fix: properly account for q= when parting the accept header

falcon/app_helpers.py

@@ -291,7 +291,12 @@ 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))
+    predefined = [MEDIA_XML, 'text/xml', MEDIA_JSON]
+    media_handlers = [mt for mt in resp.options.media_handlers if mt not in predefined]
+    # NOTE(caselit) add all the registered before the predefined ones. This ensures that
+    # in case of equal match the last one (json) is selected and that the q= is taken
+    # into consideration when selecting the media
+    preferred = req.client_prefers(media_handlers + predefined)
 
     if preferred is None:
         # NOTE(kgriffs): See if the client expects a custom media
@@ -311,10 +316,6 @@ def default_serialize_error(req: Request, resp: Response, exception: HTTPError)
             preferred = MEDIA_JSON
         elif '+xml' in accept:
             preferred = MEDIA_XML
-        else:
-            # NOTE(caselit): if nothing else matchers try using the media handlers
-            # registered in the response
-            preferred = req.client_prefers(resp.options.media_handlers)
 
     if preferred is not None:
         handler, _, _ = resp.options.media_handlers._resolve(

falcon/request.py

@@ -28,7 +28,6 @@
     Mapping,
     Optional,
     overload,
-    Sequence,
     TextIO,
     Tuple,
     Type,

tests/test_httperror.py

@@ -962,7 +962,7 @@ def test_kw_only(self):
 XML = (MEDIA_XML, MEDIA_XML, XML_CONTENT)
 CUSTOM_XML = ('custom/any+xml', MEDIA_XML, XML_CONTENT)
 
-YAML = (MEDIA_YAML, MEDIA_YAML, (b'title: 410 Gone!'))
+YAML = (MEDIA_YAML, MEDIA_YAML, b'title: 410 Gone!')
 ASYNC_ONLY = ('application/only_async', 'application/only_async', b'this is async')
 ASYNC_WITH_SYNC = (
     'application/async_with_sync',
@@ -1040,3 +1040,34 @@ def test_json_async_only_error(self, util):
         client = testing.TestClient(app)
         with pytest.raises(NotImplementedError, match='requires the sync interface'):
             client.simulate_get()
+
+    def test_add_xml_handler(self, client):
+        client.app.resp_options.media_handlers[MEDIA_XML] = FakeYamlMediaHandler()
+        res = client.simulate_get(headers={'Accept': 'application/xhtml+xml'})
+        assert res.content_type == MEDIA_XML
+        assert res.content == YAML[-1]
+
+    @pytest.mark.parametrize(
+        'accept, content_type',
+        [
+            (
+                # firefox
+                'text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,'
+                'image/webp,image/png,image/svg+xml,*/*;q=0.8',
+                MEDIA_XML,
+            ),
+            (
+                # safari / chrome
+                'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,'
+                'image/apng,*/*;q=0.8',
+                MEDIA_XML,
+            ),
+            ('text/html, application/xhtml+xml, image/jxr, */*', MEDIA_JSON),  # edge
+            (f'text/html,{MEDIA_YAML};q=0.8,*/*;q=0.7', MEDIA_YAML),
+            (f'text/html,{MEDIA_YAML};q=0.8,{MEDIA_JSON};q=0.8', MEDIA_JSON),
+        ],
+    )
+    def test_hard_content_types(self, client, accept, content_type):
+        client.app.resp_options.media_handlers[MEDIA_YAML] = FakeYamlMediaHandler()
+        res = client.simulate_get(headers={'Accept': accept})
+        assert res.content_type == content_type