day-12

Classes and Objects

From Day 12-14, we'll start learning about Object-Oriented Programming (OOP), and for today, we'll talk about classes and objects.

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What is Object-Oriented Programming?

Object-Oriented Programming, abbreviated as OOP, stands out as one of the most popular programming paradigms widely used by developers in software development today. It utilizes the concepts of classes and objects to organize code into reusable blueprints or classes which are then used to initialize instances of an object.

Think of a class as a blueprint for creating a structure of a particular data or object. It has a state (attributes) and behaviours (methods).

For example, we want to create a structure that stores data from a student.

student1_name = "Juan Dela Cruz"
student1_age = 20
student1_sex = "Male"
student1_is_enrolled = True
student1_classes = []
student1_offenses = []

Without classes, we may do something like the code snippet above, copying and pasting it for every additional student info that we will create.

Fortunately, it can be done easily with the use of classes.

Class Declaration

To use a class in Python:

class Student:
    def __init__(self):
        pass    

Now that's a lot of new stuffs going on here. It isn't complete yet, and it doesn't do anything yet, but let's examine every line of the code.

class Student:

We use the keyword class to declare a class followed by the class name,

    def __init__(self):

we use def to declare a function, and we name it __init__ to indicate that we are implementing the initialization of the class. The self being a default parameter for a class method which represents the object of the class itself,

    pass

then use pass as a placeholder for now.

Now, we will initialize some attributes to the object inside the __init__ method we just declared.

Constructor

class Student:
    def __init__(self, name, age, 
        is_enrolled, offenses):
        self.name = name
        self.age = age
        self.is_enrolled = is_enrolled
        self.classes = classes
        self.offenses = offenses

Let's pause here again as we just added a bunch of texts in our previous code.

    def __init__(self, name, age, 
        is_enrolled, classes, offenses):

First, we added some additional parameters in our __init__ method. You will notice that we added the info that we need in a student, and then assign it to a same name in the self:

        self.name = name
        self.age = age
        self.is_enrolled = is_enrolled
        self.classes = classes
        self.offenses = offenses

Remember that the self is referring to the object that gets created when we instantiate it, and then we'll store the student info being passed as parameters to that object's states, which can be accessed in self.

In case you're wondering, we aren't strictly restricted to only use the same name as the parameter, but that's just a simpler way of doing it.

If you are new with this concept and is still struggling to absorb it, this may help you understand it more clearly.

Instantiation

Now that we're done with defining the class and its __init__ method, we can now instantiate an object.

To instantiate an object, we can do this:

student1 = Student("Juan Dela Cruz", 20, True, [], [])

We are essentially calling the class Student, and passing some arguments into it. The arguments will be received by the __init__ method of the class, which will then store its value in the student1, the name of the new object we just instantiated.

If we want to access student1's properties, we will use the . dot notation.

print(f"Name: {student1.name}")
print(f"Age: {student1.age}")
print(f"Status: {'Enrolled' if student1.is_enrolled else 'Not Enrolled Yet'}")
print(f"Classes: {student1.classes}")
print(f"Offenses: {student1.offenses}")

^{You can try running the code in day-12-classes.py located in this folder.}

After running the code, you can see that it outputs the arguments that we passed when we call the class.

Class Methods

We can also create methods in the class to have more behaviours in the object.

The __init__ is a special class method that gets executed when we instantiate an object, and we can declare another method just like __init__:

class Student:
    def __init__(self, name, age, 
        is_enrolled, classes, offenses):
        self.name = name
        self.age = age
        self.is_enrolled = is_enrolled
        self.classes = classes
        self.offenses = offenses
    
    # add a new class to the student
    def add_class(self, new_class):
        self.classes.append(new_class)

We created a new method that adds new classes to the student. To use it:

student1.add_class("Math")

and the "Math" class is now added to the student's list of classes. We can check it by printing out the student's classes:

print(student1.classes)

And that's it! We now learned about creating classes, instantiating objects, declaring constructors, and adding methods.

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Additional Resources

• What is object-oriented programming? OOP explained in depth
• Python Classes and Objects