对于含多字节的字符串，进行截断的时候，要判断截断处是几字节字符，不能将多字节从中分割，避免截断后乱码
下面给出utf8和gb18030上的实现， 用任何一种都可以，可以先进行转码，用encode, decode;
方法1：对utf8: 参考：http://blog.csdn.net/marising/article/details/3452971
def subString(string,length):
if length >= len(string):
return string
result = ''
i = 0
p = 0
while True:
ch = ord(string[i])
#1111110x
if ch >= 252:
p = p + 6
#111110xx
elif ch >= 248:
p = p + 5
#11110xxx
elif ch >= 240:
p = p + 4
#1110xxxx
elif ch >= 224:
p = p + 3
#110xxxxx
elif ch >= 192:
p = p + 2
else:
p = p + 1
if p >= length:
break;
else:
i = p
return string[0:i]
方法2：对gb18030编码
def cut_string_off(string,s_len):
if len(string)==0 or s_len <=0:
return string
elif len(string)==1 or s_len >= len(string):
return string
elif s_len < len(string):
len_num=0
while len_num < s_len:
tmp_c=ord(string[len_num])
if tmp_c >0 and tmp_c <=0x7F:
len_num+=1
continue
tmp_nextc=ord(string[len_num+1])
if tmp_c >= 0x81 and tmp_c <=0xFE and tmp_nextc>=0x40 and tmp_nextc<=0xFE:
len_num+=2
continue
else:
len_num +=1;
continue
break
tmp = string[0:len_num]
# print utf2gbk(tmp)
return tmp