@Classmethod decorator

Catalog

Example

The main role

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Example

General create class （ It contains N A way ） after , You need to use the methods inside to achieve a specific function , Need to instantiate class , Then access its methods through the class after the instance , for example ：

class test_classmethod(object):

    def printd(self,a ,b):
        c = a+b
        print (c)


if __name__ == '__main__':
    c =test_classmethod()
    c.printd(3,5)

The running result is 8

If you don't want to instantiate , Can be in def printd(self,a ,b) Add... To it @classmethod, as follows ：

class test_classmethod(object):

    @classmethod
    def printd(cls,a ,b):
        c = a+b
        print (c)


if __name__ == '__main__':
    test_classmethod.printd(3,5)

classmethod  The function corresponding to the modifier does not need to be instantiated , Unwanted self Parameters , But the first parameter needs to represent the class itself cls Parameters , You can call the properties of a class , Class method , Instanced objects, etc . as follows ：

class A():

    a = 1

    def def1(self):
        print("foo")

    @classmethod
    def def2(cls):
        print('def2')
        print(cls.a)
        cls().def1()  #  call  foo  Method 


A.def2()

Running results ：

def2
1
foo

The variables defined in the class are required , If it is written in the following way, an error will be reported ：

class A():

    def __init__(self):
        a = 1

    def def1(self):
        print("foo")

    @classmethod
    def def2(cls):
        print('def2')
        print(cls.a)
        cls().def1()  #  call  foo  Method 


A.def2()

class A():

    def __init__(self):
        self.a = 1

    def def1(self):
        print("foo")

    @classmethod
    def def2(cls):
        print('def2')
        print(cls.a)
        cls().def1()  #  call  foo  Method 


A.def2()

The main role

Decorators are mainly completely different from them , The main function is outside this method , Add some extra features （ Without changing the original code ）, for example , I write some basic methods ,a function （ Belong to A A group of programmers ）、b function （ Belong to B A group of programmers ）、c function （ Belong to C A group of programmers ） You need to call this method , One day the company added D Group 、E Group 、F Group . But the leadership rules , Can only A、B、C Group call , Other groups cannot call this function , Risk exists . Then I need to write a verification method , Decorate this function

Decorator is the method to be called a Wrapped in another way W1 in , adopt w1 Execute after passing the function execution of a, Otherwise, the called... Will not be executed a function .