通过dict.viewkkeys()，dict.viewvalues（）以及dict.viewitems（）的对象称之为视图对象。它们提供了字典对象的一个动态视图，也就是说当字典中内容改变的时候，视图也跟着改变。
>>> dishes ={‘eggs‘:2,‘sausage‘:1,‘bacon‘:1,‘spam‘:500}
>>> keys = dishes.viewkeys()
>>> values = dishes.viewvalues()
>>># iteration
>>> n =0
>>>for val in values:
... n += val
>>>print(n)
504
>>># keys and values are iterated over in the same order
>>> list(keys)
[‘eggs‘,‘bacon‘,‘sausage‘,‘spam‘]
>>> list(values)
[2,1,1,500]
>>># view objects are dynamic and reflect dict changes
>>>del dishes[‘eggs‘]
>>>del dishes[‘sausage‘]
>>> list(keys)
[‘spam‘,‘bacon‘]
>>># set operations
>>> keys &{‘eggs‘,‘bacon‘,‘salad‘}
{‘bacon‘}
原文：http://www.cnblogs.com/fireflow/p/4868859.html