add negative indexing support

mmcv/video/io.py

@@ -241,6 +241,11 @@ class VideoReader(object):
     def __getitem__(self, index):
         if isinstance(index, slice):
             return [self.get_frame(i) for i in range(*index.indices(self.frame_cnt))]
+        # support negative indexing
+        if index < 0:
+            index += self.frame_cnt
+            if index < 0:
+                raise IndexError('index out of range')
         return self.get_frame(index)
 
     def __iter__(self):

tests/test_video.py

@@ -62,12 +62,18 @@ class TestVideo(object):
         assert int(round(img.mean())) == 94
         img = v[64]
         assert int(round(img.mean())) == 205
+        img = v[-104]
+        assert int(round(img.mean())) == 205
         img = v[63]
         assert int(round(img.mean())) == 94
+        img = v[-105]
+        assert int(round(img.mean())) == 94
         img = v.read()
         assert int(round(img.mean())) == 205
         with pytest.raises(IndexError):
             v.get_frame(self.num_frames + 1)
+        with pytest.raises(IndexError):
+            v[-self.num_frames - 1]
 
     def test_slice(self):
         v = mmcv.VideoReader(self.video_path)