🦺 Update implementarion and docs for RequestValidationError in pydantic v2

docs/en/docs/tutorial/handling-errors.md

@@ -172,7 +172,7 @@ These are technical details that you might skip if it's not important for you no
 
 ///
 
-`RequestValidationError` is a sub-class of Pydantic's <a href="https://docs.pydantic.dev/latest/concepts/models/#error-handling" class="external-link" target="_blank">`ValidationError`</a>.
+`RequestValidationError` is morally a sub-class of Pydantic's <a href="https://docs.pydantic.dev/latest/concepts/models/#error-handling" class="external-link" target="_blank">`ValidationError`</a>.
 
 **FastAPI** uses it so that, if you use a Pydantic model in `response_model`, and your data has an error, you will see the error in your log.
 

fastapi/_compat.py

@@ -18,12 +18,19 @@
     Union,
 )
 
-from fastapi.exceptions import RequestErrorModel
+from fastapi.exceptions import ErrorDetails, RequestErrorModel
 from fastapi.types import IncEx, ModelNameMap, UnionType
 from pydantic import BaseModel, create_model
 from pydantic.version import VERSION as P_VERSION
 from starlette.datastructures import UploadFile
-from typing_extensions import Annotated, Literal, get_args, get_origin
+from typing_extensions import (
+    Annotated,
+    Literal,
+    TypeAlias,
+    assert_never,
+    get_args,
+    get_origin,
+)
 
 # Reassign variable to make it reexported for mypy
 PYDANTIC_VERSION = P_VERSION
@@ -68,7 +75,7 @@
         )
     except ImportError:  # pragma: no cover
         from pydantic_core.core_schema import (
-            general_plain_validator_function as with_info_plain_validator_function,  # noqa: F401
+            general_plain_validator_function as with_info_plain_validator_function,
         )
 
     Required = PydanticUndefined
@@ -83,6 +90,9 @@ class BaseConfig:
     class ErrorWrapper(Exception):
         pass
 
+    # See https://github.com/pydantic/pydantic/blob/07b6473/pydantic/v1/error_wrappers.py#L45-L47
+    ErrorList: TypeAlias = Union[Sequence["ErrorList"], ErrorWrapper]
+
     @dataclass
     class ModelField:
         field_info: FieldInfo
@@ -116,22 +126,25 @@ def get_default(self) -> Any:
                 return Undefined
             return self.field_info.get_default(call_default_factory=True)
 
+        # See https://github.com/pydantic/pydantic/blob/07b6473/pydantic/v1/fields.py#L850-L852 for the signature.
         def validate(
             self,
             value: Any,
             values: Dict[str, Any] = {},  # noqa: B006
             *,
             loc: Tuple[Union[int, str], ...] = (),
-        ) -> Tuple[Any, Union[List[Dict[str, Any]], None]]:
+        ) -> Tuple[Any, Union[ErrorList, Sequence[ErrorDetails], None]]:
             try:
                 return (
                     self._type_adapter.validate_python(value, from_attributes=True),
                     None,
                 )
             except ValidationError as exc:
-                return None, _regenerate_error_with_loc(
-                    errors=exc.errors(include_url=False), loc_prefix=loc
-                )
+                errors: List[ErrorDetails] = [
+                    {**err, "loc": loc + err.get("loc", ())}  # type: ignore [typeddict-unknown-key]
+                    for err in exc.errors(include_url=False)
+                ]
+                return None, errors
 
         def serialize(
             self,
@@ -168,8 +181,16 @@ def get_annotation_from_field_info(
     ) -> Any:
         return annotation
 
-    def _normalize_errors(errors: Sequence[Any]) -> List[Dict[str, Any]]:
-        return errors  # type: ignore[return-value]
+    def _normalize_errors(
+        errors: Union[ErrorList, Sequence[ErrorDetails]],
+    ) -> List[ErrorDetails]:
+        assert isinstance(errors, Sequence), type(errors)
+        use_errors: List[ErrorDetails] = []
+        for error in errors:
+            assert not isinstance(error, ErrorWrapper)
+            assert not isinstance(error, Sequence)
+            use_errors.append(error)
+        return use_errors
 
     def _model_rebuild(model: Type[BaseModel]) -> None:
         model.model_rebuild()
@@ -266,12 +287,12 @@ def serialize_sequence_value(*, field: ModelField, value: Any) -> Sequence[Any]:
         assert issubclass(origin_type, sequence_types)  # type: ignore[arg-type]
         return sequence_annotation_to_type[origin_type](value)  # type: ignore[no-any-return]
 
-    def get_missing_field_error(loc: Tuple[str, ...]) -> Dict[str, Any]:
+    def get_missing_field_error(loc: Tuple[str, ...]) -> ErrorDetails:
         error = ValidationError.from_exception_data(
             "Field required", [{"type": "missing", "loc": loc, "input": {}}]
         ).errors(include_url=False)[0]
         error["input"] = None
-        return error  # type: ignore[return-value]
+        return error
 
     def create_body_model(
         *, fields: Sequence[ModelField], model_name: str
@@ -290,14 +311,17 @@ def get_model_fields(model: Type[BaseModel]) -> List[ModelField]:
     from fastapi.openapi.constants import REF_PREFIX as REF_PREFIX
     from pydantic import AnyUrl as Url  # noqa: F401
     from pydantic import (  # type: ignore[assignment]
-        BaseConfig as BaseConfig,  # noqa: F401
+        BaseConfig as BaseConfig,
     )
-    from pydantic import ValidationError as ValidationError  # noqa: F401
+    from pydantic import ValidationError as ValidationError
     from pydantic.class_validators import (  # type: ignore[no-redef]
-        Validator as Validator,  # noqa: F401
+        Validator as Validator,
     )
     from pydantic.error_wrappers import (  # type: ignore[no-redef]
-        ErrorWrapper as ErrorWrapper,  # noqa: F401
+        ErrorList as ErrorList,
+    )
+    from pydantic.error_wrappers import (  # type: ignore[no-redef]
+        ErrorWrapper as ErrorWrapper,
     )
     from pydantic.errors import MissingError
     from pydantic.fields import (  # type: ignore[attr-defined]
@@ -311,31 +335,31 @@ def get_model_fields(model: Type[BaseModel]) -> List[ModelField]:
     )
     from pydantic.fields import FieldInfo as FieldInfo
     from pydantic.fields import (  # type: ignore[no-redef,attr-defined]
-        ModelField as ModelField,  # noqa: F401
+        ModelField as ModelField,
     )
     from pydantic.fields import (  # type: ignore[no-redef,attr-defined]
-        Required as Required,  # noqa: F401
+        Required as Required,
     )
     from pydantic.fields import (  # type: ignore[no-redef,attr-defined]
         Undefined as Undefined,
     )
     from pydantic.fields import (  # type: ignore[no-redef, attr-defined]
-        UndefinedType as UndefinedType,  # noqa: F401
+        UndefinedType as UndefinedType,
     )
     from pydantic.schema import (
         field_schema,
         get_flat_models_from_fields,
         get_model_name_map,
         model_process_schema,
     )
-    from pydantic.schema import (  # type: ignore[no-redef]  # noqa: F401
+    from pydantic.schema import (  # type: ignore[no-redef]
         get_annotation_from_field_info as get_annotation_from_field_info,
     )
     from pydantic.typing import (  # type: ignore[no-redef]
-        evaluate_forwardref as evaluate_forwardref,  # noqa: F401
+        evaluate_forwardref as evaluate_forwardref,
     )
     from pydantic.utils import (  # type: ignore[no-redef]
-        lenient_issubclass as lenient_issubclass,  # noqa: F401
+        lenient_issubclass as lenient_issubclass,
     )
 
     GetJsonSchemaHandler = Any  # type: ignore[assignment,misc]
@@ -425,18 +449,23 @@ def is_pv1_scalar_sequence_field(field: ModelField) -> bool:
             return True
         return False
 
-    def _normalize_errors(errors: Sequence[Any]) -> List[Dict[str, Any]]:
-        use_errors: List[Any] = []
-        for error in errors:
-            if isinstance(error, ErrorWrapper):
-                new_errors = ValidationError(  # type: ignore[call-arg]
-                    errors=[error], model=RequestErrorModel
+    def _normalize_errors(
+        errors: Union[ErrorList, Sequence[ErrorDetails]],
+    ) -> List[ErrorDetails]:
+        use_errors: List[ErrorDetails] = []
+        if isinstance(errors, ErrorWrapper):
+            use_errors.extend(
+                ValidationError(  # type: ignore[call-arg]
+                    errors=[errors], model=RequestErrorModel
                 ).errors()
-                use_errors.extend(new_errors)
-            elif isinstance(error, list):
+            )
+        elif isinstance(errors, Sequence):
+            for error in errors:
+                assert not isinstance(error, dict)
                 use_errors.extend(_normalize_errors(error))
-            else:
-                use_errors.append(error)
+            return use_errors
+        else:
+            assert_never(errors)  # pragma: no cover
         return use_errors
 
     def _model_rebuild(model: Type[BaseModel]) -> None:
@@ -507,10 +536,10 @@ def copy_field_info(*, field_info: FieldInfo, annotation: Any) -> FieldInfo:
     def serialize_sequence_value(*, field: ModelField, value: Any) -> Sequence[Any]:
         return sequence_shape_to_type[field.shape](value)  # type: ignore[no-any-return,attr-defined]
 
-    def get_missing_field_error(loc: Tuple[str, ...]) -> Dict[str, Any]:
+    def get_missing_field_error(loc: Tuple[str, ...]) -> ErrorDetails:
         missing_field_error = ErrorWrapper(MissingError(), loc=loc)  # type: ignore[call-arg]
         new_error = ValidationError([missing_field_error], RequestErrorModel)
-        return new_error.errors()[0]  # type: ignore[return-value]
+        return new_error.errors()[0]
 
     def create_body_model(
         *, fields: Sequence[ModelField], model_name: str
@@ -524,17 +553,6 @@ def get_model_fields(model: Type[BaseModel]) -> List[ModelField]:
         return list(model.__fields__.values())  # type: ignore[attr-defined]
 
 
-def _regenerate_error_with_loc(
-    *, errors: Sequence[Any], loc_prefix: Tuple[Union[str, int], ...]
-) -> List[Dict[str, Any]]:
-    updated_loc_errors: List[Any] = [
-        {**err, "loc": loc_prefix + err.get("loc", ())}
-        for err in _normalize_errors(errors)
-    ]
-
-    return updated_loc_errors
-
-
 def _annotation_is_sequence(annotation: Union[Type[Any], None]) -> bool:
     if lenient_issubclass(annotation, (str, bytes)):
         return False

fastapi/dependencies/utils.py

@@ -22,11 +22,10 @@
 from fastapi import params
 from fastapi._compat import (
     PYDANTIC_V2,
-    ErrorWrapper,
     ModelField,
     Required,
     Undefined,
-    _regenerate_error_with_loc,
+    _normalize_errors,
     copy_field_info,
     create_body_model,
     evaluate_forwardref,
@@ -52,6 +51,7 @@
     contextmanager_in_threadpool,
 )
 from fastapi.dependencies.models import Dependant, SecurityRequirement
+from fastapi.exceptions import ErrorDetails
 from fastapi.logger import logger
 from fastapi.security.base import SecurityBase
 from fastapi.security.oauth2 import OAuth2, SecurityScopes
@@ -553,7 +553,7 @@ async def solve_generator(
 @dataclass
 class SolvedDependency:
     values: Dict[str, Any]
-    errors: List[Any]
+    errors: List[ErrorDetails]
     background_tasks: Optional[StarletteBackgroundTasks]
     response: Response
     dependency_cache: Dict[Tuple[Callable[..., Any], Tuple[str]], Any]
@@ -572,7 +572,7 @@ async def solve_dependencies(
     embed_body_fields: bool,
 ) -> SolvedDependency:
     values: Dict[str, Any] = {}
-    errors: List[Any] = []
+    errors: List[ErrorDetails] = []
     if response is None:
         response = Response()
         del response.headers["content-length"]
@@ -648,7 +648,8 @@ async def solve_dependencies(
     values.update(query_values)
     values.update(header_values)
     values.update(cookie_values)
-    errors += path_errors + query_errors + header_errors + cookie_errors
+    for errors_ in (path_errors, query_errors, header_errors, cookie_errors):
+        errors.extend(errors_)
     if dependant.body_params:
         (
             body_values,
@@ -687,17 +688,15 @@ async def solve_dependencies(
 
 def _validate_value_with_model_field(
     *, field: ModelField, value: Any, values: Dict[str, Any], loc: Tuple[str, ...]
-) -> Tuple[Any, List[Any]]:
+) -> Tuple[Any, List[ErrorDetails]]:
     if value is None:
         if field.required:
             return None, [get_missing_field_error(loc=loc)]
         else:
             return deepcopy(field.default), []
     v_, errors_ = field.validate(value, values, loc=loc)
-    if isinstance(errors_, ErrorWrapper):
-        return None, [errors_]
-    elif isinstance(errors_, list):
-        new_errors = _regenerate_error_with_loc(errors=errors_, loc_prefix=())
+    if errors_ is not None:
+        new_errors = _normalize_errors(errors_)
         return None, new_errors
     else:
         return v_, []
@@ -730,9 +729,9 @@ def _get_multidict_value(
 def request_params_to_args(
     fields: Sequence[ModelField],
     received_params: Union[Mapping[str, Any], QueryParams, Headers],
-) -> Tuple[Dict[str, Any], List[Any]]:
+) -> Tuple[Dict[str, Any], List[ErrorDetails]]:
     values: Dict[str, Any] = {}
-    errors: List[Dict[str, Any]] = []
+    errors: List[ErrorDetails] = []
 
     if not fields:
         return values, errors
@@ -867,9 +866,9 @@ async def request_body_to_args(
     body_fields: List[ModelField],
     received_body: Optional[Union[Dict[str, Any], FormData]],
     embed_body_fields: bool,
-) -> Tuple[Dict[str, Any], List[Dict[str, Any]]]:
+) -> Tuple[Dict[str, Any], List[ErrorDetails]]:
     values: Dict[str, Any] = {}
-    errors: List[Dict[str, Any]] = []
+    errors: List[ErrorDetails] = []
     assert body_fields, "request_body_to_args() should be called with fields"
     single_not_embedded_field = len(body_fields) == 1 and not embed_body_fields
     first_field = body_fields[0]

fastapi/exceptions.py

@@ -1,9 +1,22 @@
-from typing import Any, Dict, Optional, Sequence, Type, Union
+from typing import Any, Dict, Optional, Sequence, Tuple, Type, Union
 
 from pydantic import BaseModel, create_model
 from starlette.exceptions import HTTPException as StarletteHTTPException
 from starlette.exceptions import WebSocketException as StarletteWebSocketException
-from typing_extensions import Annotated, Doc
+from typing_extensions import Annotated, Doc, TypedDict
+
+
+class ErrorDetails(TypedDict):
+    """
+    The common subset shared by `ErrorDict` in Pydantic V1[0] and `ErrorDetails` in Pydantic V2[1].
+
+    [0] https://github.com/pydantic/pydantic/blob/4d7bef62aeff10985fe67d13477fe666b13ba070/pydantic/v1/error_wrappers.py#L21-L22
+    [1] https://github.com/pydantic/pydantic-core/blob/e1fc99dd3207157aad77defc20ab6873fd268b5b/python/pydantic_core/__init__.py#L73-L91
+    """
+
+    loc: Tuple[Union[int, str], ...]
+    msg: str
+    type: str
 
 
 class HTTPException(StarletteHTTPException):
@@ -147,15 +160,15 @@ class FastAPIError(RuntimeError):
 
 
 class ValidationException(Exception):
-    def __init__(self, errors: Sequence[Any]) -> None:
+    def __init__(self, errors: Sequence[ErrorDetails]) -> None:
         self._errors = errors
 
-    def errors(self) -> Sequence[Any]:
+    def errors(self) -> Sequence[ErrorDetails]:
         return self._errors
 
 
 class RequestValidationError(ValidationException):
-    def __init__(self, errors: Sequence[Any], *, body: Any = None) -> None:
+    def __init__(self, errors: Sequence[ErrorDetails], *, body: Any = None) -> None:
         super().__init__(errors)
         self.body = body
 
@@ -165,7 +178,7 @@ class WebSocketRequestValidationError(ValidationException):
 
 
 class ResponseValidationError(ValidationException):
-    def __init__(self, errors: Sequence[Any], *, body: Any = None) -> None:
+    def __init__(self, errors: Sequence[ErrorDetails], *, body: Any = None) -> None:
         super().__init__(errors)
         self.body = body