✨ Use TypeAdapter.validate_json for Pydantic v2

fastapi/_compat.py

@@ -1,3 +1,4 @@
+import json
 from collections import deque
 from copy import copy
 from dataclasses import dataclass, is_dataclass
@@ -135,6 +136,22 @@ if PYDANTIC_V2:
                     errors=exc.errors(include_url=False), loc_prefix=loc
                 )
 
+        def validate_json(
+            self,
+            value: Any,
+            *,
+            loc: Tuple[Union[int, str], ...] = (),
+        ) -> Tuple[Any, Union[List[Dict[str, Any]], None]]:
+            try:
+                return (
+                    self._type_adapter.validate_json(value),
+                    None,
+                )
+            except ValidationError as exc:
+                return None, _regenerate_error_with_loc(
+                    errors=exc.errors(include_url=False), loc_prefix=loc
+                )
+
         def serialize(
             self,
             value: Any,
@@ -292,6 +309,18 @@ if PYDANTIC_V2:
             for name, field_info in model.model_fields.items()
         ]
 
+    def parse_json_field(
+        field: ModelField,
+        data: bytes,
+        values: Dict[str, Any],
+        loc: Tuple[str, ...] = (),
+    ) -> Tuple[Any, Union[List[Dict[str, Any]], None]]:
+        value, errors = field.validate_json(data)
+        if isinstance(errors, list):
+            new_errors = _regenerate_error_with_loc(errors=errors, loc_prefix=loc)
+            return None, new_errors
+        return value, errors
+
 else:
     from fastapi.openapi.constants import REF_PREFIX as REF_PREFIX
     from pydantic import AnyUrl as Url  # noqa: F401
@@ -530,6 +559,17 @@ else:
     def get_model_fields(model: Type[BaseModel]) -> List[ModelField]:
         return list(model.__fields__.values())  # type: ignore[attr-defined]
 
+    def parse_json_field(
+        field: ModelField,
+        data: bytes,
+        values: Dict[str, Any],
+        loc: Tuple[str, ...] = (),
+    ) -> Tuple[Any, Union[List[Dict[str, Any]], None]]:
+        parsed_value, errors = parse_json(data, loc)
+        if errors:
+            return None, errors
+        return field.validate(parsed_value, values, loc=loc)
+
 
 def _regenerate_error_with_loc(
     *, errors: Sequence[Any], loc_prefix: Tuple[Union[str, int], ...]
@@ -662,3 +702,26 @@ def is_uploadfile_sequence_annotation(annotation: Any) -> bool:
 @lru_cache
 def get_cached_model_fields(model: Type[BaseModel]) -> List[ModelField]:
     return get_model_fields(model)
+
+
+def parse_json(
+    data: bytes,
+    loc: Tuple[str, ...],
+) -> Tuple[Any, List[Dict[str, Any]]]:
+    try:
+        return json.loads(data), []
+    except json.JSONDecodeError as e:
+        return None, [
+            {
+                "type": "value_error.jsondecode",
+                "loc": loc + (e.pos,),
+                "msg": f"Invalid JSON: {e.msg}",
+                "ctx": {
+                    "msg": e.msg,
+                    "doc": data,
+                    "pos": e.pos,
+                    "lineno": e.lineno,
+                    "colno": e.colno,
+                },
+            }
+        ]

fastapi/dependencies/utils.py

@@ -42,15 +42,14 @@ from fastapi._compat import (
     is_uploadfile_or_nonable_uploadfile_annotation,
     is_uploadfile_sequence_annotation,
     lenient_issubclass,
+    parse_json,
+    parse_json_field,
     sequence_types,
     serialize_sequence_value,
     value_is_sequence,
 )
 from fastapi.background import BackgroundTasks
-from fastapi.concurrency import (
+from fastapi.concurrency import asynccontextmanager, contextmanager_in_threadpool
-    asynccontextmanager,
-    contextmanager_in_threadpool,
-)
 from fastapi.dependencies.models import Dependant, SecurityRequirement
 from fastapi.logger import logger
 from fastapi.security.base import SecurityBase
@@ -573,7 +572,8 @@ async def solve_dependencies(
     *,
     request: Union[Request, WebSocket],
     dependant: Dependant,
-    body: Optional[Union[Dict[str, Any], FormData]] = None,
+    body: Optional[Union[bytes, Dict[str, Any], FormData]] = None,
+    is_body_json: bool = False,
     background_tasks: Optional[StarletteBackgroundTasks] = None,
     response: Optional[Response] = None,
     dependency_overrides_provider: Optional[Any] = None,
@@ -616,6 +616,7 @@ async def solve_dependencies(
             request=request,
             dependant=use_sub_dependant,
             body=body,
+            is_body_json=is_body_json,
             background_tasks=background_tasks,
             response=response,
             dependency_overrides_provider=dependency_overrides_provider,
@@ -666,6 +667,7 @@ async def solve_dependencies(
         ) = await request_body_to_args(  # body_params checked above
             body_fields=dependant.body_params,
             received_body=body,
+            is_body_json=is_body_json,
             embed_body_fields=embed_body_fields,
         )
         values.update(body_values)
@@ -695,6 +697,24 @@ async def solve_dependencies(
     )
 
 
+def _validate_json_body_as_model_field(
+    *, field: ModelField, value: Any, values: Dict[str, Any], loc: Tuple[str, ...]
+) -> Tuple[Any, List[Any]]:
+    if value is None:
+        if field.required:
+            return None, [get_missing_field_error(loc=loc)]
+        else:
+            return deepcopy(field.default), []
+    v_, errors_ = parse_json_field(field, value, values=values, loc=loc)
+    if isinstance(errors_, ErrorWrapper):
+        return None, [errors_]
+    elif isinstance(errors_, list):
+        new_errors = _regenerate_error_with_loc(errors=errors_, loc_prefix=())
+        return None, new_errors
+    else:
+        return v_, []
+
+
 def _validate_value_with_model_field(
     *, field: ModelField, value: Any, values: Dict[str, Any], loc: Tuple[str, ...]
 ) -> Tuple[Any, List[Any]]:
@@ -904,7 +924,8 @@ async def _extract_form_body(
 
 async def request_body_to_args(
     body_fields: List[ModelField],
-    received_body: Optional[Union[Dict[str, Any], FormData]],
+    received_body: Optional[Union[bytes, Dict[str, Any], FormData]],
+    is_body_json: bool,
     embed_body_fields: bool,
 ) -> Tuple[Dict[str, Any], List[Dict[str, Any]]]:
     values: Dict[str, Any] = {}
@@ -924,16 +945,28 @@ async def request_body_to_args(
 
     if single_not_embedded_field:
         loc: Tuple[str, ...] = ("body",)
+        if is_body_json:
+            v_, errors_ = _validate_json_body_as_model_field(
+                field=first_field, value=body_to_process, values=values, loc=loc
+            )
+        else:
             v_, errors_ = _validate_value_with_model_field(
                 field=first_field, value=body_to_process, values=values, loc=loc
             )
         return {first_field.name: v_}, errors_
+
+    if is_body_json and isinstance(received_body, bytes):
+        body_to_process, errors = parse_json(received_body, loc=("body",))
+
+    if errors:
+        return values, errors
+
     for field in body_fields:
         loc = ("body", field.alias)
         value: Optional[Any] = None
         if body_to_process is not None:
             try:
-                value = body_to_process.get(field.alias)
+                value = body_to_process.get(field.alias)  # type: ignore[union-attr]
             # If the received body is a list, not a dict
             except AttributeError:
                 errors.append(get_missing_field_error(loc))

fastapi/routing.py

@@ -2,7 +2,6 @@ import asyncio
 import dataclasses
 import email.message
 import inspect
-import json
 from contextlib import AsyncExitStack, asynccontextmanager
 from enum import Enum, IntEnum
 from typing import (
@@ -25,7 +24,6 @@ from typing import (
 from fastapi import params
 from fastapi._compat import (
     ModelField,
-    Undefined,
     _get_model_config,
     _model_dump,
     _normalize_errors,
@@ -243,42 +241,23 @@ def get_request_handler(
         async with AsyncExitStack() as file_stack:
             try:
                 body: Any = None
+                is_body_json = False
                 if body_field:
                     if is_body_form:
                         body = await request.form()
                         file_stack.push_async_callback(body.close)
                     else:
-                        body_bytes = await request.body()
+                        body = await request.body() or None
-                        if body_bytes:
+                        if body:
-                            json_body: Any = Undefined
-                            content_type_value = request.headers.get("content-type")
-                            if not content_type_value:
-                                json_body = await request.json()
-                            else:
                             message = email.message.Message()
-                                message["content-type"] = content_type_value
+                            message["content-type"] = request.headers.get(
+                                "content-type", "application/json"
+                            )
                             if message.get_content_maintype() == "application":
                                 subtype = message.get_content_subtype()
-                                    if subtype == "json" or subtype.endswith("+json"):
+                                is_body_json = subtype == "json" or subtype.endswith(
-                                        json_body = await request.json()
+                                    "+json"
-                            if json_body != Undefined:
-                                body = json_body
-                            else:
-                                body = body_bytes
-            except json.JSONDecodeError as e:
-                validation_error = RequestValidationError(
-                    [
-                        {
-                            "type": "json_invalid",
-                            "loc": ("body", e.pos),
-                            "msg": "JSON decode error",
-                            "input": {},
-                            "ctx": {"error": e.msg},
-                        }
-                    ],
-                    body=e.doc,
                                 )
-                raise validation_error from e
             except HTTPException:
                 # If a middleware raises an HTTPException, it should be raised again
                 raise
@@ -293,6 +272,7 @@ def get_request_handler(
                     request=request,
                     dependant=dependant,
                     body=body,
+                    is_body_json=is_body_json,
                     dependency_overrides_provider=dependency_overrides_provider,
                     async_exit_stack=async_exit_stack,
                     embed_body_fields=embed_body_fields,

tests/test_tutorial/test_body/test_tutorial001.py

@@ -4,6 +4,7 @@ from unittest.mock import patch
 import pytest
 from dirty_equals import IsDict
 from fastapi.testclient import TestClient
+from starlette.routing import Request
 
 from ...utils import needs_py310
 
@@ -206,12 +207,10 @@ def test_post_broken_body(client: TestClient):
             "detail": [
                 {
                     "type": "json_invalid",
-                    "loc": ["body", 1],
+                    "loc": ["body"],
-                    "msg": "JSON decode error",
+                    "msg": "Invalid JSON: key must be a string at line 1 column 2",
-                    "input": {},
+                    "input": "{some broken json}",
-                    "ctx": {
+                    "ctx": {"error": "key must be a string at line 1 column 2"},
-                        "error": "Expecting property name enclosed in double quotes"
-                    },
                 }
             ]
         }
@@ -221,7 +220,7 @@ def test_post_broken_body(client: TestClient):
             "detail": [
                 {
                     "loc": ["body", 1],
-                    "msg": "Expecting property name enclosed in double quotes: line 1 column 2 (char 1)",
+                    "msg": "Invalid JSON: Expecting property name enclosed in double quotes",
                     "type": "value_error.jsondecode",
                     "ctx": {
                         "msg": "Expecting property name enclosed in double quotes",
@@ -383,7 +382,7 @@ def test_wrong_headers(client: TestClient):
 
 
 def test_other_exceptions(client: TestClient):
-    with patch("json.loads", side_effect=Exception):
+    with patch.object(Request, "body", side_effect=Exception):
         response = client.post("/items/", json={"test": "test2"})
         assert response.status_code == 400, response.text
 

tests/test_tutorial/test_custom_request_and_route/test_tutorial002.py

@@ -20,7 +20,7 @@ def test_exception_handler_body_access():
                     {
                         "type": "list_type",
                         "loc": ["body"],
-                        "msg": "Input should be a valid list",
+                        "msg": "Input should be a valid array",
                         "input": {"numbers": [1, 2, 3]},
                     }
                 ],

tests/test_tutorial/test_handling_errors/test_tutorial005.py

@@ -19,7 +19,31 @@ def test_post_validation_error():
                     "input": "XL",
                 }
             ],
-            "body": {"title": "towel", "size": "XL"},
+            "body": '{"title":"towel","size":"XL"}',
+        }
+    ) | IsDict(
+        {
+            "detail": [
+                {
+                    "type": "int_parsing",
+                    "loc": ["body", "size"],
+                    "msg": "Input should be a valid integer, unable to parse string as an integer",
+                    "input": "XL",
+                }
+            ],
+            "body": '{"title": "towel", "size": "XL"}',
+        }
+    ) | IsDict(
+        # TODO: remove when deprecating Pydantic v1
+        {
+            "detail": [
+                {
+                    "loc": ["body", "size"],
+                    "msg": "value is not a valid integer",
+                    "type": "type_error.integer",
+                }
+            ],
+            "body": '{"title":"towel","size":"XL"}',
         }
     ) | IsDict(
         # TODO: remove when deprecating Pydantic v1
@@ -31,7 +55,7 @@ def test_post_validation_error():
                     "type": "type_error.integer",
                 }
             ],
-            "body": {"title": "towel", "size": "XL"},
+            "body": '{"title": "towel", "size": "XL"}',
         }
     )