python class attributes, static methods and classes Method

1. Class Properties

• 1.1 Definitions

  • An outer class method 属性名 = 属性值defined attributes
  • interview method:
    • 类名.属性名
    • 对象名.属性名

      class Student:
          cls_id = 102
          
      stu = Student()
      print(Student.cls_id)
      print(stu.cls_id)
      
      print("---------")
      
      Student.cls_id = 103
      print(Student.cls_id)
      print(stu.cls_id)
      
      print("---------")
      
      stu.cls_id = 104
      print(Student.cls_id)
      print(stu.cls_id)
      

      operation result:

      102
      102
      ---------
      103
      103
      ---------
      103
      104

• 1.2 the difference between the object and the class attribute properties

  • (1) different numbers in memory
    • Object properties: the number of objects created, how many there are in memory
    • Attribute class: class exists only in memory there is only one, but the object can be read
  • (2) define different positions
    • Object attributes: the class defined in the method
    • Attribute class: class methods defined within an outer
  • Different (3) access method
    • Object properties:对象名.属性名
    • Class properties:类名.属性名
      • Read class attribute, by对象名.属性名
  • (4) different life cycle
    • Object Properties: Create Object -> Object recovered by the system
    • Class properties: Load the global namespace -> End program
  • (5) Different owner
    • Object Properties: objects belonging
    • Class Attribute: belong to the class

2. static method

• A class method: Concepts
• Format: In the method of adding the above@staticmethod
• Parameters: You can have parameters, no parameters
• And the code is generally used regardless of the class of objects, and the object instance: application scenario
• Use:
  • (1)类名.类方法名
  • (2)对象名.类方法名
• Example usage scenarios:
  • Such as: the main menu to show student management system

3. Class Methods

• The concept: no method can be called directly by the class instantiation
• Format: In the method of adding the above@classmethod
• Parameters: first argument must be received class itself
  • Parametric methods for cls can also be other names, but generally the default is cls
  • cls class object point (i.e. below examples Goods)
• Scenario: When a process involving only static methods can use the class attributes (attribute-based method is used to modify the class)
• Use:
  • (1)对象名.类方法名
  • (2)类名.类方法名
• Example usage scenarios:
  • Modify discount on all merchandise

    class Goods():
        discount = 1
        def __init__(self,price,name):
            self.name = name
            self.price = price
        def price_discount(self):
            return self.price * Goods.discount
        @classmethod
        def change_discount(cls,new_discount):
            cls.discount = new_discount
        
        goods = Goods(20, "audi")
        print(goods.price_discount())
        Goods.change_discount(0.5)
        print(goods.price_discount())
    

    The result: 
    20 
    10.0