当我在Python中实例化类时会发生什么？(What happens when I instantiate class in Python?)

当我在Python中实例化类时会发生什么？(What happens when I instantiate class in Python?)

你能澄清Python类和类实例背后的一些想法吗？

考虑一下：

class A(): name = 'A' a = A() a.name = 'B' # point 1 (instance of class A is used here) print a.name print A.name

打印：

B A

如果相反在point 1我使用类名，输出是不同的：

A.name = 'B' # point 1 (updated, class A itself is used here)

打印：

B B

即使Python中的类是类实例的某种原型，我也希望已经创建的实例保持不变，即输出如下：

A B

你能解释一下究竟发生了什么吗？

Could you clarify some ideas behind Python classes and class instances?

Consider this:

class A(): name = 'A' a = A() a.name = 'B' # point 1 (instance of class A is used here) print a.name print A.name

prints:

B A

if instead in point 1 I use class name, output is different:

A.name = 'B' # point 1 (updated, class A itself is used here)

prints:

B B

Even if classes in Python were some kind of prototype for class instances, I'd expect already created instances to remain intact, i.e. output like this:

A B

Can you explain what is actually going on?

最满意答案

首先，Python中创建实例字段（而不是类字段）的正确方法是使用__init__方法。 我相信你已经知道了。

Python不会限制您为对象的非声明字段赋值。 例如，请考虑以下代码：

class Empty: pass e = Empty() e.f = 5 print e.f # shows 5

那么代码中的内容是：

使用A指定的静态字段name创建A类。 您创建A的实例， a 。 您为对象a创建一个新字段（但不为其他A实例创建）并为其分配B 您打印a.name的值，该值对象a是唯一a 。 您打印属于该类的静态字段A.name的值

First of all, the right way in Python to create fields of an instance (rather than class fields) is using the __init__ method. I trust that you know that already.

Python does not limit you in assigning values to non-declared fields of an object. For example, consider the following code:

class Empty: pass e = Empty() e.f = 5 print e.f # shows 5

So what's going in your code is:

You create the class A with a static field name assigned with A. You create an instance of A, a. You create a new field for the object a (but not for other instances of A) and assign B to it You print the value of a.name, which is unique to the object a. You print the value of the static field A.name, which belongs to the class