Support serialization of deque when passed to Sequence[blah blah blah]

pydantic/_internal/_generate_schema.py

@@ -1434,6 +1434,8 @@ def _subclass_schema(self, type_: Any) -> core_schema.CoreSchema:
 
     def _sequence_schema(self, sequence_type: Any) -> core_schema.CoreSchema:
         """Generate schema for a Sequence, e.g. `Sequence[int]`."""
+        from ._std_types_schema import serialize_sequence_via_list
+
         item_type = self._get_first_arg_or_any(sequence_type)
         item_type_schema = self.generate_schema(item_type)
         list_schema = core_schema.list_schema(item_type_schema)
@@ -1445,7 +1447,13 @@ def _sequence_schema(self, sequence_type: Any) -> core_schema.CoreSchema:
             python_schema = core_schema.chain_schema(
                 [python_schema, core_schema.no_info_wrap_validator_function(sequence_validator, list_schema)],
             )
-        return core_schema.json_or_python_schema(json_schema=list_schema, python_schema=python_schema)
+
+        serialization = core_schema.wrap_serializer_function_ser_schema(
+            serialize_sequence_via_list, schema=item_type_schema, info_arg=True
+        )
+        return core_schema.json_or_python_schema(
+            json_schema=list_schema, python_schema=python_schema, serialization=serialization
+        )
 
     def _iterable_schema(self, type_: Any) -> core_schema.GeneratorSchema:
         """Generate a schema for an `Iterable`."""

pydantic/_internal/_std_types_schema.py

@@ -275,6 +275,30 @@ def dequeue_validator(
         return collections.deque(handler(input_value), maxlen=maxlen)
 
 
+def serialize_sequence_via_list(
+    v: Any, handler: core_schema.SerializerFunctionWrapHandler, info: core_schema.SerializationInfo
+) -> Any:
+    items: list[Any] = []
+
+    mapped_origin = SEQUENCE_ORIGIN_MAP.get(type(v), None)
+    if mapped_origin is None:
+        # we shouldn't hit this branch, should probably add a serialization error or something
+        return v
+
+    for index, item in enumerate(v):
+        try:
+            v = handler(item, index)
+        except PydanticOmit:
+            pass
+        else:
+            items.append(v)
+
+    if info.mode_is_json():
+        return items
+    else:
+        return mapped_origin(items)
+
+
 @dataclasses.dataclass(**slots_true)
 class SequenceValidator:
     mapped_origin: type[Any]
@@ -283,23 +307,6 @@ class SequenceValidator:
     max_length: int | None = None
     strict: bool | None = None
 
-    def serialize_sequence_via_list(
-        self, v: Any, handler: core_schema.SerializerFunctionWrapHandler, info: core_schema.SerializationInfo
-    ) -> Any:
-        items: list[Any] = []
-        for index, item in enumerate(v):
-            try:
-                v = handler(item, index)
-            except PydanticOmit:
-                pass
-            else:
-                items.append(v)
-
-        if info.mode_is_json():
-            return items
-        else:
-            return self.mapped_origin(items)
-
     def __get_pydantic_core_schema__(self, source_type: Any, handler: GetCoreSchemaHandler) -> CoreSchema:
         if self.item_source_type is Any:
             items_schema = None
@@ -344,7 +351,7 @@ def __get_pydantic_core_schema__(self, source_type: Any, handler: GetCoreSchemaH
             )
 
             serialization = core_schema.wrap_serializer_function_ser_schema(
-                self.serialize_sequence_via_list, schema=items_schema or core_schema.any_schema(), info_arg=True
+                serialize_sequence_via_list, schema=items_schema or core_schema.any_schema(), info_arg=True
             )
 
             strict = core_schema.chain_schema([check_instance, coerce_instance_wrap(constrained_schema)])

tests/test_types.py

@@ -6636,3 +6636,12 @@ class Model(BaseModel):
     obj = Model.model_validate({'my_model': {'my_enum': 'a'}, 'other_model': {'my_enum': 'a'}})
     assert not isinstance(obj.my_model.my_enum, MyEnum)
     assert isinstance(obj.other_model.my_enum, MyEnum)
+
+
+def test_can_serialize_deque_passed_to_sequence() -> None:
+    ta = TypeAdapter(Sequence[int])
+    my_dec = ta.validate_python(deque([1, 2, 3]))
+    assert my_dec == deque([1, 2, 3])
+
+    assert ta.dump_python(my_dec) == my_dec
+    assert ta.dump_json(my_dec) == b'[1,2,3]'