python类实例变量和类变量

问题描述

im有一个问题，了解如何类/实例变量在python中工作。我不明白为什么当我尝试这个代码列表变量似乎是一个类变量

  class testClass（）：
 list = [] 
 def __init __（self）：
 self.list.append（'thing'）
 
p = testClass（）
 print p.list 
 
f = testClass（）
 print f.list 
  

  ['thing'] 
 ['thing'，'thing'] 
  
 
 
 
当我这样做它似乎是一个实例变量
  class testClass（）：
 def __init __（self）：
 self.list = [] 
 self.list.append（'thing'）
 
p = testClass（）
 print p.list 
 
f = testClass（）
 print f.list 
  
输出：
  ['thing'] 
 ['thing'] 
  
非常感谢
 
 
 
 jon 
解决方案
这是因为Python使用解析名称。。当你写 self.list  Python运行时试图解析 list 名称首先在实例对象中查找它， 
 
 code> self.list.append（1）
  
 
 
 
 
 
在对象 self 中有列表名称？ 
 
 
 
是：使用它！ 
 
   
 
有 list  name添加到对象 self 的类实例中？ 
 
 
 
是：使用它！完成
 
  
  
  >但是当你绑定一个名字的东西是不同的：
  self.list = [] 
  
 
 
 
 
 
 列表 c $ c> self ？ 
 
 
 
是：覆盖它！
 
 
否：绑定！
 
 
 
 
 
所以，这总是一个实例变量。
 
 
 
你的第一个例子创建一个 list 到类实例中，因为这是当前的活动范围（任何 self ）。但是你的第二个例子在 self 范围内显式创建一个列表。
 
 
 
更有趣的例子：
  class testClass（）：
 list = ['foo '] 
 def __init __（self）：
 self.list = [] 
 self.list.append（'thing'）
 
x = testClass $ b print x.list 
 print testClass.list 
 del x.list 
 print x.list 
  
这将打印：
  ['thing'] 
 ['foo' ] 
 ['foo'] 
  
删除实例名称的时候，通过 self 引用可见。
 
im having a problem understanding how class / instance variables work in python. I dont understand why when i try this code the list variable seems to be a class variable
class testClass():
    list = []
    def __init__(self):
        self.list.append('thing')

p = testClass()
print p.list

f = testClass()
print f.list
output:
['thing']
['thing', 'thing']
and when i do this it seems to be an instance variable
class testClass():
    def __init__(self):
        self.list = []
        self.list.append('thing')

p = testClass()
print p.list

f = testClass()
print f.list
output:
['thing']
['thing']
many thanks 
jon
 解决方案 
This is because of the way Python resolves names with the .. When you write self.list the Python runtime tries to resolve the list name first looking for it in the instance object, and if not found in the class instance.
Let's look into it step by step
self.list.append(1)
Is there a list name into the object self?
Yes: Use it! Finish.
No: Go to 2.
Is there a list name into the class instance of object self?
Yes: Use it! Finish
No: Error!
But when you bind a name things are different:
self.list = []
Is there a list name into the object self?
Yes: Overwrite it!
No: Bind it!
So, that is always an instance variable.
Your first example creates a list into the class instance, as this is the active scope at the time (no self anywhere). But your second example creates a list explicitly in the scope of self.
More interesting would be the example:
class testClass():
    list = ['foo']
    def __init__(self):
        self.list = []
        self.list.append('thing')

x = testClass()
print x.list
print testClass.list
del x.list
print x.list
That will print:
['thing']
['foo']
['foo']
The moment you delete the instance name the class name is visible through the self reference.
                        
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