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== 13.3#h(0.6em)Methods


  
    
=== Learning objectives


    By the end of this section you should be able to


    - Write overridden methods to change behavior of inherited methods.
- Use super() to access superclass methods.
- Identify applications of polymorphism in method and function use.


  
  
    
=== Overriding methods


    Sometimes a programmer wants to change the functions a subclass inherits. Mint is a subclass that has the same functionality as Plant, except for one function. A subclass can #strong[override] a superclass method by defining a method with the same name as the superclass method.


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Overriding a superclass method]


      #link("https://www.openstax.org/r/overriding-superclass")[Overriding a superclass method; ch 13, video 5]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Overriding methods]
]


  
  
    
=== super()


    super() is a special method that provides a temporary superclass object instance for a subclass to use. super() is commonly used to call superclass methods from a subclass. super() is commonly used in a subclass's \_\_init\_\_() to assign inherited instance attributes. Ex: super().\_\_init\_\_().


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Using super() to call the superclass \_\_init\_\_() method]


      #link("https://www.openstax.org/r/using-super")[Using super() to call the superclass \_init\_() method; ch 13, video 6]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Using super()]


      Consider the following program.


      
    class Polygon:
      def \_\_init\_\_(self, num\_sides=3):
        self.num\_sides = num\_sides

    class Rectangle(Polygon):
      def \_\_init\_\_(self, ln=1, wd=1):
        super().\_\_init\_\_(4)
        self.length = ln
        self.width = wd

    class Square(Rectangle):
      def \_\_init\_\_(self, side=1):
        super().\_\_init\_\_(side, side)
        self.side = side

    sq\_1 = Square(5)
    print(sq\_1.num\_sides)
]


  
  
    
=== Polymorphism


    #strong[Polymorphism] is the concept of having many forms. In programming, a single name can be used for multiple functionalities. Within inheritance, polymorphism is the basis of method overriding, as multiple methods have the same name.


    #examplebox("Example 1")[Polymorphism and inheritance][
The name display() maps to multiple methods. The class type of the calling object determines which display() method is executed.


      
    class Plant:
      def display(self):
        print("I'm a plant")

    class Mint(Plant):
      def display(self):
        print("I'm a mint")

    class Lavender(Mint):
      def display(self):
        print("I'm a lavender")

    mint\_1 = Mint()
    mint\_1.display()

    lavender\_1 = Lavender()
    lavender\_1.display()
      The code's output is:


      
    I'm a mint
    I'm a lavender
]


    Polymorphism can also be used with methods of unrelated classes. The class type of the calling object determines the method executed.


    #examplebox("Example 2")[Polymorphism and methods][
Tax and ContractTax are unrelated classes that each define calc\_tax(). calc\_tax() isn't overridden as ContractTax isn't inherited from Tax.


      
    class Tax:
      def \_\_init\_\_(self, value):
        self.value = value
  
      def calc\_tax(self):
        print("Calculating tax")
        total = 0.10 \* self.value  \# To replace with calculation
        return total
  
    class ContractTax:
      def \_\_init\_\_(self, value):
        self.value = value
  
      def calc\_tax(self):
          print("Calculating contracts tax")
          total = 0.15 \* self.value  \# To replace with calculation
          return total
  
    my\_tax = ContractTax(value=1000)  \# Replace 1000 with any value
    result = my\_tax.calc\_tax()
    print(f"Total tax: \${result}")
      The code's output is:


      
    Calculating contracts tax
    Total tax: \$150.00
]


    Polymorphism allows methods to be called with different parameter types. Many built-in operators and functions have this utility.


    #examplebox("Example 3")[Polymorphism and functions][
len() can be called with multiple types, including lists and strings.


      
    tree\_list = \["ash", "hazel", "oak", "yew"\]
    tree\_1 = tree\_list\[0\]
    print(len(tree\_list))
    print(len(tree\_1))
      The code's output is:


      
    4
    3
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Polymorphism in practice]
]


  
  #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Overriding methods]


    Given the Pet class, create a Bird class inherited from Pet and a Finch class inherited from Bird. Override the display() method in Bird and Finch such that the program output is:


    
    Pet type: Bird
    Bird type: Finch
    class Pet():
    def display(self):
        print("Clinic patient: Pet")

\# Define Bird class

\# Define Finch class

bird\_1 = Bird()
bird\_1.display()
finch\_1 = Finch()
finch\_1.display()
]


  #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Using super()]


    Given the Employee class, create a Developer class inherited from Employee with methods \_\_init\_\_() and print\_info() such that:


    \_\_init\_\_()


    - Uses super() to call Employee's \_\_init\_\_() to initialize e\_id and hire\_year.
- Assigns lang\_xp with list parameter lang\_xp.


    print\_info()


    - Uses super() to print e\_id and hire\_year.
- Prints "Language(s):" followed by lang\_xp.


    class Employee():
    count = 0    
    def \_\_init\_\_(self):
        Employee.count += 1
        self.e\_id = Employee.count
        self.hire\_year = 2023

    def print\_info(self):
        print("Employee", self.e\_id, "was hired in", self.hire\_year)

\# Define Developer class

python\_dev = Developer()
python\_dev.print\_info()
]