Python类属性及其初始化 [英] Python class attributes and their initialization

问题描述

我在Python和在这些日子里，我探索类是新的。我有关于内部类属性和变量的问题：什么是通过刚 Q = 1 在类的主体定义属性之间，并通过定义<$ C $的区别C> self.q = 1 的 __ __的init 里面呢？例如，是以下两种可能性之间的区别？

I'm quite new in python and during these days I'm exploring classes. I have a question concerning attributes and variables inside classes: What is the difference between defining an attribute via just q=1 in the body of the class and via defining self.q=1 inside the __init__? For example, what is the difference between the following two possibilities?

class MyClass1:
    q=1
    def __init__(self,p):
        self.p=p
    def AddSomething(self,x):
        self.q = self.q+x

和

class MyClass2:
    def __init__(self,p):
        self.q=1
        self.p=p
    def AddSomething(self,x):
        self.q = self.q+x

对于示例的输出：

The output of for example:

>>> my=MyClass1(2)
>>> my.p
2
>>> my.q
1
>>> my.AddSomething(7)
>>> my.q
8

不取决于是否 MyClass1的或 MyClass2 被使用。无论是在 MyClass1的也不 MyClass2 并发生一个错误。

does not depend on whether MyClass1 or MyClass2 is used. Neither in MyClass1 nor in MyClass2 does an error occur.

推荐答案

Q = 1 里面的类是类属性，用类作为一个整体，而不是相关的类的任何特定实例。它使用的是类本身最清楚访问： MyClass1.q

q=1 inside the class is a class attribute, associated with the class as a whole and not any particular instance of the class. It is most clearly accessed using the class itself: MyClass1.q.

一个实例属性是直接在 __通过分配到自（分配到一个类的实例，一般的init __ 如用 self.p = p ），但你可以在任何时间分配属性的实例。

A instance attribute is assigned directly to an instance of a class, usually in __init__ by assigning to self (such as with self.p = p), but you can assign attributes to an instance at any time.

类属性可以的阅读的无论是使用类绑定（ MyClass.q ）或实例结合（我的.Q ，假设它不是由一个实例阴影具有相同名称的属性）。他们只能的设置的，但是使用一类具有约束力。设定值与实例绑定的总是的修改实例属性，创建它，如果必要的。考虑这个例子：

Class attributes can be read either using the class binding (MyClass.q) or an instance binding (my.q, assuming it is not shadowed by an instance attribute with the same name). They can only be set, however, using a class binding. Setting a value with an instance binding always modifies an instance attribute, creating it if necessary. Consider this example:

>>> a = MyClass1()
>>> a.q
1
>>> a.q = 3    # Create an instance attribute that shadows the class attribute
3
>>> MyClass1.q
1
>>> b = MyClass1()
>>> b.q   # b doesn't have an instance attribute q, so access the class's
1

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