Python清单错误或我错了吗? [英] Python list bug or am I wrong?

问题描述

我已经构建了一个3级嵌套列表

I have constructed a 3 level nested list

#run on Python 3.2.3  32 bit on Win 7
L2=list(0 for i in range(2))
L3=list(L2 for i in range(3))
L4=list(L3 for i in range(4))
#give a new value to the very first number in list:
L4[0][0][0]=5
print("L4:")
print(L4)
#outputs erronously:
#[[[5, 0], [5, 0], [5, 0]], [[5, 0], [5, 0], [5, 0]], [[5, 0], [5, 0], [5, 0]], [[5, 0], [5, 0], [5, 0]]]

明确给出的相同列表

#the same L4 given explicitly:
anotherL4=[[[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]]]
print("anotherL4:")
#give a new value to the very first number:
anotherL4[0][0][0]=5
print(anotherL4)
#outputs correctly:
#[[[5, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]]]

推荐答案

当您编写 list(range(4)中的i为L3)时，您告诉它在上产生相同的列表L3生成器理解的每次迭代.当您随后修改该列表时，由于所有修改都是对同一列表的引用，因此修改会随处显示.

When you write list(L3 for i in range(4)), you are telling it to yield the same list L3 on each iteration of the generator comprehension. When you subsequently modify that list, the modifications show up everywhere, because all of them are references to the same list.

这样做可以达到您想要的效果

You could get the effect you seem to want by doing

list(list(L3) for i in range(4))

因为使用 list(L3)会创建一个新列表.

since using list(L3) makes a new list.

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