Python 3列表行为

Question

我在练习蟒蛇，发现下面的行为，但我不明白为什么。谁能解释一下原因吗？

案例1

a = [[] for i in range(5)]
b = [[]] * 5
a[3].append(3)
b[3].append(3)
print(a) # Output: [[], [], [], [3], []]
print(b) # Output: [[3], [3], [3], [3], [3]]

我在a和b上看到了不同的行为，a和b之间有什么区别？

案例2

def test(sentences):
    root = {}
    for sentence in sentences:
        base = root
        for word in sentence.split(' '):
            if not base.get(word):
                base[word] = {}
            base = base[word]
    return root

print(test(["Hello world", "Hello there"]))
# Output: {'Hello': {'world': {}, 'there': {}}}

也许是个问题，但是在第二种情况下，当你没有修改根的时候，它是如何被修改的呢？

Answer 1

列表上的*操作将创建一个列表的副本。这是证据，

In [1]: a = [[] for i in range(5)]
   ...: b = [[]] * 5

In [2]: [id(i) for i in a]
Out[2]: [4649788160, 4640976128, 4647308224, 4647305856, 4643089472]

In [3]: [id(i) for i in b]
Out[3]: [4648966336, 4648966336, 4648966336, 4648966336, 4648966336]

对于case2也是如此。