對象創建

Question

這僅僅是「學習的樂趣」。我完全從書本和教程中自學，而且對編程還很陌生。我試圖探索一個從列表創建對象的概念。以下是我有：

class Obj: # Creates my objects 
    def __init__(self, x): 
     self.name = x 
     print('You have created a new object:', self.name) 

objList = [] 
choice = 'y' 

while choice != 'n': # Loop that runs until user chooses, 'n' to quit 
    for i in objList: 
     print(i) # Iterates through the list showing all of the objects added 
    for i in objList: 
     if Obj(i): 
      print(i, 'has already been created.') # Checks for existance of object, if so skips creation 
     else: 
      createObj = Obj(i) # Creates object if it doesn't exist 
    choice = input('Add object? (y/n): ') 
    if choice == 'y': 
     newObject = input('Name of object to add: ') 
     if newObject in objList: # Checks for existance of user enrty in list 
      print(newObject, 'already exists.') # Skips .append if item already in list 
     else: 
      objList.append(newObject) # Adds entry if not already in list 

print('Goodbye!') 

當我跑，我得到：

Add object? (y/n): y 
Name of object to add: apple 
apple 
You have created a new object: apple # At this point, everything is correct 
apple has already been created. # Why is it giving me both conditions for my "if" statement? 
Add object? (y/n): y 
Name of object to add: pear 
apple 
pear 
You have created a new object: apple # Was not intending to re-create this object 
apple has already been created. 
You have created a new object: pear # Only this one should be created at this point 
pear has already been created. # Huh??? 
Add object? (y/n): n 
Goodbye! 

我已經做了一些研究和閱讀有關創建一個字典做它似乎我一些意見米試圖做。我已經建立了一個使用字典來實現這個功能的程序，但是爲了學習目的，我試圖瞭解這是否可以通過創建對象來完成。它看起來好像一切正​​常，除了程序通過遍歷列表來檢查對象的存在時，它就會失敗。

我那麼做：

>>> Obj('dog') 
You have created a new object: dog 
<__main__.Obj object at 0x02F54B50> 
>>> if Obj('dog'): 
    print('exists') 

You have created a new object: dog 
exists 

這導致我的理論。當我放入「if」語句時，它是否創建了一個名爲「dog」的對象的新實例？如果是這樣，我該如何檢查物體的存在？如果我將對象存儲在一個變量中，那麼每次迭代時我的頂級片段中的循環是否會覆蓋該變量？我的「打印」語句是因爲對象存在還是因爲它的下一行代碼而運行？對不起，我的問題的長度，但我相信如果我提供更好的信息，我可以得到更好的答案。

Answer 1

對象只是數據和函數的容器。儘管Obj("dog")和Obj("dog")是相同的，但它們並不相同。換句話說，每次你撥打__init__你都會得到一個全新的副本。所有不爲None，0或False的對象評估爲True，因此您的if聲明成功。

您仍然必須使用字典來查看您是否曾經創建過一隻狗。例如，

objects = { "dog" : Obj("dog"), "cat" : Obj("cat") } 
if "cat" in objects: 
    print objects["cat"].x # prints cat