__delattr__")
+            {
+                if let Some(CodeGeneratorKind::DataclassLike(_)) =
+                    CodeGeneratorKind::from_class(db, self, None)
+                    && self.has_dataclass_param(db, field_policy, DataclassFlags::FROZEN)
+                {
+                    if let Some(builder) = context.report_lint(
+                        &CANNOT_OVERWRITE_ATTRIBUTE,
+                        literal.node(db, context.file(), context.module()),
+                    ) {
+                        let mut diagnostic = builder.into_diagnostic(format_args!(
+                            "Cannot overwrite attribute `{}` in class `{}`",
+                            name,
+                            self.name(db)
+                        ));
+                        diagnostic.info(name);
+                    }
+                }
+            }
+        }
+    }
+
     /// Returns a list of all annotated attributes defined in the body of this class. This is similar
     /// to the `__annotations__` attribute at runtime, but also contains default values.
     ///

crates/ty_python_semantic/src/types/class.rs

@@ -51,6 +51,7 @@ pub(crate) fn register_lints(registry: &mut LintRegistryBuilder) {
     registry.register_lint(&AMBIGUOUS_PROTOCOL_MEMBER);
     registry.register_lint(&CALL_NON_CALLABLE);
     registry.register_lint(&POSSIBLY_MISSING_IMPLICIT_CALL);
+    registry.register_lint(&CANNOT_OVERWRITE_ATTRIBUTE);
     registry.register_lint(&CONFLICTING_ARGUMENT_FORMS);
     registry.register_lint(&CONFLICTING_DECLARATIONS);
     registry.register_lint(&CONFLICTING_METACLASS);
@@ -393,6 +394,32 @@ declare_lint! {
     }
 }
 
+declare_lint! {
+    /// ## What it does
+    /// Checks for dataclass definitions that have both `frozen=True` and a custom `__setattr__`
+    /// method defined.
+    ///
+    /// ## Why is this bad?
+    /// Frozen dataclasses synthesize `__setattr__` and `__delattr__` methods which raise a
+    /// `FrozenInstanceError` to emulate immutability.
+    ///
+    /// Overriding either of these methods raises a runtime error.
+    ///
+    /// ## Examples
+    /// ```python
+    /// from dataclasses import dataclass
+    ///
+    /// @dataclass(frozen=True)
+    /// class A:
+    ///     def __setattr__(self, name: str, value: object) -> None: ...
+    /// ```
+    pub(crate) static CANNOT_OVERWRITE_ATTRIBUTE = {
+        summary: "detects dataclasses with `frozen=True` that have a custom `__setattr__` or `__delattr__` implementation",
+        status: LintStatus::preview("1.0.0"),
+        default_level: Level::Error,
+    }
+}
+
 declare_lint! {
     /// ## What it does
     /// Checks for classes definitions which will fail at runtime due to

crates/ty_python_semantic/src/types/diagnostic.rs

@@ -990,6 +990,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             if let Some(protocol) = class.into_protocol_class(self.db()) {
                 protocol.validate_members(&self.context);
             }
+
+            class.validate_members(&self.context);
         }
     }
 

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -31,6 +31,7 @@ Settings: Settings {
         "ambiguous-protocol-member": Warning (Default),
         "byte-string-type-annotation": Error (Default),
         "call-non-callable": Error (Default),
+        "cannot-overwrite-attribute": Error (Default),
         "conflicting-argument-forms": Error (Default),
         "conflicting-declarations": Error (Default),
         "conflicting-metaclass": Error (Default),