Fix Sequence ignoring discriminator (pydantic#9980)

pydantic/_internal/_generate_schema.py

@@ -92,7 +92,7 @@
 from ._mock_val_ser import MockCoreSchema
 from ._schema_generation_shared import CallbackGetCoreSchemaHandler
 from ._typing_extra import is_finalvar, is_self_type, is_zoneinfo_type
-from ._utils import lenient_issubclass
+from ._utils import lenient_issubclass, smart_deepcopy
 
 if TYPE_CHECKING:
     from ..fields import ComputedFieldInfo, FieldInfo
@@ -1658,6 +1658,7 @@ def _sequence_schema(self, sequence_type: Any) -> core_schema.CoreSchema:
         item_type_schema = self.generate_schema(item_type)
         list_schema = core_schema.list_schema(item_type_schema)
 
+        json_schema = smart_deepcopy(list_schema)
         python_schema = core_schema.is_instance_schema(typing.Sequence, cls_repr='Sequence')
         if item_type != Any:
             from ._validators import sequence_validator
@@ -1670,7 +1671,7 @@ def _sequence_schema(self, sequence_type: Any) -> core_schema.CoreSchema:
             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
+            json_schema=json_schema, python_schema=python_schema, serialization=serialization
         )
 
     def _iterable_schema(self, type_: Any) -> core_schema.GeneratorSchema:

tests/test_discriminated_union.py

@@ -14,6 +14,7 @@
 from pydantic.dataclasses import dataclass as pydantic_dataclass
 from pydantic.errors import PydanticUserError
 from pydantic.fields import FieldInfo
+from pydantic.functional_validators import model_validator
 from pydantic.json_schema import GenerateJsonSchema
 from pydantic.types import Tag
 
@@ -1379,6 +1380,90 @@ class Model(BaseModel):
     }
 
 
+def test_sequence_discriminated_union_validation():
+    """
+    Related issue: https://github.com/pydantic/pydantic/issues/9872
+    """
+
+    class A(BaseModel):
+        type: Literal['a']
+        a_field: str
+
+    class B(BaseModel):
+        type: Literal['b']
+        b_field: str
+
+    class Model(BaseModel):
+        items: Sequence[Annotated[Union[A, B], Field(discriminator='type')]]
+
+    import json
+
+    data_json = '{"items": [{"type": "b"}]}'
+    data_dict = json.loads(data_json)
+
+    expected_error = {
+        'type': 'missing',
+        'loc': ('items', 0, 'b', 'b_field'),
+        'msg': 'Field required',
+        'input': {'type': 'b'},
+    }
+
+    # missing field should be `b_field` only, not including `a_field`
+    # also `literal_error` should not be reported on `type`
+
+    with pytest.raises(ValidationError) as exc_info:
+        Model.model_validate(data_dict)
+    assert exc_info.value.errors(include_url=False) == [expected_error]
+
+    with pytest.raises(ValidationError) as exc_info:
+        Model.model_validate_json(data_json)
+    assert exc_info.value.errors(include_url=False) == [expected_error]
+
+
+def test_sequence_discriminated_union_validation_with_validator():
+    """
+    This is the same as the previous test, but add validators to both class.
+    """
+
+    class A(BaseModel):
+        type: Literal['a']
+        a_field: str
+
+        @model_validator(mode='after')
+        def check_a(self):
+            return self
+
+    class B(BaseModel):
+        type: Literal['b']
+        b_field: str
+
+        @model_validator(mode='after')
+        def check_b(self):
+            return self
+
+    class Model(BaseModel):
+        items: Sequence[Annotated[Union[A, B], Field(discriminator='type')]]
+
+    import json
+
+    data_json = '{"items": [{"type": "b"}]}'
+    data_dict = json.loads(data_json)
+
+    expected_error = {
+        'type': 'missing',
+        'loc': ('items', 0, 'b', 'b_field'),
+        'msg': 'Field required',
+        'input': {'type': 'b'},
+    }
+
+    # missing field should be `b_field` only, not including `a_field`
+    # also `literal_error` should not be reported on `type`
+
+    with pytest.raises(ValidationError) as exc_info:
+        Model.model_validate(data_dict)
+    assert exc_info.value.errors(include_url=False) == [expected_error]
+
+
 @pytest.fixture(scope='session', name='animals')
 def callable_discriminated_union_animals() -> SimpleNamespace:
     class Cat(BaseModel):