✨ We can now set an item and get the correct error code

Author: JoaquimEsteves

mypy/checkexpr.py

@@ -3740,7 +3740,9 @@ def nonliteral_tuple_index_helper(self, left_type: TupleType, index: Expression)
             return self.chk.named_generic_type("builtins.tuple", [union])
         return union
 
-    def visit_typeddict_index_expr(self, td_type: TypedDictType, index: Expression) -> Type:
+    def visit_typeddict_index_expr(
+        self, td_type: TypedDictType, index: Expression, setitem: bool = False
+    ) -> Type:
         if isinstance(index, StrExpr):
             key_names = [index.value]
         else:
@@ -3769,7 +3771,7 @@ def visit_typeddict_index_expr(self, td_type: TypedDictType, index: Expression)
         for key_name in key_names:
             value_type = td_type.items.get(key_name)
             if value_type is None:
-                self.msg.typeddict_key_not_found(td_type, key_name, index)
+                self.msg.typeddict_key_not_found(td_type, key_name, index, setitem)
                 return AnyType(TypeOfAny.from_error)
             else:
                 value_types.append(value_type)

mypy/checkmember.py

@@ -1068,7 +1068,9 @@ def analyze_typeddict_access(
         if isinstance(mx.context, IndexExpr):
             # Since we can get this during `a['key'] = ...`
             # it is safe to assume that the context is `IndexExpr`.
-            item_type = mx.chk.expr_checker.visit_typeddict_index_expr(typ, mx.context.index)
+            item_type = mx.chk.expr_checker.visit_typeddict_index_expr(
+                typ, mx.context.index, setitem=True
+            )
         else:
             # It can also be `a.__setitem__(...)` direct call.
             # In this case `item_type` can be `Any`,

mypy/messages.py

@@ -1653,8 +1653,16 @@ def typeddict_key_must_be_string_literal(self, typ: TypedDictType, context: Cont
         )
 
     def typeddict_key_not_found(
-        self, typ: TypedDictType, item_name: str, context: Context
+        self, typ: TypedDictType, item_name: str, context: Context, setitem: bool = False
     ) -> None:
+        """
+            Handles error messages.
+
+            Note, that we differentiate in between reading a value and setting
+            a value.
+            Setting a value on a TypedDict is an 'unknown-key' error,
+            whereas reading it is the more serious/general 'item' error.
+        """
         if typ.is_anonymous():
             self.fail(
                 '"{}" is not a valid TypedDict key; expected one of {}'.format(
@@ -1666,7 +1674,7 @@ def typeddict_key_not_found(
             self.fail(
                 f'TypedDict {format_type(typ)} has no key "{item_name}"',
                 context,
-                code=codes.TYPPEDICT_UNKNOWN_KEY,
+                code=codes.TYPPEDICT_UNKNOWN_KEY if setitem else codes.TYPEDDICT_ITEM,
             )
             matches = best_matches(item_name, typ.items.keys())
             if matches:

test-data/unit/check-errorcodes.test

@@ -465,7 +465,7 @@ d: D = {'x': '', 'y': 1}  # E: Extra key "y" for TypedDict "D"  [typeddict-unkno
 
 a['y'] = 1  # E: TypedDict "D" has no key "y"  [typeddict-unknown-key]
 a['x'] = 'x'  # E: Value of "x" has incompatible type "str"; expected "int"  [typeddict-item]
-a['y']  # E: TypedDict "D" has no key "y"  [typeddict-unknown-key]
+a['y']  # E: TypedDict "D" has no key "y"  [typeddict-item]
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-typeddict.pyi]