切片原型 strs = ‘abcdefg’ Strs[start: end:step] 切片的三个参数分别表开始,结束,步长 第一位下标为0,end位不取,如strs[1:3] = ‘bc’ 如果start,end超出现有数组范围,按实际范围截断strs[-100:100]=’abcdefg’ Step为空时,缺省值为1 Strs[1:5] = ‘bcde’ strs[1:5:2] = ‘bd’ Step为正时,start<end,否则为空 Strs[5:1] = ‘’ Start为空,默认为负无穷 strs[:4] = ‘abcd’ End为空,默认为正无穷 strs[2:] = ‘c