Preserve null artpath in edit (#6839)

Fixes #2438.

When editing album metadata, `artpath: null` was converted through the
string parsing path and could become a path ending in `None`.

This keeps YAML null values as `None` when dumping and applying edit
data, and adds a regression test for editing album metadata while
`artpath` is missing.

- Added a regression test.
- Added a changelog entry.

beetsplug/edit.py

@@ -91,6 +91,8 @@ def _safe_value(obj, key, value):
     This ensures that values do not change their type when the user edits their
     YAML representation.
     """
+    if value is None:
+        return True
     typ = obj._type(key)
     return isinstance(typ, SAFE_TYPES) and isinstance(value, typ.model_type)
 

docs/changelog.rst

@@ -43,6 +43,9 @@ New features
 Bug fixes
 ~~~~~~~~~
 
+- :doc:`plugins/edit`: Preserve missing album art paths when editing album
+  metadata, instead of turning ``artpath: null`` into a path ending in ``None``.
+  :bug:`2438`
 - :doc:`plugins/subsonicupdate`: Log a clearer error when the Subsonic server
   returns a non-JSON response. :bug:`5635`
 - :doc:`plugins/missing`: Honor the ``-f``/``--format`` option (and the

test/plugins/test_edit.py

@@ -268,6 +268,25 @@ class EditCommandTest(IOMixin, EditMixin, BeetsTestCase):
             self.album.items(), self.items_orig, ["albumartist", "mtime"]
         )
 
+    def test_a_album_edit_preserves_missing_artpath(self, mock_write):
+        """Album query (-a), edit album field, preserve missing artpath."""
+        self.config["edit"]["albumfields"] = "album artpath"
+
+        self.run_mocked_command(
+            {"replacements": {"\u00e4lbum": "modified \u00e4lbum"}},
+            # Apply changes.
+            ["a"],
+            args=["-a"],
+        )
+
+        self.album.load()
+        assert mock_write.call_count == self.TRACK_COUNT
+        assert self.album.album == "modified \u00e4lbum"
+        assert self.album.artpath is None
+        self.assertItemFieldsModified(
+            self.album.items(), self.items_orig, ["album", "mtime"]
+        )
+
     def test_malformed_yaml(self, mock_write):
         """Edit the yaml file incorrectly (resulting in a malformed yaml
         document)."""