复合赋值给Python类和实例变量 [英] Compound assignment to Python class and instance variables

问题描述

我一直在试图理解Python对类和实例变量的处理。特别是，我发现这个答案非常有帮助。基本上，它说如果你声明一个类变量，然后你做一个赋值 [instance] .property ，你将被分配到一个不同的变量 -

所以我考虑 - 如果我想让我的类的每个实例都有一个默认值（如零）的成员，我应该这样做：

  class Foo：
 num = 0 
  
 
 
 
还是这样？
  class Foo ：
 def __init __（self）：
 self.num = 0 
  
在我之前读到的，我认为第二个例子将初始化正确变量（实例而不是类变量）。但是，我发现第一种方法也很好地工作：
  class Foo：
 num = 0 
 
 bar = Foo（）
 bar.num + = 1＃好，没有错误，意味着bar有一个属性'num'
 bar.num 
> ;> 1 
 Foo.num 
>>>> 0＃但类变量没有修改！所以我刚才补充的是什么？ 
  
那么，为什么这么做？我没有得到什么？ FWIW，我之前对OOP的理解来自于C ++，所以通过类比来解释（或指向它的分解）可能是有用的。
解决方案
就我个人而言，我发现Shalabh Chaturvedi的这些文件非常实用
 
 
 
  bar.num + = 1 是 bar.num = bar.num + 1 。这是在右侧拾取类变量 Foo.num ，并将其赋给一个实例变量 bar.num  。
 
I've been trying to understand Python's handling of class and instance variables. In particular, I found this answer quite helpful. Basically it says that if you declare a class variable, and then you do an assignment to [instance].property, you will be assigning to a different variable altogether -- one in a different namespace from the class variable.

So then I considered -- if I want every instance of my class to have a member with some default value (say zero), should I do it like this:
class Foo:
    num = 0
or like this?
class Foo:
    def __init__(self):
        self.num = 0
Based on what I'd read earlier, I'd think that the second example would be initializing the 'right' variable (the instance instead of the class variable). However, I find that the first method works perfectly well too:
class Foo:
    num = 0

bar = Foo()
bar.num += 1 # good, no error here, meaning that bar has an attribute 'num'
bar.num
>>> 1
Foo.num
>>> 0 # yet the class variable is not modified! so what 'num' did I add to just now?
So.. why does this work? What am I not getting? FWIW, my prior understanding of OOP has come from C++, so explanation by analogy (or pointing where it breaks down) might be useful.
解决方案
Personally, I've found these documents by Shalabh Chaturvedi extremely useful and informative regarding this subject matter.

bar.num += 1 is a shorthand for bar.num = bar.num + 1. This is picking up the class variable Foo.num on the righthand side and assigning it to an instance variable bar.num.

                        
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