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Python Classes and objects

Classes and objects are fundamental concepts in object-oriented programming (OOP) in Python. They allow you to create reusable, modular code by defining templates (classes) for creating specific instances (objects).

Defining a Class

A class in Python is defined using the class keyword followed by the class name and a colon. The class body contains methods (functions) and attributes (variables).

Example:

class Dog:
    # Class attribute
    species = "Canis familiaris"

    # Initializer / Instance attributes
    def __init__(self, name, age):
        self.name = name
        self.age = age

    # Instance method
    def description(self):
        return f"{self.name} is {self.age} years old"

    # Another instance method
    def speak(self, sound):
        return f"{self.name} says {sound}"

Creating Objects

An object is an instance of a class. You create an object by calling the class as if it were a function.

Example:

class Dog:
    # Class attribute
    species = "Canis familiaris"

    # Initializer / Instance attributes
    def __init__(self, name, age):
        self.name = name
        self.age = age

    # Instance method
    def description(self):
        return f"{self.name} is {self.age} years old"

    # Another instance method
    def speak(self, sound):
        return f"{self.name} says {sound}"

Class and Instance Attributes

  • Class attributes are shared among all instances of a class.
  • Instance attributes are specific to each object created from the class.

Example:

print(dog1.species)  # Output: Canis familiaris
print(dog2.species)  # Output: Canis familiaris

# Changing the species attribute for all instances
Dog.species = "Canine"
print(dog1.species)  # Output: Canine
print(dog2.species)  # Output: Canine

# Changing the name attribute for dog1
dog1.name = "Max"
print(dog1.name)  # Output: Max
print(dog2.name)  # Output: Lucy

Inheritance

Inheritance allows you to create a new class based on an existing class. The new class (child class) inherits attributes and methods from the existing class (parent class).

Example:

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        raise NotImplementedError("Subclass must implement abstract method")

class Dog(Animal):
    def speak(self):
        return f"{self.name} says Woof"

class Cat(Animal):
    def speak(self):
        return f"{self.name} says Meow"

# Creating instances of Dog and Cat
dog = Dog("Buddy")
cat = Cat("Whiskers")

print(dog.speak())  # Output: Buddy says Woof
print(cat.speak())  # Output: Whiskers says Meow


Encapsulation

Encapsulation restricts direct access to some of an object's attributes and methods. This is achieved using private attributes and methods.

Example:

class Car:
    def __init__(self, make, model):
        self.make = make
        self.model = model
        self.__engine_running = False  # Private attribute

    def start_engine(self):
        self.__engine_running = True

    def stop_engine(self):
        self.__engine_running = False

    def engine_status(self):
        return self.__engine_running

car = Car("Toyota", "Corolla")
car.start_engine()
print(car.engine_status())  # Output: True
car.stop_engine()
print(car.engine_status())  # Output: False


Polymorphism

Polymorphism allows different classes to be treated as instances of the same class through a common interface. This is typically achieved by method overriding.

Example:

class Bird:
    def fly(self):
        raise NotImplementedError("Subclass must implement abstract method")

class Sparrow(Bird):
    def fly(self):
        return "Sparrow flies"

class Penguin(Bird):
    def fly(self):
        return "Penguin can't fly"

# Using polymorphism
def describe_flight(bird):
    print(bird.fly())

sparrow = Sparrow()
penguin = Penguin()

describe_flight(sparrow)  # Output: Sparrow flies
describe_flight(penguin)  # Output: Penguin can't fly

Summary

  • Classes: Blueprints for creating objects. They encapsulate data (attributes) and behavior (methods).
  • Objects: Instances of classes.
  • Attributes: Variables that belong to a class or an object.
  • Methods: Functions that belong to a class and typically operate on its objects.
  • Inheritance: Mechanism for creating a new class based on an existing class.
  • Encapsulation: Restricts access to certain details of an object, promoting modularity and maintenance.
  • Polymorphism: Allows objects of different classes to be treated through the same interface.

Using these principles, you can create structured, modular, and reusable code in Python.

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