Python classmethod装饰器原理及用法解析

英文文档：

classmethod(function)

Return a class method for function.

A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom:

class C:
@classmethod
def f(cls, arg1, arg2, ...): ...
The @classmethod form is a function decorator – see the description of function definitions in Function definitions for details.

It can be called either on the class (such as C.f()) or on an instance (such as C().f()). The instance is ignored except for its class. If a class method is called for a derived class, the derived class object is passed as the implied first argument.

Class methods are different than C++ or Java static methods. If you want those, see staticmethod() in this section.

　 标记方法为类方法的装饰器

说明：

　　1. classmethod 是一个装饰器函数，用来标示一个方法为类方法

　　2. 类方法的第一个参数是类对象参数，在方法被调用的时候自动将类对象传入，参数名称约定为cls

　　3. 如果一个方法被标示为类方法，则该方法可被类对象调用(如 C.f())，也可以被类的实例对象调用(如 C().f())

> class C:
  @classmethod
  def f(cls,arg1):
    print(cls)
    print(arg1)
    
> C.f('类对象调用类方法')
<class '__main__.C'>
类对象调用类方法

> c = C()
> c.f('类实例对象调用类方法')
<class '__main__.C'>
类实例对象调用类方法

　　4. 类被继承后，子类也可以调用父类的类方法，但是第一个参数传入的是子类的类对象

> class D(C):
  pass

> D.f("子类的类对象调用父类的类方法")
<class '__main__.D'>
子类的类对象调用父类的类方法

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