[ty] Disallow read-only fields in TypedDict updates (#24128)

charliermarsh · web-flow · commit 6c8101a5b3ca · 2026-03-24T16:31:48.000-04:00
## Summary Closes astral-sh/ty#3098.
diff --git a/crates/ty_python_semantic/resources/mdtest/typed_dict.md b/crates/ty_python_semantic/resources/mdtest/typed_dict.md
@@ -1657,6 +1657,47 @@ config["host"] = "127.0.0.1"
 config["port"] = 80
 ```
 
+## `update()` with `ReadOnly` items
+
+`update()` also cannot write to `ReadOnly` items, unless the source key is bottom-typed and
+therefore cannot be present:
+
+```py
+from typing_extensions import Never, NotRequired, ReadOnly, TypedDict
+
+class ReadOnlyPerson(TypedDict):
+    id: ReadOnly[int]
+    age: int
+
+class AgePatch(TypedDict, total=False):
+    age: int
+
+class IdPatch(TypedDict, total=False):
+    id: int
+
+class ImpossibleIdPatch(TypedDict, total=False):
+    id: NotRequired[Never]
+
+person: ReadOnlyPerson = {"id": 1, "age": 30}
+age_patch: AgePatch = {"age": 31}
+id_patch: IdPatch = {"id": 2}
+impossible_id_patch: ImpossibleIdPatch = {}
+
+person.update(age_patch)
+
+# error: [invalid-argument-type]
+person.update(id_patch)
+
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+person.update({"id": 2})
+
+# error: [invalid-argument-type]
+person.update(id=2)
+
+person.update(impossible_id_patch)
+```
+
 ## Methods on `TypedDict`
 
 ```py
diff --git a/crates/ty_python_semantic/src/types/class.rs b/crates/ty_python_semantic/src/types/class.rs
@@ -2027,14 +2027,14 @@ pub(super) fn synthesize_typed_dict_update_member<'db>(
     instance_ty: Type<'db>,
     keyword_parameters: &[Parameter<'db>],
 ) -> Type<'db> {
-    let partial_ty = if let Type::TypedDict(typed_dict) = instance_ty {
-        Type::TypedDict(typed_dict.to_partial(db))
+    let update_patch_ty = if let Type::TypedDict(typed_dict) = instance_ty {
+        Type::TypedDict(typed_dict.to_update_patch(db))
     } else {
         instance_ty
     };
 
     let value_ty = UnionBuilder::new(db)
-        .add(partial_ty)
+        .add(update_patch_ty)
         .add(KnownClass::Iterable.to_specialized_instance(
             db,
             &[Type::heterogeneous_tuple(
diff --git a/crates/ty_python_semantic/src/types/class/static_literal.rs b/crates/ty_python_semantic/src/types/class/static_literal.rs
@@ -1978,15 +1978,20 @@ impl<'db> StaticClassLiteral<'db> {
                 )))
             }
             (CodeGeneratorKind::TypedDict, "update") => {
-                let keyword_parameters: Vec<_> = self
-                    .fields(db, specialization, field_policy)
-                    .iter()
-                    .map(|(name, field)| {
-                        Parameter::keyword_only(name.clone())
-                            .with_annotated_type(field.declared_ty)
-                            .with_default_type(field.declared_ty)
-                    })
-                    .collect();
+                let keyword_parameters: Vec<_> = if let Type::TypedDict(typed_dict) = instance_ty {
+                    typed_dict
+                        .to_update_patch(db)
+                        .items(db)
+                        .iter()
+                        .map(|(name, field)| {
+                            Parameter::keyword_only(name.clone())
+                                .with_annotated_type(field.declared_ty)
+                                .with_default_type(field.declared_ty)
+                        })
+                        .collect()
+                } else {
+                    Vec::new()
+                };
 
                 Some(synthesize_typed_dict_update_member(
                     db,
diff --git a/crates/ty_python_semantic/src/types/typed_dict.rs b/crates/ty_python_semantic/src/types/typed_dict.rs
@@ -151,6 +151,27 @@ impl<'db> TypedDictType<'db> {
         Self::from_patch_items(db, items)
     }
 
+    /// Returns a patch version of this `TypedDict` for `TypedDict.update()`.
+    ///
+    /// All fields become optional, and read-only fields become bottom-typed. This preserves the
+    /// PEP 705 rule that `update()` must reject any source that can write a read-only key, while
+    /// still accepting `NotRequired[Never]` placeholders for keys that cannot be present.
+    pub(crate) fn to_update_patch(self, db: &'db dyn Db) -> Self {
+        let items: TypedDictSchema<'db> = self
+            .items(db)
+            .iter()
+            .map(|(name, field)| {
+                let mut field = field.clone().with_required(false);
+                if field.is_read_only() {
+                    field.declared_ty = Type::Never;
+                }
+                (name.clone(), field)
+            })
+            .collect();
+
+        Self::from_patch_items(db, items)
+    }
+
     pub fn definition(self, db: &'db dyn Db) -> Option<Definition<'db>> {
         match self {
             TypedDictType::Class(defining_class) => defining_class.definition(db),
