Python 定义类时, 为何该值不是所有实例共享的 ?

• 类的属性是所有类实例共享的

• 类实例修改类属性，会发生"copy on write"

In [15]: class C:
    ...:     var = 1
    ...:     

In [16]: c1 = C()

In [17]: c2 = C()

In [18]: c1.var
Out[18]: 1

In [19]: c2.var
Out[19]: 1

In [20]: C.var = 2

In [21]: c1.var
Out[21]: 2

In [22]: c2.var
Out[22]: 2

In [23]: c2.var = 3

In [24]: c2.var
Out[24]: 3

In [25]: c1.var
Out[25]: 2

可以借助一个辅助单例模式的类

class Attr():
    attr = {}
    def __init__(self):
        self.__dict__ = self.attr
class Myclass():
    def __init__(self):
        self.attr = Attr()
    @property
    def value(self):
        return self.attr.value
    @value.setter
    def value(self, value):
        self.attr.value = value

然后所有的Myclass实例共享一个attr,就有类似的效果。

                                  
In [47]: a = Myclass()            
                                  
In [48]: b = Myclass()            
                                  
In [49]: a.value = 1              
                                  
In [50]: b.value                  
Out[50]: 1                        
                                  
In [51]: b.value = 2              
                                  
In [52]: a.value, b.value         
Out[52]: (2, 2)                   

Python3

__class__

>>> class C(object):
    v=1

    
>>> c1=C()
>>> c2=C()
>>> c1.v,c2.v
(1, 1)
>>> c2.v=2
>>> c1.v,c2.v
(1, 2)
>>> c2.__class__
<class '__main__.C'>
>>> c2.__class__.v
1
>>> c1.__class__.v
1
>>> c2.__class__.v=99
>>> c1.__class__.v
99
>>> 

题主通过c2.CLASS_VALUE = 2赋值的时候实际上是动态的给实例对象c2增加了CLASS_VALUE这个实例属性
所以打印c2.CLASS_VALUE的时候会访问到刚刚动态创建的实例属性。通过打印__dict__就可以看到
而类属性没有改变，通过c1.__class__.CLASS_VALUE还是c2.__class__.CLASS_VALUE都是一样的。想改变类属性的值可以通过类来改变C.CLASS_VALUE = value

In [2]: class C(object):
   ...:     CLASS_VALUE = 1
   ...:     

In [3]: 

In [3]: c1 = C()

In [4]: c2 = C()

In [5]: c1.CLASS_VALUE
Out[5]: 1

In [6]: c1.__dict__
Out[6]: {}

In [7]: C.__dict__
Out[7]: 
dict_proxy({'CLASS_VALUE': 1,
            '__dict__': <attribute '__dict__' of 'C' objects>,
            '__doc__': None,
            '__module__': '__main__',
            '__weakref__': <attribute '__weakref__' of 'C' objects>})

In [8]: c1.CLASS_VALUE = 2

In [9]: c1.__dict__
Out[9]: {'CLASS_VALUE': 2}

In [10]: c1.__class__.CLASS_VALUE
Out[10]: 1

In [11]: c2.CLASS_VALUE
Out[11]: 1

In [12]: C.CLASS_VALUE = 10

In [13]: c1.__class__.CLASS_VALUE
Out[13]: 10

In [14]: c2.CLASS_VALUE
Out[14]: 10

In [15]: c2.__class__.CLASS_VALUE
Out[15]: 10