python一切皆对象。
列表的元素可以修改，元组的不能修改。
# python2.7
name = 'The world is like a mirror: when you frown at it, it frowns at you; when smile, it smiles too. '
print name[0:2]
print name[10:]
print name[1:2:2]
切片重新创建新对象
a = raw_input('please input sth:')
print(a)
