Use media handlers to serialize errors by accepted content type

falcon/app_helpers.py

@@ -15,8 +15,10 @@
 """Utilities for the App class."""
 from __future__ import annotations
 
+from collections import OrderedDict
 from inspect import iscoroutinefunction
 from typing import IO, Iterable, List, Tuple
+from typing import Optional
 
 from falcon import util
 from falcon.constants import MEDIA_JSON
@@ -227,14 +229,14 @@ def default_serialize_error(req: Request, resp: Response, exception: HTTPError)
         resp: Instance of ``falcon.Response``
         exception: Instance of ``falcon.HTTPError``
     """
-    preferred = _negotiate_preffered_media_type(req)
+    preferred = _negotiate_preffered_media_type(req, resp)
 
     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)
+        handler, _, _ = resp.options.media_handlers._resolve(
+            preferred, MEDIA_JSON, raise_not_found=False
+        )
+        if handler:
+            resp.data = handler.serialize(exception.to_dict(OrderedDict), preferred)
         else:
             resp.data = exception.to_xml()
 
@@ -245,8 +247,10 @@ def default_serialize_error(req: Request, resp: Response, exception: HTTPError)
     resp.append_header('Vary', 'Accept')
 
 
-def _negotiate_preffered_media_type(req: Request) -> str:
-    preferred = req.client_prefers((MEDIA_XML, 'text/xml', MEDIA_JSON))
+def _negotiate_preffered_media_type(req: Request, resp: Response) -> Optional[str]:
+    supported = {MEDIA_XML, 'text/xml', MEDIA_JSON}
+    supported.update(set(resp.options.media_handlers.keys()))
+    preferred = req.client_prefers(supported)
     if preferred is None:
         # NOTE(kgriffs): See if the client expects a custom media
         # type based on something Falcon supports. Returning something

falcon/typing.py

@@ -46,6 +46,12 @@ def _resolve(
         ) -> Tuple[Serializer, Optional[Callable], Optional[Callable]]:
             ...
 
+        def __setattr__(self, key: str, value: Serializer) -> None:
+            ...
+
+        def __delattr__(self, key: str) -> None:
+            ...
+
 
 Link = Dict[str, str]
 

tests/test_app_helpers.py

@@ -1,12 +1,16 @@
 import pytest
 
 from falcon import HTTPNotFound
+from falcon import ResponseOptions
 from falcon.app_helpers import default_serialize_error
+from falcon.media import BaseHandler
+from falcon.media import Handlers
 from falcon.request import Request
 from falcon.response import Response
 from falcon.testing import create_environ
 
-JSON = ('application/json', 'application/json', b'{"title": "404 Not Found"}')
+JSON_CONTENT = b'{"title": "404 Not Found"}'
+JSON = ('application/json', 'application/json', JSON_CONTENT)
 XML = (
     'application/xml',
     'application/xml',
@@ -15,7 +19,7 @@
         b'<error><title>404 Not Found</title></error>'
     ),
 )
-CUSTOM_JSON = ('custom/any+json', 'application/json', b'{"title": "404 Not Found"}')
+CUSTOM_JSON = ('custom/any+json', 'application/json', JSON_CONTENT)
 
 CUSTOM_XML = (
     'custom/any+xml',
@@ -26,21 +30,48 @@
     ),
 )
 
+YAML = (
+    'application/yaml',
+    'application/yaml',
+    (b'error:\n' b'    title: 404 Not Found'),
+)
+
+
+class FakeYamlMediaHandler(BaseHandler):
+    def serialize(self, media: object, content_type: str) -> bytes:
+        return b'error:\n' b'    title: 404 Not Found'
+
 
 class TestDefaultSerializeError:
+    def test_if_no_content_type_and_accept_fall_back_to_json(self) -> None:
+        response = Response()
+        default_serialize_error(
+            Request(env=(create_environ())),
+            response,
+            HTTPNotFound(),
+        )
+        assert response.content_type == 'application/json'
+        assert response.headers['vary'] == 'Accept'
+        assert response.data == JSON_CONTENT
+
     @pytest.mark.parametrize(
         'accept, content_type, data',
         (
             JSON,
             XML,
             CUSTOM_JSON,
             CUSTOM_XML,
+            YAML,
         ),
     )
-    def test_serializes_error_to_preffered_by_sender(
+    def test_serializes_error_to_preferred_by_sender(
         self, accept, content_type, data
     ) -> None:
-        response = Response()
+        handlers = Handlers()
+        handlers['application/yaml'] = FakeYamlMediaHandler()
+        options = ResponseOptions()
+        options.media_handlers = handlers
+        response = Response(options=options)
         default_serialize_error(
             Request(env=(create_environ(headers={'accept': accept}))),
             response,

tests/test_media_handlers.py

@@ -353,6 +353,20 @@ def on_get(self, req, resp):
     assert result.json == falcon.HTTPForbidden().to_dict()
 
 
+def test_handlers_include_new_media_handlers_in_resolving() -> None:
+    class FakeHandler:
+        ...
+
+    handlers = media.Handlers({falcon.MEDIA_URLENCODED: media.URLEncodedFormHandler()})
+    handler = FakeHandler()
+    handlers['application/yaml'] = handler
+    resolved, _, _ = handlers._resolve(
+        'application/yaml', 'application/json', raise_not_found=False
+    )
+    assert resolved.__class__.__name__ == handler.__class__.__name__
+    assert resolved == handler
+
+
 class TestBaseHandler:
     def test_defaultError(self):
         h = media.BaseHandler()