Add prav_track and tests

mopidy/backends/__init__.py

@@ -127,3 +127,17 @@ class BasePlaybackController(object):
                 self.playlist_position + 1]
         except IndexError:
             return None
+
+    @property
+    def previous_track(self):
+        if self.current_track is None:
+            return None
+
+        if self.playlist_position - 1 < 0:
+            return None
+
+        try:
+            return self.backend.current_playlist.playlist.tracks[
+                self.playlist_position - 1]
+        except IndexError:
+            return None

tests/backends/basetests.py

@@ -261,3 +261,19 @@ class BasePlaybackControllerTest(object):
         tracks = self.backend.current_playlist.playlist.tracks
         self.playback.play()
         self.assertEqual(self.playback.next_track, tracks[1])
+
+    @populate_playlist
+    def test_previous_track_before_play(self):
+        self.assertEqual(self.playback.previous_track, None)
+
+    @populate_playlist
+    def test_previous_track_after_play(self):
+        self.playback.play()
+        self.assertEqual(self.playback.previous_track, None)
+
+    @populate_playlist
+    def test_previous_track_after_next(self):
+        tracks = self.backend.current_playlist.playlist.tracks
+        self.playback.play()
+        self.playback.next()
+        self.assertEqual(self.playback.previous_track, tracks[0])