python None对象

Question

您能解释一下我用这行代码在python中得到什么吗？该代码创建什么？

 NP = 2
 NB = 2  
 ND = 2 

 N  = NP*NB*ND 
 M  = 2*N + NP*NB
 res =  [[None] * 3] * (M)

谢谢！

Answer 1

如果您实际运行代码，您将得到

>>> res
[[None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None], [None, None, None]]

但是，值得一提的是，顶层列表中的列表实际上是一个重复多次的单个列表，因此，对一个变量进行变异会使其全部变异：

>>> res[0][1] = 'x'
>>> res
[[None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None], [None, 'x', None]]