if len(s) == 0:
    return ''
while(s[0] == " "):
    if len(s) == 1:
        return ''
    else:
        s = s[1:]
while(s[-1] == " "):
        s = s[:-1]
return s



while(s[:1]== ' '):
    s=s[1:]
while(s[-1:]==' '):
    s=s[:-1]
return s

这两个是等效的,原因就在于切片操作不会超出索引范围,