def trim(s): if s == '': return s i = 0

#切除头部的空格
while s[i] == ' ':
    if len(s) < 2:   #只剩下一个空格元素
        s = ''
        return s
    else:
        s = s[i + 1:]
j = 1
#切除尾部的空格
while s[-j] == ' ':
    if len(s) < 2:
        s = ''
        return s
    else:
        s = s[:-j] 
return s