🐛 Fix bug overriding custom HTTPException and RequestValidationError from exception_handlers (#1924)

fastapi/applications.py

@@ -66,6 +66,10 @@ class FastAPI(Starlette):
         self.exception_handlers = (
             {} if exception_handlers is None else dict(exception_handlers)
         )
+        self.exception_handlers.setdefault(HTTPException, http_exception_handler)
+        self.exception_handlers.setdefault(
+            RequestValidationError, request_validation_exception_handler
+        )
 
         self.user_middleware = [] if middleware is None else list(middleware)
         self.middleware_stack = self.build_middleware_stack()
@@ -165,10 +169,6 @@ class FastAPI(Starlette):
                 )
 
             self.add_route(self.redoc_url, redoc_html, include_in_schema=False)
-        self.add_exception_handler(HTTPException, http_exception_handler)
-        self.add_exception_handler(
-            RequestValidationError, request_validation_exception_handler
-        )
 
     async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         if self.root_path:

tests/test_exception_handlers.py

@@ -0,0 +1,44 @@
+from fastapi import FastAPI, HTTPException
+from fastapi.exceptions import RequestValidationError
+from fastapi.testclient import TestClient
+from starlette.responses import JSONResponse
+
+
+def http_exception_handler(request, exception):
+    return JSONResponse({"exception": "http-exception"})
+
+
+def request_validation_exception_handler(request, exception):
+    return JSONResponse({"exception": "request-validation"})
+
+
+app = FastAPI(
+    exception_handlers={
+        HTTPException: http_exception_handler,
+        RequestValidationError: request_validation_exception_handler,
+    }
+)
+
+client = TestClient(app)
+
+
+@app.get("/http-exception")
+def route_with_http_exception():
+    raise HTTPException(status_code=400)
+
+
+@app.get("/request-validation/{param}/")
+def route_with_request_validation_exception(param: int):
+    pass  # pragma: no cover
+
+
+def test_override_http_exception():
+    response = client.get("/http-exception")
+    assert response.status_code == 200
+    assert response.json() == {"exception": "http-exception"}
+
+
+def test_override_request_validation_exception():
+    response = client.get("/request-validation/invalid")
+    assert response.status_code == 200
+    assert response.json() == {"exception": "request-validation"}