Python: Object Oriented Programming (OOP)

Python Object Oriented Programming (OOP)

Python Object Oriented Programming (OOP)

1 Overview

📘 Object Oriented Programming (OOP)

In Python, Object Oriented Programming (OOP) is a programming paradigm based on the concept of “objects”, which can contain data in the form of fields (often known as attributes or properties), and code, in the form of procedures (often known as methods).

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2 Object Oriented Programming (OOP)

• Organizes code like a community of interacting individuals.
• Benefits: Manageable, maintainable, reusable code.

3 Python Classes and Objects

• Classes: Blueprints (like architectural plans).
• Objects: Instances built from the blueprint (like houses).
• Use a class blueprint to create multiple objects.
• It is like building multiple houses (objects) from the same architectural plan (class).

4 Object Intuition

• Objects represent “real-life” entities.
• They have properties (attributes) and behaviors (methods/“functions”).
• Defined by the programmer.

5 Example 1: Tic-Tac-Toe Entities

5.1 Classes:

• Player Attributes: name Methods: update name
• Board Attributes: number of squares (e.g., 3x3) Methods: Display board, set piece, check legal moves
• Game Attributes: board, list of players Methods: play game, reset game

5.2 Object Instances:

• 2 players
• 1 board
• 1 game

6 Example 2: Store Entities

6.1 Classes:

• Product Attributes: quantity, description, brand Methods: update stock, apply discount
• Client Attributes: name, phone number, client score, order history Methods: update contact info, view order history
• Employee Attributes: name, phone number, bank account, salary, role Methods: increase salary, update bank account
• Order Attributes: products, client, total price Methods: add product, remove product, process payment

7 Example 3: Python is Object-First

• Everything in Python is an object.
• Strings, integers, functions are objects.
• For example, creating a string object:

variable = str()   # Equivalent to: variable = ''

8 String Class Example

• Attributes: __doc__, __class__, …
• Methods: lower(), replace(), count(), …

9 Defining a Class in Python

• Classes usually use UpperCamelCase names.
• The class definition is the blueprint.
• Instances are objects built from the blueprint.

Example:

class Product:
    pass

p1 = Product()
p2 = Product()
print(p1)
# <__main__.Product object at 0x7c2b933f7c10>

10 __init__ Method

• Automatically called when a new object is created.
• Initializes object attributes.
• self refers to the instance being created.

Example:

class Product:
    def __init__(self, name, quantity=1):
        self.name = name
        self.quantity = quantity
        self.active = True

p = Product('Cake')
print(p.name)      # Cake
print(p.quantity)  # 1
print(p.active)    # True

11 self Keyword

• Refers to the object instance.
• Used to access object attributes.
• Think of self as a way for methods to refer to the object itself.

12 Changing Object Attributes

Example:

p1 = Product('Cake', 5)
p2 = Product('Soup', 2)
p1.name = 'Milk'
print(p1.name)  # Milk
print(p2.name)  # Soup

• Changing p1.name does not affect p2.name.

You can add new attributes to objects dynamically:

p1.brand = 'Pascual'
print(p1.brand)  # Pascual
print(p2.brand)  # AttributeError: 'Product' object has no attribute 'brand'

13 Class Attributes

• Shared across all instances.

Example:

class Product:
    tax_rate = 0.21  # class attribute

    def __init__(self, name, quantity=1):
        self.name = name
        self.quantity = quantity
        self.active = True

p1 = Product('Cake')
p2 = Product('Soup')
print(p1.tax_rate)  # 0.21
print(p2.tax_rate)  # 0.21

• If you change a class attribute, all instances reflect the change:

Product.tax_rate = 0.1
print(p1.tax_rate)  # 0.1
print(p2.tax_rate)  # 0.1

• Beware: Assigning to an instance attribute with the same name shadows the class attribute:

p1.tax_rate = 0.4
print(p1.tax_rate)  # 0.4
print(p2.tax_rate)  # 0.1

14 Class Methods

• Define actions the object can take.
• self must always be the first parameter.

Example:

class Product:
    def __init__(self, name, brand, quantity=1):
        self.name = name
        self.brand = brand
        self.quantity = quantity

    def update_stock(self, amount):
        self.quantity += amount
        print(f'New quantity: {self.quantity}')

Creating an object:

cake = Product('Cake', 'Pascual', 10)
cake.update_stock(5)  # New quantity: 15

When calling a method, you do not pass self explicitly; Python passes it behind the scenes.

15 More Methods Example

class Product:
    def __init__(self, name, unit_price, quantity=1):
        self.name = name
        self.unit_price = unit_price
        self.quantity = quantity

    def update_stock(self, amount):
        self.quantity += amount
        print(f'New quantity: {self.quantity}')

    def get_discounted_price(self, discount):
        return self.unit_price - self.unit_price * discount

    def describe(self):
        print(self.name)
        print(f'Quantity: {self.quantity}')
        print(f'Unit price: {self.unit_price}€')

cake = Product('Cake', unit_price=1.79, quantity=10)
cake.describe()

16 Dunder (Double Underscore) Methods

• Special methods surrounded by double underscores.
• Have specific purposes, like overriding Python behaviors.
• __init__ called automatically on object creation.
• Examples: __str__, __add__.

Example:

class Product:
    # Other methods...

    def __str__(self):
        return f'{self.name}: {self.quantity}'

Usage:

pizza = Product('Pizza', unit_price=3.95, quantity=20)
print(pizza)  # Pizza: 20

17 Operator Overloading

• Enables use of operators (+, -, *) with custom behaviors for objects.
• By default, adding two custom objects results in a TypeError.

Example of enabling + operator:

class Product:
    # Other methods...

    def __add__(self, other):
        same_name = self.name == other.name
        same_price = self.unit_price == other.unit_price

        if same_name and same_price:
            total = self.quantity + other.quantity
            return Product(self.name, self.unit_price, total)
        else:
            raise Exception("Cannot add these two products.")

Example of use:

pizza1 = Product('Pizza - Tarradellas', unit_price=3.95, quantity=8)
pizza2 = Product('Pizza - Tarradellas', unit_price=3.95, quantity=3)
pizza3 = Product('Pizza - Buittoni', unit_price=4.10, quantity=10)

new_pizza = pizza1 + pizza2
print(new_pizza)  # Pizza - Tarradellas: 11

new_pizza = pizza1 + pizza3
# Exception raised: Cannot add these two products.

18 Complex Example: Library System

Classes for managing a book library:

class Book:
    def __init__(self, tit, auth):
        self.title = tit
        self.author = auth
        self.availability = True

    def borrow(self):
        if self.availability:
            self.availability = False
            print(f"{self.title} has been borrowed.")
        else:
            print(f"{self.title} is not available.")

    def return_book(self):
        self.availability = True
        print(f"{self.title} has been returned.")

    def __str__(self):
        return f"{self.title} - {self.author}"

class Library:
    def __init__(self, name, loc):
        self.books = []
        self.name = name
        self.location = loc

    def add_book(self, book):
        self.books.append(book)
        return f"{book.title} added to the library."

    def get_book(self, title):
        for book in self.books:
            if book.title == title:
                if book.availability:
                    return book
                else:
                    print(f"{title} is currently borrowed.")
                    return None
        print(f"{title} not found in the library.")
        return None

Example usage:

pilarin_bayes = Library("Pilarin Bayés", "Vic")
joan_miro = Library("Joan Miró", "Barcelona")

hp1 = Book("Harry Potter 1", "J.K. Rowling")
hp2 = Book("Harry Potter 2", "J.K. Rowling")
hp3 = Book("Harry Potter 3", "J.K. Rowling")
hp4 = Book("Harry Potter 4", "J.K. Rowling")

pilarin_bayes.add_book(hp1)
pilarin_bayes.add_book(hp2)

joan_miro.add_book(hp2)
joan_miro.add_book(hp3)
joan_miro.add_book(hp4)

my_hp1 = joan_miro.get_book("Harry Potter 1")
print(my_hp1)  # Harry Potter 1 not found in the library. None

my_hp1 = pilarin_bayes.get_book("Harry Potter 1")
print(my_hp1)  # Harry Potter 1 - J.K. Rowling

my_hp1.borrow()  # Harry Potter 1 has been borrowed.

my_hp1 = pilarin_bayes.get_book("Harry Potter 1")
print(my_hp1)  # Harry Potter 1 is currently borrowed. None