Extend Subsonic XML renderer to allow explicit XML child tags

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2695>

api/funkwhale_api/subsonic/renderers.py

@@ -6,6 +6,10 @@ from rest_framework import renderers
 import funkwhale_api
 
 
+class TagValue(str):
+    """Use this for string values that must be rendered as tags instead of attributes in XML."""
+
+
 # from https://stackoverflow.com/a/8915039
 # because I want to avoid a lxml dependency just for outputting cdata properly
 # in a RSS feed
@@ -83,6 +87,10 @@ def dict_to_xml_tree(root_tag, d, parent=None):
                     el = ET.Element(key)
                     el.text = str(obj)
                 root.append(el)
+        elif isinstance(value, TagValue):
+            el = ET.Element(key)
+            el.text = str(value)
+            root.append(el)
         else:
             if key == "value":
                 root.text = str(value)

api/tests/subsonic/test_renderers.py

@@ -79,9 +79,10 @@ def test_xml_renderer_dict_to_xml():
         "hello": "world",
         "item": [{"this": 1, "value": "text"}, {"some": "node"}],
         "list": [1, 2],
+        "some-tag": renderers.TagValue("foo"),
     }
     expected = """<?xml version="1.0" encoding="UTF-8"?>
-<key hello="world"><item this="1">text</item><item some="node" /><list>1</list><list>2</list></key>"""
+<key hello="world"><item this="1">text</item><item some="node" /><list>1</list><list>2</list><some-tag>foo</some-tag></key>"""  # noqa
     result = renderers.dict_to_xml_tree("key", payload)
     exp = ET.fromstring(expected)
     assert ET.tostring(result) == ET.tostring(exp)