@requires_duration
@apply_to_mask
def time_symmetrize(clip):
 return concatenate_videoclips([clip, clip.fx(time_mirror)])

@ requires_duration @ apply_to_mask def time_symmetrize ( clip ) : return concatenate_videoclips ( [ clip , clip . fx ( time_mirror ) ] )
returns a clip that plays the current clip once forwards and then once backwards .