Python:字典列表在每次迭代中仅存储最后附加的值 [英] Python: List of dictionary stores only last appended value in every iteration

问题描述

我有这个字典清单:

MylistOfdict = [{'Word': 'surveillance',
  'Word No': 1},
 {'Word': 'equivocal',
  'Word No': 2}]

我想创建一个新的字典列表(word_db2)，其中MylistOfdict中的每个字典都有3个字典.除了MylistOfdict的键和值之外，每个词典中还应具有值分别为Type 1，Type 2，Type 3和'Card Key'的'Card Type'键

I want to create a new list of dictionary (word_db2) that has 3 dictionaries for each dictionary in MylistOfdict. In addition to key and values of MylistOfdict, each of those dictionary should have 'Card Type' key with value Type 1, Type 2, Type 3 and 'Card Key' key with incremental value

代码:

word_db2 = []

key = 1
for i in MylistOfdict:
    for j in range(1, 4):
        i['Card Type'] = 'Type '+str(j)
        i['Card Key'] = key
        print(i)

        word_db2.append(i)
        key += 1

输出:

{'Word No': 1, 'Card Key': 1, 'Word': 'surveillance', 'Card Type': 'Type 1'}
{'Word No': 1, 'Card Key': 2, 'Word': 'surveillance', 'Card Type': 'Type 2'}
{'Word No': 1, 'Card Key': 3, 'Word': 'surveillance', 'Card Type': 'Type 3'}
{'Word No': 2, 'Card Key': 4, 'Word': 'equivocal', 'Card Type': 'Type 1'}
{'Word No': 2, 'Card Key': 5, 'Word': 'equivocal', 'Card Type': 'Type 2'}
{'Word No': 2, 'Card Key': 6, 'Word': 'equivocal', 'Card Type': 'Type 3'}

此输出正确，但word_db2在每次迭代中仅存储最后附加的值

This output is correct, but word_db2 stores only last appended value in every iteration

print(word_db2)

输出:

[{'Card Key': 3, 'Card Type': 'Type 3', 'Word': 'surveillance', 'Word No': 1},
 {'Card Key': 3, 'Card Type': 'Type 3', 'Word': 'surveillance', 'Word No': 1},
 {'Card Key': 3, 'Card Type': 'Type 3', 'Word': 'surveillance', 'Word No': 1},
 {'Card Key': 6, 'Card Type': 'Type 3', 'Word': 'equivocal', 'Word No': 2},
 {'Card Key': 6, 'Card Type': 'Type 3', 'Word': 'equivocal', 'Word No': 2},
 {'Card Key': 6, 'Card Type': 'Type 3', 'Word': 'equivocal', 'Word No': 2}]

推荐答案

让我们逐步回顾一下循环主体的逻辑:

Let's review the loop body logic step by step:

1. 接受其中一项命令
2. 修改
3. 将其添加到列表的末尾

因此，您错过的关键点是修改并附加在第一步中选择的相同对象.在代码片段的末尾，word_db2包含六个对象引用，但只有两个唯一.结果，输出显示相似的行.

So the key point you missed is that you modify and append the same object that was selected on the first step. And at the end of the snippet word_db2 contains six object refs, but only two unique. As a result, the output shows similar rows.

您可以先对字典进行浅拷贝，然后再对其进行修改和附加:

You can make a shallow copy of a dict before modifying and appending it:

for j in range(1, 4):
    i = dict(i)
    i['Card Type'] = 'Type '+str(j)
    i['Card Key'] = key
    print(i)

    word_db2.append(i)
    key += 1

请进一步注意，如果字典包含其他可变对象(例如嵌套字典)，则应制作一个深层副本:

As further note, if the dict contains other mutable objects like nested dicts, you should make a deep copy:

import copy
old_dict = {'a': [1, 2, 3], 'b': [4, 5, 6]}
new_dict = copy.deepcopy(old_dict)
old_dict['a'][1] = 7
new_dict['a'][1] # 2

这篇关于Python:字典列表在每次迭代中仅存储最后附加的值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！