How to keep a dictionary in order in python (code)

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1. Requirements

We want to create a dictionary, and at the same time, when iterating or serializing the dictionary, we can also control the order of the elements in it.

2. Solution

To control the order of elements in the dictionary, you can use the OrderedDict class in the collections module. When iterating over a dictionary, it does so strictly in the order in which the elements were originally added.

from collections import OrderedDict
d=OrderedDict()
d['a']=1
d['b']=2
d['c']=3
d['d']=4
#根据插入删除输出
for key in d:
print(key,d[key])

Result:

a 1
b 2
c 3
d 4

OrderedDict is particularly useful when you want to build a mapping structure that can later be serialized or encoded into another format. For example: If you want to accurately control the order of each field when encoding JSON, then you only need to construct the data in OrderedDict first:

from collections import OrderedDict
import json

d=OrderedDict()
d['a']=1
d['b']=2
d['c']=3
d['d']=4
j=json.dumps(d)
print(j)

Result:

{"a": 1, "b": 2, "c": 3, "d": 4}

OrderedDict internally maintains a doubly linked list, which will Arrange the positions of keys according to the order in which elements are added. The first newly added element is placed at the end of the linked list, and then the existing keys are reassigned without changing the position of the keys.

Please note: OrderedDict is 2 times the size of an ordinary dictionary. This is due to the additional linked list it creates. Therefore, if you plan to build a data structure that involves a large number of OrderedDict instances (for example, reading 100,000 rows from a CSV file into an OrderedDict list), you will need to perform a requirements analysis of the application to determine the benefits of using OrderedDict. Can the disadvantages of additional memory overhead be overcome?

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