curds: Make Tracks indexable

This lets us convert string fields into the corresponding track tag. I
intend to use this so playlists can have a custom sort function.

I eventually intend to add the matching __setitem__() function for
changing tags.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

curds/tags.py

@@ -55,6 +55,12 @@ class Track(Tag):
         if match:
             self.discnumber = int(discno_map.get(match.group(), match.group()))
 
+    def __getitem__(self, key):
+        item = vars(self).get(key)
+        if not item or item == self.album:
+            item = vars(self.album).get(key)
+        return item
+
     def __hash__(self):
         return hash((hash(self.album), self.title, self.artist,
                           self.tracknumber, self.discnumber))

curds/test_tags_track.py

@@ -61,3 +61,18 @@ class TestTrackTag(unittest.TestCase):
             u = tags.Track.add(os.path.join(test_tracks, "01 - Test Track.ogg"))
             self.assertIsNone(u)
         self.assertEqual(len(tags.tag_map), 2)
+
+    def test_track_getitem(self):
+        p = [ test_tracks, "01 - Test Track.ogg" ]
+        t = tags.Track(os.path.join(*p))
+        self.assertEqual(t["path"],         os.path.join(*p))
+        self.assertEqual(t["title"],       "Test Track")
+        self.assertEqual(t["artist"],      "Test Artist")
+        self.assertEqual(t["tracknumber"],  1)
+        self.assertEqual(t["discnumber"],   1)
+        self.assertEqual(t["length"],       10)
+        self.assertEqual(t["album"],       "Test Album")
+        self.assertEqual(t["date"],         2019)
+        self.assertEqual(t["genre"],       "Test")
+        self.assertEqual(t["albumartist"], "Test Artist")
+        self.assertIsNone(t["nokey"])