Class & magic method

• class

  • make a class and instance

    # make a rectangle
    # self parameter is always used, because it refers to the initalized object itself
    class rectangle:
        def __init__(self, a, b):
            self.side1=a
            self.side2=b
    
    		def area(self):
    		        a=self.side1 * self.side2
    		        print("area=",a)
    
    >>> r1=rectangle(10, 20) #make an instance by calling the class like calling the function
    >>> r1.area()
    area=200
    

  • magic method

    • init()

      • Constructor, call when an instance is initialized (what’s why always used define parameter!)

    • new()

      • do somethings before initialize the object

      class Employee:
       
          def __init__(self):
              print ("__init__ magic method is called")
       
          def __new__(random):
              print ("__new__ magic method is called")
              inst = object.__new__(random)
              return inst
       
      emp = Employee()

    • str()

      • returned class object which is represented as a designed string

      #WITHOUT the **str**() function:
      class Person:
        def __init__(self, name, age):
          self.name = name
          self.age = age
       
      p1 = Person("John", 36)
      print(p1)    # result: <__main__.Person object at 0x2b2a41db3100>
       
      #WITHOUT the **str**() function:
      class Person:
        def __init__(self, name, age):
          self.name = name
          self.age = age
       
        def __str__(self):
          return f"{self.name}({self.age})"
       
      p1 = Person("John", 36)
      print(p1)     # result: John(36)

    • del()

      • Destructor, call when an instance is deleted

      class Example:
      # Initializing
          def __init__(self):
              print("Example Instance.")
       
      # Calling destructor
          def __del__(self):
              print("Destructor called, Example deleted.")
       
      obj = Example()
      del obj

  • Inheritance

    • make a child class

      class parent_name:
          statements
      
      class child_name(parent_name):
          statements
      

    • Pass the parameter of child class to the parent using super() function

      #make a rectangle class from quadriLateral class, where attributes of the rectangle can be applied to quadriLateral
      
      class quadriLateral:
          def __init__(self, a, b, c, d):
              self.side1=a
              self.side2=b
              self.side3=c
              self.side4=d
      
      		def perimeter(self):
      		        p=self.side1 + self.side2 + self.side3 + self.side4
      		        print("perimeter=",p)
      
      class rectangle(quadriLateral):
          def __init__(self, a, b):
              super().__init__(a, b, a, b)
      
      >>> r1=rectangle(10, 20)
      >>> r1.perimeter()
      perimeter=60
      

    • Overriding (modify the functionality of the parent)

      class rectangle(QuadriLateral):
          def __init__(self, a,b):
              super().__init__(a, b, a, b)
      
          def area(self):
              a = self.side1 * self.side2
              print("area of rectangle=", a)
      
      #tranform a rectangle class into a square class
      
      class square(rectangle):
          def __init__(self, a):
              super().__init__(a, a)
      
          def area(self):  #define a function of which the name is the same as parent (overrided)
              a=pow(self.side1, 2)
              print('Area of Square: ', a)
      
      >>>s=Square(10)
      >>>s.area()
      Area of Square: 100
      

• parenthesis on function and class

  • With parenthesis: call the function
  • Without parenthesis: reference **the function (point it to a variable)