allow templates/formatting when setting fields with modify

beets/ui/commands.py

@@ -1411,9 +1411,6 @@ def modify_items(lib, mods, dels, query, write, move, album, confirm):
     # Parse key=value specifications into a dictionary.
     model_cls = library.Album if album else library.Item
 
-    for key, value in mods.items():
-        mods[key] = model_cls._parse(key, value)
-
     # Get the items to modify.
     items, albums = _do_query(lib, query, album, False)
     objs = albums if album else items
@@ -1424,7 +1421,9 @@ def modify_items(lib, mods, dels, query, write, move, album, confirm):
            .format(len(objs), 'album' if album else 'item'))
     changed = []
     for obj in objs:
-        if print_and_modify(obj, mods, dels) and obj not in changed:
+        obj_mods = {key: model_cls._parse(key, format(obj, value))
+                    for key, value in mods.items()}
+        if print_and_modify(obj, obj_mods, dels) and obj not in changed:
             changed.append(obj)
 
     # Still something to do?

docs/reference/cli.rst

@@ -263,6 +263,9 @@ artist="Tom Tom Club"`` will change the artist for the track "Genius of Love."
 To remove fields (which is only possible for flexible attributes), follow a
 field name with an exclamation point: ``field!``.
 
+Values support the same template syntax as beets'
+:doc:`path formats <pathformat>`.
+
 The ``-a`` switch operates on albums instead of individual tracks. Without
 this flag, the command will only change *track-level* data, even if all the
 tracks belong to the same album. If you want to change an *album-level* field,

test/test_ui.py

@@ -287,6 +287,13 @@ class ModifyTest(unittest.TestCase, TestHelper):
         self.assertEqual(len(list(original_items)), 3)
         self.assertEqual(len(list(new_items)), 7)
 
+    def test_modify_formatted(self):
+        item = self.lib.items().get()
+        orig_title = item.title
+        self.modify("title=${title} - append")
+        item.load()
+        self.assertEqual(item.title, f"{orig_title} - append")
+
     # Album Tests
 
     def test_modify_album(self):
@@ -318,6 +325,13 @@ class ModifyTest(unittest.TestCase, TestHelper):
         item.read()
         self.assertNotIn(b'newAlbum', item.path)
 
+    def test_modify_album_formatted(self):
+        item = self.lib.items().get()
+        orig_album = item.album
+        self.modify("--album", "album=${album} - append")
+        item.load()
+        self.assertEqual(item.album, f"{orig_album} - append")
+
     # Misc
 
     def test_write_initial_key_tag(self):