一入python深似海--Views in Python3.

在Python3中，Views（视图）的作用，类似于数据库中视图的一个作用，能够反映所对应对象的变化。就像数据库表变化后，视图查询的结果是变化后的值一样，python中通过Views查询的值也是这样的。

Python3中，dict.keys(),dict.items(),dict.values()返回的是Views，而不是list。可以通过list()函数将Views()转换为list。

下面是一个说明Views作用的例子：

>>> dishes = {"eggs": 2, "sausage": 1, "bacon": 1, "spam": 500}
>>> keys = dishes.keys()
>>> values = dishes.values()

>>> # 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"}