class Demo(object):
 def instance_method(self, your_para):
 """
 this is an instance_method
 you should call it like the follow:
 a = Demo()
 a.instance_method(your_para)
 plus: in python, we denote 'cls' as latent para of Class
 while 'self' as latent para of the instance of the Class
 :param your_para: 
 :return: 
 """
 print("call instance_method and get:", your_para)
 @classmethod
 def class_method(cls, your_para):
 """
 this is a class_method
 you can call it like the follow:
 method1:
 a = Demo()
 a.class_method(your_para)
 method2:
 Demo.class_method
 plus: in python, we denote 'cls' as latent para of Class
 while 'self' as latent para of the instance of the Class
 :param your_para: 
 :return: 
 """
 print("call class_method and get:", your_para)
 @staticmethod
 def static_method(your_para):
 """
 this is a static_method and you can call it like the 
 methods of class_method
 :param your_para: 
 :return: 
 """
 print("call static_method and get:", your_para)

class Fruit(object):
 total = 0 # 這是一個類屬性
 @classmethod
 def print_total(cls):
 print('this is the ', cls, '.total:', cls.total, ' and its id: ', id(cls.total)) # cls是類本身，打印類屬性total的值
 print('this is the Fruit.total:', Fruit.total, 'and its id: ', id(Fruit.total))
 print("=======================")
 @classmethod
 def set(cls, value):
 cls.total = value
class Apple(Fruit):
 pass
class Orange(Fruit):
 pass
app1 = Apple()
app1.set(10)
app1.print_total()
Apple.print_total()
Fruit.set(2)
app1.print_total()
Fruit.print_total()
"""
output:
this is the <class '__main__.Apple'> .total: 10 and its id: 1355201264
this is the Fruit.total: 0 and its id: 1355200944
=======================
this is the <class '__main__.Apple'> .total: 10 and its id: 1355201264
this is the Fruit.total: 0 and its id: 1355200944
=======================
this is the <class '__main__.Apple'> .total: 10 and its id: 1355201264
this is the Fruit.total: 2 and its id: 1355201008
=======================
this is the <class '__main__.Fruit'> .total: 2 and its id: 1355201008
this is the Fruit.total: 2 and its id: 1355201008
=======================
"""