ValidationException.errors() are ErrorDetails

Update the documentation to explain that `RequestValidationError` isn't
literally a subclass since Pydantic V2.

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

@@ -162,7 +162,7 @@ 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

@@ -4,6 +4,7 @@ from dataclasses import dataclass, is_dataclass
 from enum import Enum
 from functools import lru_cache
 from typing import (
+    TYPE_CHECKING,
     Any,
     Callable,
     Deque,
@@ -23,7 +24,14 @@ from fastapi.types import IncEx, ModelNameMap, UnionType
 from pydantic import BaseModel, create_model
 from pydantic.version import VERSION as PYDANTIC_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,
+)
 
 PYDANTIC_VERSION_MINOR_TUPLE = tuple(int(x) for x in PYDANTIC_VERSION.split(".")[:2])
 PYDANTIC_V2 = PYDANTIC_VERSION_MINOR_TUPLE[0] == 2
@@ -60,6 +68,7 @@ if PYDANTIC_V2:
     from pydantic.json_schema import GenerateJsonSchema as GenerateJsonSchema
     from pydantic.json_schema import JsonSchemaValue as JsonSchemaValue
     from pydantic_core import CoreSchema as CoreSchema
+    from pydantic_core import ErrorDetails as ErrorDetails
     from pydantic_core import PydanticUndefined, PydanticUndefinedType
     from pydantic_core import Url as Url
 
@@ -84,6 +93,9 @@ if PYDANTIC_V2:
     class ErrorWrapper(Exception):
         pass
 
+    # See https://github.com/pydantic/pydantic/blob/bb18ac5/pydantic/error_wrappers.py#L45-L47.
+    ErrorList: TypeAlias = Union[Sequence["ErrorList"], ErrorWrapper]
+
     @dataclass
     class ModelField:
         field_info: FieldInfo
@@ -117,22 +129,25 @@ if PYDANTIC_V2:
                 return Undefined
             return self.field_info.get_default(call_default_factory=True)
 
+        # See https://github.com/pydantic/pydantic/blob/bb18ac5/pydantic/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["loc"]}
+                    for err in exc.errors(include_url=False)
+                ]
+                return None, errors
 
         def serialize(
             self,
@@ -169,7 +184,13 @@ if PYDANTIC_V2:
     ) -> Any:
         return annotation
 
-    def _normalize_errors(errors: Sequence[Any]) -> List[Dict[str, Any]]:
+    def _normalize_errors(
+        errors: Union[ErrorList, Sequence[ErrorDetails]],
+    ) -> List[ErrorDetails]:
+        assert isinstance(errors, Sequence), type(errors)
+        for error in errors:
+            assert not isinstance(error, ErrorWrapper)
+            assert not isinstance(error, Sequence)
         return errors  # type: ignore[return-value]
 
     def _model_rebuild(model: Type[BaseModel]) -> None:
@@ -267,12 +288,12 @@ if PYDANTIC_V2:
         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]:
-        error = ValidationError.from_exception_data(
+    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]
+        ).errors(include_url=False, include_input=False)
         error["input"] = None
-        return error  # type: ignore[return-value]
+        return error
 
     def create_body_model(
         *, fields: Sequence[ModelField], model_name: str
@@ -297,6 +318,14 @@ else:
     from pydantic.class_validators import (  # type: ignore[no-redef]
         Validator as Validator,
     )
+
+    if TYPE_CHECKING:  # pragma: nocover
+        from pydantic.error_wrappers import (  # type: ignore[no-redef]
+            ErrorDict as ErrorDetails,
+        )
+    from pydantic.error_wrappers import (  # type: ignore[no-redef]
+        ErrorList as ErrorList,
+    )
     from pydantic.error_wrappers import (  # type: ignore[no-redef]
         ErrorWrapper as ErrorWrapper,
     )
@@ -427,18 +456,23 @@ else:
             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:
@@ -509,10 +543,10 @@ else:
     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]
+        [new_error] = ValidationError([missing_field_error], RequestErrorModel).errors()
+        return new_error
 
     def create_body_model(
         *, fields: Sequence[ModelField], model_name: str
@@ -526,17 +560,6 @@ else:
         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

@@ -3,6 +3,7 @@ from contextlib import AsyncExitStack, contextmanager
 from copy import copy, deepcopy
 from dataclasses import dataclass
 from typing import (
+    TYPE_CHECKING,
     Any,
     Callable,
     Coroutine,
@@ -22,11 +23,10 @@ import anyio
 from fastapi import params
 from fastapi._compat import (
     PYDANTIC_V2,
-    ErrorWrapper,
     ModelField,
     RequiredParam,
     Undefined,
-    _regenerate_error_with_loc,
+    _normalize_errors,
     copy_field_info,
     create_body_model,
     evaluate_forwardref,
@@ -46,6 +46,9 @@ from fastapi._compat import (
     serialize_sequence_value,
     value_is_sequence,
 )
+
+if TYPE_CHECKING:  # pragma: nocover
+    from fastapi._compat import ErrorDetails
 from fastapi.background import BackgroundTasks
 from fastapi.concurrency import (
     asynccontextmanager,
@@ -563,7 +566,7 @@ 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]
@@ -582,7 +585,7 @@ 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"]
@@ -658,7 +661,8 @@ 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,
@@ -697,17 +701,15 @@ 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_, []
@@ -740,9 +742,9 @@ 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
@@ -877,9 +879,9 @@ 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

@@ -1,5 +1,7 @@
-from typing import Any, Dict, Optional, Sequence, Type, Union
+from typing import TYPE_CHECKING, Any, Dict, Optional, Sequence, Type, Union
 
+if TYPE_CHECKING:  # pragma: nocover
+    from fastapi._compat import ErrorDetails
 from pydantic import BaseModel, create_model
 from starlette.exceptions import HTTPException as StarletteHTTPException
 from starlette.exceptions import WebSocketException as StarletteWebSocketException
@@ -147,15 +149,15 @@ 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 +167,7 @@ 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
 

fastapi/routing.py

@@ -153,7 +153,6 @@ async def serialize_response(
     is_coroutine: bool = True,
 ) -> Any:
     if field:
-        errors = []
         if not hasattr(field, "serialize"):
             # pydantic v1
             response_content = _prepare_response_content(
@@ -163,15 +162,11 @@ async def serialize_response(
                 exclude_none=exclude_none,
             )
         if is_coroutine:
-            value, errors_ = field.validate(response_content, {}, loc=("response",))
+            value, errors = field.validate(response_content, {}, loc=("response",))
         else:
-            value, errors_ = await run_in_threadpool(
+            value, errors = await run_in_threadpool(
                 field.validate, response_content, {}, loc=("response",)
             )
-        if isinstance(errors_, list):
-            errors.extend(errors_)
-        elif errors_:
-            errors.append(errors_)
         if errors:
             raise ResponseValidationError(
                 errors=_normalize_errors(errors), body=response_content
@@ -286,7 +281,6 @@ def get_request_handler(
                     status_code=400, detail="There was an error parsing the body"
                 )
                 raise http_error from e
-            errors: List[Any] = []
             async with AsyncExitStack() as async_exit_stack:
                 solved_result = await solve_dependencies(
                     request=request,
@@ -340,9 +334,7 @@ def get_request_handler(
                             response.body = b""
                         response.headers.raw.extend(solved_result.response.headers.raw)
             if errors:
-                validation_error = RequestValidationError(
-                    _normalize_errors(errors), body=body
-                )
+                validation_error = RequestValidationError(errors, body=body)
                 raise validation_error
         if response is None:
             raise FastAPIError(
@@ -376,9 +368,7 @@ def get_websocket_app(
                 embed_body_fields=embed_body_fields,
             )
             if solved_result.errors:
-                raise WebSocketRequestValidationError(
-                    _normalize_errors(solved_result.errors)
-                )
+                raise WebSocketRequestValidationError(solved_result.errors)
             assert dependant.call is not None, "dependant.call must be a function"
             await dependant.call(**solved_result.values)