fix: Support user defined http and request validation exception handlers.

Author: EdgyEdgemond

src/fastapi_problem/handler.py

@@ -199,13 +199,12 @@ def add_exception_handler(  # noqa: PLR0913
     generic_swagger_defaults: bool = True,
     strict_rfc9457: bool = False,
 ) -> ExceptionHandler:
+    handlers_ = {
+        HTTPException: http_exception_handler,
+        RequestValidationError: request_validation_handler,
+    }
     handlers = handlers or {}
-    handlers.update(
-        {
-            HTTPException: http_exception_handler,
-            RequestValidationError: request_validation_handler,
-        },
-    )
+    handlers_.update(handlers)
     pre_hooks = pre_hooks or []
     post_hooks = post_hooks or []
 
@@ -216,7 +215,7 @@ def add_exception_handler(  # noqa: PLR0913
     eh = ExceptionHandler(
         logger=logger,
         unhandled_wrappers=unhandled_wrappers,
-        handlers=handlers,
+        handlers=handlers_,
         pre_hooks=pre_hooks,
         post_hooks=post_hooks,
         documentation_uri_template=documentation_uri_template,

tests/test_handler.py

@@ -679,3 +679,25 @@ async def status(_a: str) -> dict:
             "description": "Validation Error",
         },
     }
+
+
+async def test_custom_http_exception_handler_in_app():
+    def custom_handler(_eh, _request, _exc) -> error.Problem:
+        return error.Problem("a problem")
+
+    app = FastAPI()
+
+    handler.add_exception_handler(
+        app=app,
+        handlers={HTTPException: custom_handler},
+    )
+
+    transport = httpx.ASGITransport(app=app, raise_app_exceptions=False, client=("1.2.3.4", 123))
+    client = httpx.AsyncClient(transport=transport, base_url="https://test")
+
+    r = await client.get("/endpoint")
+    assert r.json() == {
+        "type": "problem",
+        "title": "a problem",
+        "status": 500,
+    }