Extract prefered response content type negotiation to function

falcon/app_helpers.py

@@ -226,8 +226,26 @@ def default_serialize_error(req: Request, resp: Response, exception: HTTPError):
         resp: Instance of ``falcon.Response``
         exception: Instance of ``falcon.HTTPError``
     """
-    preferred = req.client_prefers((MEDIA_XML, 'text/xml', MEDIA_JSON))
+    preferred = _negotiate_preffered_media_type(req)
+
+    if preferred is not None:
+        if preferred == MEDIA_JSON:
+            handler, _, _ = resp.options.media_handlers._resolve(
+                MEDIA_JSON, MEDIA_JSON, raise_not_found=False
+            )
+            resp.data = exception.to_json(handler)
+        else:
+            resp.data = exception.to_xml()
+
+        # NOTE(kgriffs): No need to append the charset param, since
+        #   utf-8 is the default for both JSON and XML.
+        resp.content_type = preferred
+
+    resp.append_header('Vary', 'Accept')
+
 
+def _negotiate_preffered_media_type(req: Request) -> str:
+    preferred = req.client_prefers((MEDIA_XML, 'text/xml', MEDIA_JSON))
     if preferred is None:
         # NOTE(kgriffs): See if the client expects a custom media
         # type based on something Falcon supports. Returning something
@@ -246,21 +264,7 @@ def default_serialize_error(req: Request, resp: Response, exception: HTTPError):
             preferred = MEDIA_JSON
         elif '+xml' in accept:
             preferred = MEDIA_XML
-
-    if preferred is not None:
-        if preferred == MEDIA_JSON:
-            handler, _, _ = resp.options.media_handlers._resolve(
-                MEDIA_JSON, MEDIA_JSON, raise_not_found=False
-            )
-            resp.data = exception.to_json(handler)
-        else:
-            resp.data = exception.to_xml()
-
-        # NOTE(kgriffs): No need to append the charset param, since
-        #   utf-8 is the default for both JSON and XML.
-        resp.content_type = preferred
-
-    resp.append_header('Vary', 'Accept')
+    return preferred
 
 
 class CloseableStreamIterator: