Interview Question: What is the difference between List and Dictionary in Python? 面经：Python 的 List 和 Dictionary 有啥区别？

justyy (82)in #cn • 7 years ago (edited)

This is a real interview question: What is the difference between List and Dictionary in Python?
问题： Python 的 List 和 Dictionary 有啥区别？

You need to answer that immediately without googling..
不许查资料，你怎么回答这个面试题？

My answer: a list is like an array whilst a dictionary stores key-value pairs.
我不加思索的回答到： List 就像数组一样而 Dictionary 是键值对的一数据结构。

The interviewer asked "is the dictionary ordered"?
面试官继续说，那么 Dictionary 是有序的么？

What? I think he meant, can we trust the sequence when iterating the dictionary?
啥？啥是有序？

With some doubt, I still said: it is unordered.. because the dictionary in Python is like hash table and a hash table is clearly unordered.
我还是犹豫了一下，说是无序的，面试官说，为什么？
我说，因为 Python 里的 Dictionary 就如哈希表一样，而哈希表是没有序的。

He seems satisfied.
面试官似乎很满意。

After I come back, I verified a little bit:
回来之后，我验证了一下。

Create a list (or array)
Python3 创建数组

a=[1,2,3,4,5]

Remove item 3
删掉3

a.remove(3)
print(a) # [1, 2, 4, 5]

Put 3 back to the list
把3加回去

a.append(3)
print(a) # [1, 2, 4, 5, 3]

So, yes, the list is 'ordered' because the new item always is appended to the end of the list. Let's take a look at the dictionary.
数组来说，是有序的，因为每次添加总是在数组的末尾，我们再来看一下字典Dictionary

a={1:1,2:2,3:3,4:4,5:5}

Remove key=3
删除3

a.pop(3) # {1: 1, 2: 2, 4: 4, 5: 5}

Put key=3 back to the dictionary
再把3加回到字典中

a[3] = 3

and we've got the original dictionary.
得到了：

print(a) # a={1:1,2:2,3:3,4:4,5:5}

However, it is inconsistent on my friend's PC, which shows {1:1,2:2,4:4,5:5,3:3}
但是在我朋友的机器上，显示却是 {1:1,2:2,4:4,5:5,3:3}

So, the order cannot be trusted when you iterate the dictionary. The following code might break!
意思是，字典里的顺序是不能被信任的，也就是说如果你

for key in a:
    do_something_with_order_matters(a[key])

做了一些顺序有区别的事，那么就很可能会出问题！

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