python 内建方法fromkeys()创建一个具有相同元素值的字典
>>> ddict = {}.fromkeys(('x','y'),-1)
>>> ddict
{'y': -1, 'x': -1}
>>> edict = {}.fromkeys(('foo','bar'))
>>> edict
{'foo': None, 'bar': None}
>>> 
>>> ddict = {}.fromkeys(('x','y'),-1)
>>> ddict
{'y': -1, 'x': -1}
>>> edict = {}.fromkeys(('foo','bar'))
>>> edict
{'foo': None, 'bar': None}
>>>