Object-oriented programming (OOP) involves creating objects that can be referenced/represented in your application using a class or object.

Defining Classes and Objects

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Classes: A blueprint for objects. Can contain methods (functions) and attributes (similar to keys in a dictionary)

Objects: An instance of a class blueprint that contains their class’s methods and properties.

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OOP aims to encapsulate your code into logical hierarchical groupings using classes.

// language: python
# Define a class named Car
class Car:
    # Class constructor to initialize the attributes
    def __init__(self, make, model, year):
        # Instance attributes
        self.make = make
        self.model = model
        self.year = year
    # Instance method
    def display_info(self):
        print(f"{self.year} {self.make} {self.model}")
# Create an object of the Car class
my_car = Car("Toyota", "Corolla", 2020)
# Use the object to call the method
my_car.display_info()  # Output: 2020 Toyota Corolla

When a new class is created, the def __init__ (self): method should also be created to initialise the class with a ‘self’ property.

Instance Attributes

‘Self’ refers to the specific instance of the class you are working with. But you can include instance attributes within your class to define attributes that make up that class. Ensure you include a new parameter of that attribute in the init definition.

// language: python
class User:
    def __init__(self, name):
        self.name = name

And then you can define an attribute within the class as a parameter.

// language: python
class User:
    def __init__(self, name):
        self.name = name
user1 = User('John')
print(user1.name) # John

If there are other attributes required then provide data for each attribute, otherwise, you will get an error.

// language: python
class User:
    def __init__(self, name, age, city):
        self.name = name
        self.age = age
        self.city = city
user1 = User('John', 18, 'Paris')
print(user1.name) # John
print(user1.age) # 18
print(user1.city) # Paris

How attributes and methods are defined in classes matters. If the attribute or method has a single underscore before it then this is a convention that means this is a private attribute or method. But it is not strictly private/secret as it can still be printed/displayed/operated. However, if there are two underscores before the property/method then this means that this is specific to a particular class based on inheritance. You cannot just print person.__msg as the name changes to being variable.class__nameOfProperty (person.Person__msg).

// language: python
class Person:
    def __init__ (self):
        self.name = "Tony"
        self._secret = "hi!"
        self.__msg = "I like turtles"
p = Person()
print(p.name) # Tony
print(p._secret) # hi!
print(p.__msg) # AttributeError: 'Person' object has no attribute '__msg'
print(p._Person__msg) # I like turtles

Instance Methods

Classes can also contain instance methods that perform a function. They are called as any function would be called using the name of the function with parentheses afterwards.

// language: python
class User:
    def __init__(self, firstname, surname, age, city):
        self.firstname = firstname
        self.surname = surname
        self.age = age
        self.city = city
    def full_name (self):
        return self.firstname + ' ' + self.surname
user1 = User('John', 'Doe', 18, 'Paris')
print(user1.full_name())

Class Attributes

You can also define attributes directly on a class once and they are shared by all instances of a class and

Articles

Object-Oriented Programming with Python: Introduction