Class

In Python, classes are defined as follows:

class ClassName:
    pass

The class keyword tells the compiler that we are creating a custom class, which is followed by the class name and the : sign.

All the properties and methods of the class will be defined within the class scope.

What is an initializer?

As the name suggests, the initializer is used to initialize an object of a class. It’s a special method that outlines the steps that are performed when an object of a class is created in the program. It’s used to define and assign values to instance variables.

The initialization method is similar to other methods but has a pre-defined name, __init__.

Info

The double underscores mean this is a special method that the Python interpreter will treat as a special case.

The initializer is a special method because it does not have a return type. The first parameter of __init__ is self, which is a way to refer to the object being initialized.

It is always a good practice to define it as the first member method in the class definition.

Defining initializers

Initializers are called when an object of a class is created. See the example below:

class Employee:
    # defining the properties and assigning them None
    def __init__(self, ID, salary, department):
        self.ID = ID
        self.salary = salary
        self.department = department


# creating an object of the Employee class with default parameters
Steve = Employee(3789, 2500, "Human Resources")

# Printing properties of Steve
print("ID :", Steve.ID)
print("Salary :", Steve.salary)
print("Department :", Steve.department)

The initializer is automatically called when an object of the class is created. Now that we will be using initializers to make objects, a good practice would be to initialize all of the object properties when defining the initializer.

Warning

It is important to define the initializer with complete parameters to avoid any errors. Similar to methods, initializers also have the provision for optional parameters.

Challenges

• Take a look on Task: Calculate Student Performance and its solution

Task 1Solution

"""
    Calculate Student Performance

    Problem statement
        Implement a class - Student - that has four properties and two methods. All these attributes (properties and methods) should be public. This problem can be broken down into three tasks.

    Task 1
        Implement a constructor to initialize the values of four properties: name, phy, chem, and bio.

    Task 2
        Implement a method – totalObtained – in the Student class that calculates total marks of a student.

    Task 3
        Using the totalObtained method, implement another method, percentage, in the Student class that calculates the percentage of students marks.
        Assume that the total marks of each subject are 100. The combined marks of three subjects are 300.
        Formula: percentage = (marks_obtained/total_marks) * 100
"""

class Student:
    def __init__(self):
        pass # to fill

    def totalObtained(self):
        pass # to fill

    def percentage(self):
        pass # to fill

class Student:
    def __init__(self, name, phy, chem, bio):
        self.name = name
        self.phy = phy
        self.chem = chem
        self.bio = bio

    def totalObtained(self):
        return(self.phy + self.chem + self.bio)

    def percentage(self):
        return((self.totalObtained() / 300) * 100)

• Let's continue to Task: Implement Calculator Class and its solution

Task 2Solution

"""
    Implement Calculator Class

    Write a Python class called Calculator by completing the tasks below:

    Task 1
        Initializer
        Implement an initializer to initialize the values of num1 and num2.

    Properties
        num1
        num2

    Task 2
        Methods
            add() is a method that returns the sum of num1 and num2.
            subtract() is a method that returns the subtraction of num1 from num2.
            multiply() is a method that returns the product of num1 and num2.
            divide() is a method that returns the division of num2 by num1.

    Input
        Pass numbers (integers or floats) in the initializer.

    Output
        addition, subtraction, division, and multiplication

    Sample input
        obj = Calculator(10, 94);
        obj.add()
        obj.subtract()
        obj.multiply()
        obj.divide()

    Sample output
        104
        84
        940
        9.4
"""

class Calculator:
    def __init__(self):
        pass

    def add(self):
        pass

    def subtract(self):
        pass

    def multiply(self):
        pass

    def divide(self):
        pass

class Calculator:
    def __init__(self, num1, num2):
        self.num1 = num1
        self.num2 = num2

    def add(self):
        return (self.num2 + self.num1)

    def subtract(self):
        return (self.num2 - self.num1)

    def multiply(self):
        return (self.num2 * self.num1)

    def divide(self):
        return (self.num2 / self.num1)


demo1 = Calculator(10, 94)
print("Addition:", demo1.add())
print("Subtraction:", demo1.subtract())
print("Mutliplication:", demo1.multiply())
print("Division:", demo1.divide())