字符串按位数分割

# 以分割银行卡账号为例，每四位以空格分割
def splitstring(numberstring, num=4, sign=' '):
    # 经常传入的数据转化为字符串
    numstring = str(numberstring)
    # 获取传入的截取长度个符号，开始的下标与结束下标
    num = num
    sign = sign
    start_index = 0
    end_index = num
    str_list =[]
    # 需要遍历的次数
    len_str=int(len(numstring))
    for i in range(int(len_str/num+1)):
        str_list.append(numstring[start_index:end_index])
        str_list.append(sign)
        start_index = end_index
        end_index += num
    outstr = "".join(str_list)
    len_outstr = len(outstr)
    if len_str % num == 0:
        last_outstr = outstr[0:len_outstr-2]
    else:
        last_outstr = outstr[0:len_outstr-1]
    return last_outstr