蟒蛇类属性 [英] python class attribute

问题描述

我有一个关于在Python类属性的问题。

i have a question about class attribute in python.

class base :
    def __init__ (self):
        pass
    derived_val = 1

t1 = base()
t2 = base ()

t2.derived_val +=1
t2.__class__.derived_val +=2
print t2.derived_val             # its value is 2
print t2.__class__.derived_val   # its value is 3

结果是不同的。我也使用id（）函数来寻找t2.derived_val和T2。类 .derived_val有不同的内存地址。
我的问题是derived_val是类属性。为什么在上面的例子有什么不同？
难道是因为类的实例复制类属性旁边自己的derived_val？

The results are different. I also use id() function to find t2.derived_val and t2.class.derived_val have different memory address. My problem is derived_val is class attribute. Why it is different in above example? Is it because the instance of class copy its own derived_val beside the class attribute?

推荐答案

有阶级属性和实例属性。
当你说

There are class attributes, and instance attributes. When you say

class base :
    derived_val = 1

您正在定义一个类属性。 derived_val 成为关键
基地.__字典__ 。

You are defining a class attribute. derived_val becomes a key in base.__dict__.

t2=base()
print(base.__dict__)
# {'derived_val': 1, '__module__': '__main__', '__doc__': None}
print(t2.__dict__)
# {}

当你说 t2.derived_val 的Python试图找到在'derived_val'T2 .__字典__ 。由于它不存在，它看起来是否有任何 T2 的基类的'derived_val 键

When you say t2.derived_val Python tries to find 'derived_val' in t2.__dict__. Since it is not there, it looks if there is a 'derived_val' key in any of t2's base classes.

print(t2.derived_val)
print(t2.__dict__)
# 1
# {}

但是，当你分配一个值 t2.derived_val ，你现在添加实例属性 T2 。 A derived_val 键添加到 T2 .__字典__ 。

But when you assign a value to t2.derived_val, you are now adding an instance attribute to t2. A derived_val key is added to t2.__dict__.

t2.derived_val = t2.derived_val+1
print(t2.derived_val)
print(t2.__dict__)
# 2
# {'derived_val': 2}

请注意，在这一点上，有两个 derived_val 属性，但只
实例属性也很方便。类属性只能通过引用变得可访问 base.derived_val 或直接访问类字典基地.__字典__ 。

Note that at this point, there are two derived_val attributes, but only the instance attribute is easily accessible. The class attribute becomes accessible only through referencing base.derived_val or direct access to the class dict base.__dict__.

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