如何在本式字典实现中使用setDefault()方法

Question

有人可以解释词典的分配"逆"在以下情况下发生吗？

def invert_dict(d):
  inverse = {}
  for key in d:
    new_key = d[key]
    inverse.setdefault(new_key, []).append(key)
  return inverse
letters_in_word = {"mine": 4, "yours": 5, "ours": 4, "sunday": 6, "friend": 6, "fun": 3, "happy": 5, "beautiful": 8}
print (invert_dict(letters_in_word))

当然，输出是正确的：

{8：['beautiful']，3：['fun']，4：['mine'，''']，5：['happhy''，'yess']，6：['sunday'，"朋友"]}

python 3.x文档说：

setDefault（key [，默认]）：

如果键在字典中，请返回其值。如果不是，请插入具有默认值和返回默认值的值。默认默认为无。

让我用一个例子来说明我做什么，而不理解：

1. 假设new_key =＆quot; happy＆quot;
2. new_key的值为5
3. setDefault（）被调用，让我们假设5已在"您的字典"中。（据我所知，由于词典是无序的" inverse.setDefault（5，[]）＆quot'已返回[yours;
4. append（）被调用，['yours'] - ＆gt;[''

我知道，到4结束时，我错了，因为实际上，我们的列表已分配给密钥" 5"。我不明白的是发生的一点 - 似乎我们刚刚返回，真的应该分配：

inverse[new_key] = inverse.setdefault(new_key, []).append(key)

但是，如果我运行了代码，我会收到错误-'NoneType' object has no attribute 'append'。

任何解释都受到赞赏 - 我想我一定会缺少两种方法的交互方式。

P.S。这是我的第一个问题，因此，如果问题/结构不是"这里的事情是如何完成"的问题，那么这是我的第一个问题。让我知道如何改进，我会尽力做到！

Answer 1

打印语句是了解程序中发生的事情的一种非常有用且简便的方法：

def invert_dict(d):
    inverse = {}
    for key in d:
        new_key = d[key]
        print('key:', key)
        print('new_key:', new_key)
        print('inverse before:', inverse)
        value = inverse.setdefault(new_key, [])
        print('inverse in the middle:', inverse)
        print('value before:', value)
        value.append(key)
        print('value after:', value)
        print('inverse after:', inverse)
    return inverse
letters_in_word = {"mine": 4, "yours": 5, "ours": 4, "sunday": 6, "friend": 6, "fun": 3, "happy": 5, "beautiful": 8}
print(invert_dict(letters_in_word))

输出：

key: beautiful
new_key: 8
inverse before: {}
inverse in the middle: {8: []}
value before: []
value after: ['beautiful']
inverse after: {8: ['beautiful']}
key: yours
new_key: 5
inverse before: {8: ['beautiful']}
inverse in the middle: {8: ['beautiful'], 5: []}
value before: []
value after: ['yours']
inverse after: {8: ['beautiful'], 5: ['yours']}
key: ours
new_key: 4
inverse before: {8: ['beautiful'], 5: ['yours']}
inverse in the middle: {8: ['beautiful'], 4: [], 5: ['yours']}
value before: []
value after: ['ours']
inverse after: {8: ['beautiful'], 4: ['ours'], 5: ['yours']}
key: sunday
new_key: 6
inverse before: {8: ['beautiful'], 4: ['ours'], 5: ['yours']}
inverse in the middle: {8: ['beautiful'], 4: ['ours'], 5: ['yours'], 6: []}
value before: []
value after: ['sunday']
inverse after: {8: ['beautiful'], 4: ['ours'], 5: ['yours'], 6: ['sunday']}
key: happy
new_key: 5
inverse before: {8: ['beautiful'], 4: ['ours'], 5: ['yours'], 6: ['sunday']}
inverse in the middle: {8: ['beautiful'], 4: ['ours'], 5: ['yours'], 6: ['sunday']}
value before: ['yours']
value after: ['yours', 'happy']
inverse after: {8: ['beautiful'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
key: fun
new_key: 3
inverse before: {8: ['beautiful'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
inverse in the middle: {8: ['beautiful'], 3: [], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
value before: []
value after: ['fun']
inverse after: {8: ['beautiful'], 3: ['fun'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
key: mine
new_key: 4
inverse before: {8: ['beautiful'], 3: ['fun'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
inverse in the middle: {8: ['beautiful'], 3: ['fun'], 4: ['ours'], 5: ['yours', 'happy'], 6: ['sunday']}
value before: ['ours']
value after: ['ours', 'mine']
inverse after: {8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday']}
key: friend
new_key: 6
inverse before: {8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday']}
inverse in the middle: {8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday']}
value before: ['sunday']
value after: ['sunday', 'friend']
inverse after: {8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday', 'friend']}
{8: ['beautiful'], 3: ['fun'], 4: ['ours', 'mine'], 5: ['yours', 'happy'], 6: ['sunday', 'friend']}

也非常有用的是一个很好的调试器，例如Pycharm中的调试器。尝试一下。