def trim(s):

 #处理极端值：空字符串与空格字符串

 bool_s = bool(s)

 if bool_s:

  for x in s:

   bool_s = bool_s and x == ' '

  if bool_s:

   return ''

 else:

  return ''

 #获取非空字符串索引，通过索引切片达到去除两端空格目的

 count = 0

 count_end = -1

 #获取第一个非空字符索引值

 while s[count] == ' ':

  count += 1

 #获取最后一个非空字符索引值

 while s[count_end] == ' ':

  count_end -= 1

 #通过切片返回新字符串

 if count_end == -1:

  return s[count:]

 else:

  return s[count:count_end+1]