playlist: Allow creating models without a Tag

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

playlist/model.py

@@ -2,7 +2,7 @@
 from gi.repository import GObject, Gio
 
 class TagModel(GObject.GObject, Gio.ListModel):
-    def __init__(self, tag):
+    def __init__(self, tag=None):
         GObject.GObject.__init__(self)
         self.tag = tag
 
@@ -10,10 +10,13 @@ class TagModel(GObject.GObject, Gio.ListModel):
         return GObject.TYPE_PYOBJECT
 
     def do_get_n_items(self):
-        return len(self.tag)
+        if self.tag:
+            return len(self.tag)
+        return 0
 
     def do_get_item(self, n):
-        return self.tag[n]
+        if self.tag:
+            return self.tag[n]
 
     def do_get_object(self, n):
         return self.tag[n]

playlist/test_model.py

@@ -21,3 +21,9 @@ class TestTagModel(unittest.TestCase):
         t.add_track(fake.Track(1))
         self.assertEqual(m.get_n_items(), 1)
         self.assertEqual(m.get_item(0), fake.Track(1))
+
+    def test_tag_model_init_none(self):
+        m = model.TagModel()
+
+        self.assertEqual(m.get_n_items(), 0)
+        self.assertIsNone(m.get_item(0))