1. Class Definition

• The properties and methods of the data type are defined in the class;
• Different objects of the same class share the methods of the class, but each has its own properties;

1.1 Defining the method

class class name:
	class body

• Key Points:
  • Class names must conform to the rules for "identifiers"; as a general rule, the first letter is capitalized, and multiple words use the "CamelCase" principle;
  • Define properties and methods in the class body;
  • Attributes are used to describe data, and methods (that is, functions) are used to describe operations related to these data;

1.2 Constructor __init__()

• The python object contains:
  • id
  • type
  • value
    • Attributes
    • method
• Key Points:
  • The name is fixed and must be: init();
  • The first parameter must be: self;
  • The constructor is used to initialize the instance properties of the instance object;
  • The constructor is called through the "class name (parameter list)", and after the call, the created object is returned to the corresponding variable;
  • init() method: Initialize the created object;
  • new() method: used to create objects, generally there is no need to redefine this method;
  • The system will provide a default __init__ method. If the __init__ method with parameters is defined, the system will not create a default __init__ method;

1.3 Instance Properties

• Properties subordinate to an instance object, also known as "instance variables";
• Key Points:
  • definition:

    self.instance property name = initial value
    

  • access:

    self.instance property name
    

  • After creating an instance object, access it through the instance object:

    obj01 = class name()   #Create an object and call __init__() to initialize properties
    
    obj01.instance property name = value    #You can assign values ​​to existing properties, or you can add new properties
    

1.4 Instance Methods

• subordinate to the method of the instance object;
• definition:

  def method name(self [, parameter list])
  	function body
  

• Key Points:
  • The first parameter must be self;
  • You do not need to pass parameters to self, and the interpreter will automatically pass parameters;
• Difference between function and method:
  • essentially the same;
  • The method belongs to the instance object and needs to be called through the object, and the function does not need it;
  • You need to pass self when the method is defined, and the function does not need to;
• Other operations:
  • dir(obj): Get all the properties and methods of the object;
  • obj.__dict__ : get the attribute dictionary of the object;
  • pass: empty statement;
  • isinstance( object, type): Determine whether the object is of the specified type;

2. Class objects

2.1 Class attributes

• Class attributes are attributes subordinate to "class objects", also known as "class variables";
• can be shared by all instance objects;
• definition:

  class class name:
  	class variable name= initial value
  

class Student:
	company = "SXT" #class attribute
	count = 0 #class attribute
	
	def __init__(self,name,score):
		self.name = name #instance properties  
		self.score = score
		Student.count = Student.count+1
	
	def say_score(self): #instance method
		print("My company is:",Student.company)
		print(self.name,'The score is:',self.score)

2.2 Class Methods

• class methods are subordinate to "class objects";

• definition:
  Class methods are defined by the decorator @classmethod;

  @classmethod
  def class method name(cls [，parameter list]):
  	function body
  

  class Student:
  company = "SXT" #class attribute
  @classmethod
  def printCompany(cls):
  	print(cls.company)
  
  Student.printCompany()
  

• Key Points:

  • @classmethod must be on the line above the method;
  • must have cls;
  • When calling, "class name. class method name (parameter list)", no need to pass a value to cls;
  • When the subclass inherits the parent class method, the incoming cls is the subclass object, not the parent class object;
  • Accessing instance properties and instance methods from within a class method method results in an error

2.3 Static methods

• Methods that have nothing to do with "class objects" are called "static methods";

• A "static method" is defined in a module in the same way as a normal function, but it is in the "class namespace" and needs to be called through the class when it is called;

• definition:
  Defined by the decorator @staticmethod

  @staticmethod
  def static method name([parameter list]):
  	function body
  

  class Student:  
  	company = "SXT" # class attribute
  	@staticmethod  
  	def add(a, b): # static method  
  		print("{0}+{1}={2}".format(a,b,(a+b)))  
  	return a+b  
  Student.add(20,30)
  

• Key Points:

  • @staticmethod must be on the line above the method;
  • When called, "class name. static method name (parameter list)";
  • Accessing instance properties and instance methods in static methods will cause errors;

2.4 __del__ method (destructor) and garbage collection mechanism

• The __del__ method is used to implement the operations required when the object is destroyed;
• Python can implement automatic garbage collection: when the object is not referenced (reference count is 0), the __del__ method is called by the garbage collector;
• The system will automatically provide the __del__ method, which generally does not require a custom destructor method;

class Person:  
	def __del__(self):  
		print("Destroy the object:{0}".format(self))  
	
p1 = Person()  
p2 = Person()  
del p2  
print("program ends")

# Destroy object: <... >
# program ends
# Destroy object: <... >  

2.5 __call__ methods and callable objects

• An object that defines the __call__ method can be called like a function;

class SalaryAccount:  
	def __call__(self, salary):
	yearSalary = salary*12
	daySalary = salary//22.5
	hourSalary = daySalary//8  
	return dict(yearSalary=yearSalary,
				monthSalary=salary,  
				daySalary=daySalary, 
				hourSalary=hourSalary)
				
s = SalaryAccount()  
print(s(30000)) 
# {'yearSalary': 360000, 'monthSalary': 30000, 'daySalary': 1333, 'hourSalary': 166}