Make request handler extensible

fastapi/routing.py

@@ -33,6 +33,7 @@ from fastapi._compat import (
 from fastapi.datastructures import Default, DefaultPlaceholder
 from fastapi.dependencies.models import Dependant
 from fastapi.dependencies.utils import (
+    SolvedDependency,
     _should_embed_body_fields,
     get_body_field,
     get_dependant,
@@ -240,52 +241,12 @@ def get_request_handler(
     async def app(request: Request) -> Response:
         response: Union[Response, None] = None
         async with AsyncExitStack() as file_stack:
-            try:
-                body: Any = None
-                if body_field:
-                    if is_body_form:
-                        body = await request.form()
-                        file_stack.push_async_callback(body.close)
-                    else:
-                        body_bytes = await request.body()
-                        if body_bytes:
-                            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
-                                if message.get_content_maintype() == "application":
-                                    subtype = message.get_content_subtype()
-                                    if subtype == "json" or subtype.endswith("+json"):
-                                        json_body = await request.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
-            except Exception as e:
-                http_error = HTTPException(
-                    status_code=400, detail="There was an error parsing the body"
+            body = await parse_request_body(
+                request,
+                body_field=body_field,
+                is_body_form=is_body_form,
+                file_stack=file_stack,
             )
-                raise http_error from e
             errors: List[Any] = []
             async with AsyncExitStack() as async_exit_stack:
                 solved_result = await solve_dependencies(
@@ -303,6 +264,29 @@ def get_request_handler(
                         values=solved_result.values,
                         is_coroutine=is_coroutine,
                     )
+                    response = await prepare_response_object(
+                        raw_response,
+                        solved_result,
+                    )
+            if errors:
+                validation_error = RequestValidationError(
+                    _normalize_errors(errors), body=body
+                )
+                raise validation_error
+        if response is None:
+            raise FastAPIError(
+                "No response object was returned. There's a high chance that the "
+                "application code is raising an exception and a dependency with yield "
+                "has a block with a bare except, or a block with except Exception, "
+                "and is not raising the exception again. Read more about it in the "
+                "docs: https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#dependencies-with-yield-and-except"
+            )
+        return response
+
+    async def prepare_response_object(
+        raw_response: Any,
+        solved_result: SolvedDependency,
+    ) -> Response:
         if isinstance(raw_response, Response):
             if raw_response.background is None:
                 raw_response.background = solved_result.background_tasks
@@ -314,16 +298,12 @@ def get_request_handler(
             # If status_code was set, use it, otherwise use the default from the
             # response class, in the case of redirect it's 307
             current_status_code = (
-                            status_code
-                            if status_code
-                            else solved_result.response.status_code
+                status_code if status_code else solved_result.response.status_code
             )
             if current_status_code is not None:
                 response_args["status_code"] = current_status_code
             if solved_result.response.status_code:
-                            response_args["status_code"] = (
-                                solved_result.response.status_code
-                            )
+                response_args["status_code"] = solved_result.response.status_code
             content = await serialize_response(
                 field=response_field,
                 response_content=raw_response,
@@ -339,24 +319,67 @@ def get_request_handler(
             if not is_body_allowed_for_status_code(response.status_code):
                 response.body = b""
             response.headers.raw.extend(solved_result.response.headers.raw)
-            if errors:
-                validation_error = RequestValidationError(
-                    _normalize_errors(errors), body=body
-                )
-                raise validation_error
-        if response is None:
-            raise FastAPIError(
-                "No response object was returned. There's a high chance that the "
-                "application code is raising an exception and a dependency with yield "
-                "has a block with a bare except, or a block with except Exception, "
-                "and is not raising the exception again. Read more about it in the "
-                "docs: https://fastapi.tiangolo.com/tutorial/dependencies/dependencies-with-yield/#dependencies-with-yield-and-except"
-            )
         return response
 
     return app
 
 
+async def parse_request_body(
+    request: Request,
+    *,
+    body_field: Optional[ModelField],
+    is_body_form: bool | None,
+    file_stack: AsyncExitStack,
+) -> Any:
+    try:
+        body: Any = None
+        if body_field:
+            if is_body_form:
+                body = await request.form()
+                file_stack.push_async_callback(body.close)
+            else:
+                body_bytes = await request.body()
+                if body_bytes:
+                    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
+                        if message.get_content_maintype() == "application":
+                            subtype = message.get_content_subtype()
+                            if subtype == "json" or subtype.endswith("+json"):
+                                json_body = await request.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
+    except Exception as e:
+        http_error = HTTPException(
+            status_code=400, detail="There was an error parsing the body"
+        )
+        raise http_error from e
+    return body
+
+
 def get_websocket_app(
     dependant: Dependant,
     dependency_overrides_provider: Optional[Any] = None,