当你创建一个对象并给它赋一个变量的时候，这个变量仅仅 引用 那个对象，而不是表示这个
对象本身！也就是说，变量名指向你计算机中存储那个对象的内存。这被称作名称到对象的绑
定。
#!/usr/bin/python
# Filename: reference.py
print 'Simple Assignment'
shoplist = ['apple', 'mango', 'carrot', 'banana']
mylist = shoplist # mylist is just another name pointing to the same object!
del shoplist[0]
print 'shoplist is', shoplist
print 'mylist is', mylist
# notice that both shoplist and mylist both print the same list without
# the 'apple' confirming that they point to the same object
print 'Copy by making a full slice'
mylist = shoplist[:] # make a copy by doing a full slice
del mylist[0] # remove first item
print 'shoplist is', shoplist
print 'mylist is', mylist
# notice that now the two lists are different
$ python reference.py
Simple Assignment
shoplist is ['mango', 'carrot', 'banana']
mylist is ['mango', 'carrot', 'banana']
Copy by making a full slice
shoplist is ['mango', 'carrot', 'banana']
mylist is ['carrot', 'banana']
你需要记住的只是如果你想要复制一个列表或者类似的序
列或者其他复杂的对象（不是如整数那样的简单 对象 ），那么你必须使用切片操作符来取得
拷贝。如果你只是想要使用另一个变量名，两个名称都 引用 同一个对象，那么如果你不小心
的话，可能会引来各种麻烦。