该方法用于创建并返回一个新的字典．两个参数，第一个为字典的键，第二个为字典的值(默认为None).

示例：
1.

>>> dict = dict.fromkeys([1,2,3])
>>> dict
{1: None, 2: None, 3: None}

>>> dict = dict.fromkeys([1,2,3],'test')
>>> dict
{1: 'test', 2: 'test', 3: 'test'}

这里需要注意！！！(没你想的那么好)

>>> dict = dict.fromkeys([1,2,3],['one','two','three'])
>>> dict
{1: ['one', 'two', 'three'], 2: ['one', 'two', 'three'], 3: ['one', 'two', 'three']}