请教一个python语法的问题？
def ifilter(before, width, height): after = [] flt_type = [0] * height flt_list = [ifilter0, ifilter1, ifilter2, ifilter3, ifilter4] for k in range(height): after.append([ord(b) for b in before[(k*(width+1)+1):((k+1)*(width+1))]]) flt_type[k] = ord(before[k * (width+1)]) after[0] = flt_list[flt_type[0]](after[0], [0] * width) for k in range(1, height): after[k] = flt_list[flt_type[k]](after[k], after[k-1]) return after
