python之切片操作，实现一个trim()函数，去除字符串首尾的空格.

# -*- coding: utf-8 -*-
def trim(s):
    if len(s)==0:
        return ''
    if s[:1]==' ':
        return trim(s[1:])
    elif s[-1:]=='':
        return trim(s[:-1])
    else:
        return s

# 测试:
if trim('hello  ') != 'hello':
    print('测试失败!')
elif trim('  hello') != 'hello':
    print('测试失败!')
elif trim('  hello  ') != 'hello':
    print('测试失败!')
elif trim('  hello  world  ') != 'hello  world':
    print('测试失败!')
elif trim('') != '':
    print('测试失败!')
elif trim('    ') != '':
    print('测试失败!')
else:
    print('测试成功!')