curds: Add a way to cancel notifications

I expect this will mostly get used by testing code as we create and
delete a bunch of different objects.

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

curds/playlist/playlist.py

@@ -11,6 +11,11 @@ class Playlist(list):
             self.append(track)
             self.notify("on-add", track)
 
+    def never_mind(name, func):
+        cb = Playlist.notifications.get(name, [])
+        if func in cb:
+            cb.remove(func)
+
     def notify_me(name, func):
         Playlist.notifications.setdefault(name, []).append(func)
 

curds/playlist/test_playlist.py

@@ -80,3 +80,13 @@ class TestPlaylist(unittest.TestCase):
         plist.add(None)
         self.assertEqual(self.cb_plist, plist)
         self.assertEqual(self.cb_track, track)
+
+        self.cb_plist = None
+        self.cb_track = None
+        track = tags.Track.add(os.path.join(path, "02 - Test Track 02.ogg"))
+        playlist.Playlist.never_mind("on-add", self.on_add1)
+        self.assertEqual(len(playlist.Playlist.notifications["on-add"]), 1)
+        playlist.Playlist.never_mind("on-add", self.on_add1)
+        plist.add(track)
+        self.assertIsNone(self.cb_plist)
+        self.assertEqual(self.cb_track, track)