import gst
import gobject

def xxx():
    pipeline.seek(-0.5, gst.FORMAT_TIME, gst.SEEK_FLAG_NONE,
                    gst.SEEK_TYPE_SET, 10 * gst.SECOND, gst.SEEK_TYPE_NONE, 0)

pipeline = gst.parse_launch('filesrc location=/path/to/file/video.mkv ! matroskademux ! decodebin ! autovideosink name=sink')
pipeline.set_state(gst.STATE_PLAYING)

gobject.timeout_add_seconds(6, xxx)

loop = gobject.MainLoop()
loop.run()