在python中自己写的数据类型使用print无法输出每个元素

Question

估计是我问题没有描述清楚，这样吧，直接上代码：

class Mylist:
    def __init__(self):
        self._mylist=list()

    def __len__(self):
        return len(self._mylist)

    def add(self,value):
        return self._mylist.append(value)

我自己模仿list的行为。写了一个基本的list，名字叫Mylist,并给他一个add方法用来添加其中的元素，添加完之后，我想输出其中的内容，然后我使用：

list1=Mylist()
list1.add(1)
list1.add(2)
print(list1)

我以为print会显示出list1中的每一项，但是发现实际没有，显示的为：
<__main__.Mylist object at 0x0071A470>
怎么样能让print(list1)显示出的结果和真实list类型一样呢？例如：
[1,2]
__str__具体怎么实现，貌似这个只能为str类型,int不行，而且我发现在pycharm 里面写的时候，提示__str__方法“overrides method in object”

Answer 1

You are using the "aggregation" method to create your own cluster data. At this time, delegation is a simple method:

class Mylist:
    def __init__(self):
        self._mylist=list()

    def __len__(self):
        return len(self._mylist)

    def add(self,value):
        return self._mylist.append(value)

    def __str__(self):
        return str(self._mylist)

__str__ is a special method in the Python class. Its return value is the value obtained by using str(x), and print(x ) is actually equivalent to print(str(x)). In fact, to be more detailed, when we call str(x), we are actually calling x.__str__(). __str__ 是 Python 類中的特殊方法，他的回傳值就是使用 str(x) 所得到的值， 而 print(x) 其實就等於是 print(str(x))．其實再講細一點，當我們呼叫 str(x) 的時候其實是呼叫 x.__str__()．

也就是說我們可以這樣想像:

print(x) === print(str(x)) === print(x.__str__())

一般我們 自定義的類，__str__ 方法的回傳值是默認的字串，比如說: <__main__.Mylist object at 0x0071A470> 用以說明 namespace, class name 和位置．如果要改變 __str__ 的回傳值，我們必須要覆寫他．

這邊我們讓 __str__ 的回傳值為 MyList 類中聚合的 list 的 __str__值，這樣就透過委託的方式讓 __str__ 的輸出跟 list 一樣了．

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多嘴補充一下，這種在 class 裡面 以雙底線開頭且以雙底線結尾 的 method (俗名叫做魔術方法或是特殊方法)，有個正式名稱叫做 "dunder method"，對於 __str__，我們可以唸作 "dunder string"．

The frequent use of a double underscores in internal identifiers in Python gave rise to the abbreviation dunder; this was coined by Mark Jacksonand independently by Tim Hochberg, within minutes of each other, both in reply to the same question in 2002. --wiki

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下方評論問的問題我回答在這裡．

首先是不要被混淆，我們利用 print 印出來的內容都是 字串，即便你看到 [1, 2] 其實這也是一個字串 '[1, 2]'，只不過內建的幾種資料型態(或我們有覆寫過 __str__ 的 class) 會想辦法輸出一個帶有該資料型態特徵的字串(通常會非常接近我們產生這些資料時所用的"字面")．

舉例，我們使用字面產生一個 list:

lst = [1, 2]

當我們打印 lst 時，Python 是會製造一個長得像該資料型態字面(甚至一模一樣)的字串讓你印出．

print(lst)

[1, 2] # 其實這是個字串，但是 lst 還是 list!

所以在這裡 str(list) 並沒有改變 list 中元素的 type，只不過將帶有其特徵的 "字串" 當成回傳值．

其次，如果想要在 Python shell (Python的互動介面)中能夠只利用變數名稱就展示用來表示 Mylist 的字串，光是 __str__ 還不夠，這必須要覆寫 __repr__

That is to say, we can imagine it like this:

print(x) === print(str(x)) === print(x.__str__())

Generally for our #🎜🎜#customized classes #🎜🎜#, the return value of the __str__ method is the default string, for example: <__main__.Mylist object at 0x0071A470> is used to describe namespace, class name and location. If we want to change the return value of __str__, we must overwrite it. #🎜🎜# #🎜🎜#Here we let the return value of __str__ be the __str__ value of the aggregated list in the MyList class, so that through delegation The method makes the output of __str__ the same as that of list. #🎜🎜#

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#🎜🎜#I would like to add that this kind of method (commonly known as magic method or special method) in the class that #🎜🎜# starts with a double underline and ends with a double underline #🎜🎜# has an official name called #🎜 🎜#"dunder method"#🎜🎜#, for __str__, we can pronounce it as "dunder string". #🎜🎜#

#🎜🎜#The frequent use of a double underscores in internal identifiers in Python gave rise to the abbreviation dunder; this was coined by Mark Jacksonand independently by Tim Hochberg, within minutes of each other, both in reply to the same question in 2002. --wiki#🎜🎜#

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#🎜🎜#I will answer the questions asked in the comments below. #🎜🎜# #🎜🎜#First of all, don’t be confused. The content we print using print is all #🎜🎜# string #🎜🎜#, even if you see [1, 2] In fact, this is also a string '[1, 2]', but it has several built-in data types (or we have overridden __str__ class) will find a way to output a string with the characteristics of the data type (usually very close to the "literal" we used to generate this data). #🎜🎜# #🎜🎜#For example, we use literals to generate a list: #🎜🎜#

class Mylist:

    def __init__(self):
        self._mylist=list()

    def __len__(self):
        return len(self._mylist)

    def add(self,value):
        return self._mylist.append(value)

    def __str__(self):
        return str(self._mylist)
        
    def __repr__(self):
        return str(self)

#🎜🎜#When we print lst, Python will create a string that looks like the data type literal (or even exactly the same) for you to print. #🎜🎜#

\>>> from test import Mylist
\>>> lst = Mylist()
\>>> lst.add(1)
\>>> lst.add(2)
\>>> lst
[1, 2]

#🎜🎜#So here str(list) does not change the type of the elements in the list, but only treats the "string" with its characteristics as the return value. #🎜🎜# #🎜🎜#Secondly, if you want to display the string used to represent Mylist in the Python shell (Python's interactive interface) using only the variable name, just __str__ Not enough, this must be overridden __repr__:#🎜🎜# rrreee #🎜🎜##🎜🎜#result#🎜🎜#:#🎜🎜# rrreee

Answer 2

__str__

Answer 3

Implement the __str__ function in the class and write the content you want to output
For example

class Mylist(object):
    def __str__(self):
        return "This is my list"
 
mylist = Mylist()
print mylist

Answer 4

The print function will call the __str__ method of the object. If you have not reimplemented the __str__ method yourself, the __str__ method inherited from the parent class will be used by default, so the output is not necessarily in the form of a list.

class MyList(list):
    pass
    
print MyList()    # []