setdefault（）方法在這個逆向字典實現中如何工作？

Question

有人可以解釋如何在下面的情況下發生字典「逆」的分配？

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)) 

輸出，當然，是正確的：

{8：[ '美麗']，3：[ '樂趣']，4：[ '我的'， '我們的']，5 ：[ '快樂'， '你']，6： '星期天'， '朋友']}

的Python 3.x的文件說：

setdefault（鍵[，默認值]）：

如果鍵在字典中，則返回其值。如果沒有，則插入具有默認值的鍵並返回默認值。默認默認爲None。

讓我用一個例子來說明我做什麼和不理解：

1. 假設new_key = 「快樂」
2. new_key有5
3. setdefault的值（）被調用讓我們假設5已經在「你的」字典中了（據我所知，由於字典無序，這不一定是這種情況，但我們假設）並將返回[「yours」] （我的猜測這裏發生的事情稍有不同，所以事實上並非如此「inverse.setdefault（5，[]）」已返回[「yours」]，就是這樣）
4. append（）被調用並且[「yours」] - > [「yours」，「happy」] - 這就是我們留下的。

我知道在4月底我錯了，因爲實際上我們的列表已經被分配給了鍵「5」。我不明白的是，發生在哪個點 - 好像我們剛剛回來，真的應該與分配：

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

但是，如果我運行代碼一樣，我得到的錯誤 - 'NoneType' object has no attribute 'append' 。

任何解釋讚賞 - 我想我必須錯過這兩種方法如何相互作用的東西。

P.S.這是我的第一個問題，如果問題的本質/結構不是'這裏的事情是怎麼做的'，那麼我很抱歉。讓我知道如何改進，我會盡我所能去做！

Answer 1

打印語句是理解什麼是程序中發生的一個非常有用而簡單的方法：

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)) 

輸出：

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']} 

也非常有用的是一個很好的調試器，如一個在PyCharm 。試試看。