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

@@ -163,7 +163,7 @@ path -> item_id
 !!! warning
     These are technical details that you might skip if it's not important for you now.
 
-`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

@@ -17,12 +17,12 @@
     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, get_args, get_origin
 
 # Reassign variable to make it reexported for mypy
 PYDANTIC_VERSION = P_VERSION
@@ -82,6 +82,9 @@ 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[Any], ErrorWrapper]
+
     @dataclass
     class ModelField:
         field_info: FieldInfo
@@ -115,13 +118,14 @@ 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, None]]:
             try:
                 return (
                     self._type_adapter.validate_python(value, from_attributes=True),
@@ -167,7 +171,7 @@ def get_annotation_from_field_info(
     ) -> Any:
         return annotation
 
-    def _normalize_errors(errors: Sequence[Any]) -> List[Dict[str, Any]]:
+    def _normalize_errors(errors: Sequence[Any]) -> List[ErrorDetails]:
         return errors  # type: ignore[return-value]
 
     def _model_rebuild(model: Type[BaseModel]) -> None:
@@ -265,12 +269,12 @@ 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
@@ -289,6 +293,9 @@ def create_body_model(
     from pydantic.class_validators import (  # type: ignore[no-redef]
         Validator as Validator,
     )
+    from pydantic.error_wrappers import (  # type: ignore[no-redef]
+        ErrorList as ErrorList,
+    )
     from pydantic.error_wrappers import (  # type: ignore[no-redef]
         ErrorWrapper as ErrorWrapper,
     )
@@ -418,7 +425,7 @@ def is_pv1_scalar_sequence_field(field: ModelField) -> bool:
             return True
         return False
 
-    def _normalize_errors(errors: Sequence[Any]) -> List[Dict[str, Any]]:
+    def _normalize_errors(errors: Sequence[Any]) -> List[ErrorDetails]:
         use_errors: List[Any] = []
         for error in errors:
             if isinstance(error, ErrorWrapper):
@@ -500,10 +507,10 @@ 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
@@ -516,7 +523,7 @@ def create_body_model(
 
 def _regenerate_error_with_loc(
     *, errors: Sequence[Any], loc_prefix: Tuple[Union[str, int], ...]
-) -> List[Dict[str, Any]]:
+) -> List[ErrorDetails]:
     updated_loc_errors: List[Any] = [
         {**err, "loc": loc_prefix + err.get("loc", ())}
         for err in _normalize_errors(errors)

fastapi/dependencies/utils.py

@@ -50,6 +50,7 @@
     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
@@ -539,7 +540,7 @@ async def solve_dependencies(
     Dict[Tuple[Callable[..., Any], Tuple[str]], Any],
 ]:
     values: Dict[str, Any] = {}
-    errors: List[Any] = []
+    errors: List[ErrorDetails] = []
     if response is None:
         response = Response()
         del response.headers["content-length"]
@@ -652,9 +653,9 @@ async def solve_dependencies(
 def request_params_to_args(
     required_params: Sequence[ModelField],
     received_params: Union[Mapping[str, Any], QueryParams, Headers],
-) -> Tuple[Dict[str, Any], List[Any]]:
+) -> Tuple[Dict[str, Any], List[ErrorDetails]]:
     values = {}
-    errors = []
+    errors: List[ErrorDetails] = []
     for field in required_params:
         if is_scalar_sequence_field(field) and isinstance(
             received_params, (QueryParams, Headers)
@@ -675,10 +676,9 @@ def request_params_to_args(
             continue
         v_, errors_ = field.validate(value, values, loc=loc)
         if isinstance(errors_, ErrorWrapper):
-            errors.append(errors_)
+            errors.extend(_regenerate_error_with_loc(errors=[errors_], loc_prefix=()))
         elif isinstance(errors_, list):
-            new_errors = _regenerate_error_with_loc(errors=errors_, loc_prefix=())
-            errors.extend(new_errors)
+            errors.extend(_regenerate_error_with_loc(errors=errors_, loc_prefix=()))
         else:
             values[field.name] = v_
     return values, errors
@@ -687,9 +687,9 @@ def request_params_to_args(
 async def request_body_to_args(
     required_params: List[ModelField],
     received_body: Optional[Union[Dict[str, Any], FormData]],
-) -> Tuple[Dict[str, Any], List[Dict[str, Any]]]:
+) -> Tuple[Dict[str, Any], List[ErrorDetails]]:
     values = {}
-    errors: List[Dict[str, Any]] = []
+    errors: List[ErrorDetails] = []
     if required_params:
         field = required_params[0]
         field_info = field.field_info
@@ -757,10 +757,12 @@ async def process_fn(
 
             v_, errors_ = field.validate(value, values, loc=loc)
 
-            if isinstance(errors_, list):
-                errors.extend(errors_)
-            elif errors_:
-                errors.append(errors_)
+            if isinstance(errors_, ErrorWrapper):
+                errors.extend(
+                    _regenerate_error_with_loc(errors=[errors_], loc_prefix=())
+                )
+            elif isinstance(errors_, list):
+                errors.extend(_regenerate_error_with_loc(errors=errors_, loc_prefix=()))
             else:
                 values[field.name] = v_
     return values, errors

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
 

fastapi/routing.py

@@ -251,7 +251,7 @@ async def app(request: Request) -> Response:
                             "msg": "JSON decode error",
                             "input": {},
                             "ctx": {"error": e.msg},
-                        }
+                        }  # type: ignore [typeddict-unknown-key]
                     ],
                     body=e.doc,
                 )