Python Mastery: From Beginner to Expert - Sykalo Eugene 2023

Classes and objects
Object-oriented programming

Introduction to Object-Oriented Programming

Object-oriented programming (OOP) is a programming paradigm that emphasizes the use of objects, which are instances of classes, to represent and manipulate data. In OOP, programs are organized around objects and data rather than actions and logic.

Some advantages of object-oriented programming over procedural programming include:

Modularity: Objects can be easily reused in different parts of a program or in different programs altogether.
Encapsulation: The data and behavior of an object are contained within the object, making it easier to manage and maintain.
Abstraction: Objects can be abstracted to represent complex real-world entities in a simplified way.
Inheritance: Objects can inherit properties and behavior from other objects, reducing the amount of code duplication.

In Python, everything is an object. This means that everything in Python has a type and can be assigned to a variable or passed as an argument to a function. Python is also an object-oriented language, which means that it supports all of the features of OOP.

Classes and Objects

In object-oriented programming, a class is a blueprint for creating objects. An object is an instance of a class, and it can have attributes (variables) and methods (functions).

What are classes and objects?

A class is a template or blueprint that defines the variables and methods common to all objects of a certain kind. For example, if we were creating a program to represent cars, we might create a Car class that defines the variables (make, model, year, etc.) and methods (start, stop, accelerate, brake, etc.) that all cars have in common.

An object is an instance of a class. When we create an object, we are creating a specific instance of the class, with its own unique set of values for its variables.

How to create classes and objects in Python

To create a class in Python, use the class keyword, followed by the name of the class. Here is an example:

class Car:
 make = ""
 model = ""
 year = 0

 def start(self):
 print("Starting the car...")

 def stop(self):
 print("Stopping the car...")

This creates a class called Car that has three variables (make, model, and year) and two methods (start and stop).

To create an object of a class, you simply call the class as if it were a function, like this:

my_car = Car()

This creates a new object of the Car class and assigns it to the variable my_car.

Understanding attributes and methods

Attributes are variables that belong to an object. They define the state of the object. In our Car class example, the make, model, and year variables are attributes of the Car object.

Methods are functions that belong to an object. They define the behavior of the object. In our Car class example, the start and stop methods are methods of the Car object.

To access an attribute or method of an object, you use the dot notation, like this:

my_car.make = "Toyota"
my_car.model = "Corolla"
my_car.year = 2021

my_car.start()
my_car.stop()

This sets the make, model, and year attributes of the my_car object and calls its start and stop methods.

Inheritance

Inheritance is a mechanism in object-oriented programming that allows new classes to be based on existing classes. The new class, called the derived class or subclass, inherits properties and behavior from the existing class, called the base class or superclass. This allows for code reuse and reduces the amount of code duplication.

Definition of inheritance in object-oriented programming

Inheritance is a way to establish a hierarchy of classes, where each subclass inherits properties and behavior from its superclass. The subclass can then add its own properties and behavior, or override those of its superclass.

Types of inheritance (Single, Multiple, Multi-level)

There are three types of inheritance in object-oriented programming:

Single inheritance: A subclass inherits properties and behavior from a single superclass.
Multiple inheritance: A subclass inherits properties and behavior from multiple superclasses.
Multi-level inheritance: A subclass inherits properties and behavior from a superclass, which in turn inherits properties and behavior from another superclass.

Implementation of inheritance in Python

To define a subclass in Python, simply specify the superclass in parentheses after the class name, like this:

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

 def speak(self):
 pass

class Dog(Animal):
 def speak(self):
 return "Woof!"

class Cat(Animal):
 def speak(self):
 return "Meow!"

In this example, Dog and Cat are subclasses of Animal. They inherit the name attribute and speak method from Animal, but override the speak method with their own implementations.

To create an object of a subclass, you simply call the subclass as if it were a function, like this:

my_dog = Dog("Fido")
my_cat = Cat("Whiskers")

This creates new objects of the Dog and Cat subclasses and assigns them to the variables my_dog and my_cat. These objects have the name attribute inherited from Animal, as well as the speak method overridden by their respective subclasses.

Polymorphism

Polymorphism is the ability of objects to take on multiple forms. In object-oriented programming, this means that an object of one class can be treated as an object of another class, as long as both classes have a common interface. Polymorphism allows for code reuse and flexibility in object-oriented programming.

Definition of polymorphism in object-oriented programming

Types of polymorphism (Overloading, Overriding)

There are two types of polymorphism in object-oriented programming:

Overloading: Overloading allows multiple methods with the same name to be defined in a class. The methods must differ in their parameter lists (number or type of parameters). When the method is called, the appropriate method is automatically selected based on the parameters passed in.
Overriding: Overriding allows a subclass to provide its own implementation of a method that is already provided by its superclass. The subclass can then use this method instead of the superclass's method.

Implementation of polymorphism in Python

To implement polymorphism in Python, we can use either overloading or overriding.

Overloading

In Python, we can simulate method overloading by using default parameter values. For example:

class MyClass:
 def my_method(self, arg1, arg2=None):
 if arg2 is None:
 # do something
 else:
 # do something else

In this example, my_method can be called with one or two arguments. If only one argument is provided, arg2 will default to None.

Overriding

To override a method in Python, we simply define a method with the same name in the subclass. For example:

class Animal:
 def speak(self):
 print("The animal speaks.")

class Dog(Animal):
 def speak(self):
 print("The dog barks.")

In this example, Dog overrides the speak method of its superclass Animal. When the speak method is called on a Dog object, it will print "The dog barks." instead of "The animal speaks."

Polymorphism allows for greater flexibility and reusability in object-oriented programming. By designing classes with a common interface, we can create more generic code that can work with objects of different classes. This makes our code more modular and easier to maintain.

Encapsulation

Encapsulation is the practice of hiding the internal details of an object and exposing only the necessary information to the outside world. In object-oriented programming, encapsulation is achieved through the use of access modifiers, such as public, private, and protected.

Definition of encapsulation in object-oriented programming

Encapsulation is the practice of hiding the implementation details of an object from the outside world, while exposing a public interface that can be used to interact with the object. This is done to protect the integrity of the object and prevent unauthorized access or modification.

Implementation of encapsulation in Python

In Python, encapsulation is achieved through the use of access modifiers. By default, all attributes and methods of a class are public, which means that they can be accessed and modified by any code that has a reference to the object.

To create private attributes and methods, we can prefix their names with two underscores, like this:

class MyClass:
 def __init__(self):
 self.public_attribute = "Public Attribute"
 self.__private_attribute = "Private Attribute"

 def public_method(self):
 print("This is a public method.")

 def __private_method(self):
 print("This is a private method.")

In this example, public_attribute and public_method are public, while __private_attribute and __private_method are private.

To access a private attribute or method, we can use name mangling. Name mangling is the process of adding a prefix to the attribute or method name to make it harder to access from outside the class. The prefix is the name of the class, with a double underscore added to the beginning. For example:

my_object = MyClass()
print(my_object.public_attribute) # Output: "Public Attribute"
print(my_object.__private_attribute) # Raises an AttributeError

In this example, we can access the public_attribute attribute of my_object, but attempting to access __private_attribute raises an AttributeError. However, we can still access the private attribute using name mangling, like this:

print(my_object._MyClass__private_attribute) # Output: "Private Attribute"

This is not recommended, however, as it violates the principle of encapsulation and can make our code more difficult to maintain.

Encapsulation is an important concept in object-oriented programming, as it allows us to protect the internal details of our objects and prevent unauthorized access or modification. By using access modifiers, we can create public interfaces that are safe and easy to use, while keeping the implementation details hidden.

Abstraction

Definition of abstraction in object-oriented programming

Abstraction is the process of simplifying complex real-world entities into a set of essential characteristics. In object-oriented programming, abstraction is achieved through the use of abstract classes and interfaces. An abstract class is a class that cannot be instantiated and is used to define a common interface for its subclasses. An interface is a set of methods that a class must implement in order to be considered an instance of the interface.

Implementation of abstraction in Python

In Python, we can define abstract classes using the abc module. Here is an example:

from abc import ABC, abstractmethod

class Animal(ABC):
 @abstractmethod
 def speak(self):
 pass

class Dog(Animal):
 def speak(self):
 print("The dog barks.")

In this example, Animal is an abstract class that defines the speak method as an abstract method. This means that any subclass of Animal must implement the speak method in order to be considered a valid class. Dog is a subclass of Animal that implements the speak method with its own implementation.

We can also define interfaces in Python using abstract classes. Here is an example:

from abc import ABC, abstractmethod

class Drawable(ABC):
 @abstractmethod
 def draw(self):
 pass

class Rectangle(Drawable):
 def draw(self):
 print("Drawing a rectangle.")

class Circle(Drawable):
 def draw(self):
 print("Drawing a circle.")

In this example, Drawable is an interface that defines the draw method as an abstract method. Rectangle and Circle are both subclasses of Drawable that implement the draw method with their own implementations.

Abstraction allows us to create more generic code that can work with objects of different classes, as long as they implement a common interface. This makes our code more modular and easier to maintain.

By the end of this chapter, you should have a strong understanding of object-oriented programming in Python, including classes and objects, inheritance, polymorphism, encapsulation, and abstraction. These concepts are fundamental to creating effective and efficient Python programs, and can be used to solve a wide variety of programming problems.