Inheritance and Composition in Object Oriented Programming: Understanding the Basics
When it comes to object-oriented programming, the concepts of inheritance and composition are crucial to understanding how to create reusable code. They allow developers to effectively reuse existing code and create new code from that foundation.
In this article, we will explore the basics of inheritance and composition, their differences, and how they help developers create solutions to programming problems.
Definition and Importance
Inheritance refers to an object-oriented programming (OOP) relationship in which one class (the derived class) inherits properties and methods from another class (the base class). On the other hand, composition also pertains to an OOP relationship, where an object is made up of other objects.
So why is this important? Inheritance and composition are two techniques in OOP that promote code reuse.
Code reuse is important because it minimizes code redundancy and makes the code easier to maintain, which leads to a more efficient development process. Moreover, it allows the reuse of tested and proven code, saving both time and resources.
What is Inheritance?
Inheritance is a crucial aspect of object-oriented programming because it makes it easy to create new classes based on existing ones.
Inheritance is a basic pattern in OOP that enables the creation of a derived class as a specialized version of a base class, which is more general. Once derived class(es) inherit from a base class, the derived class will have all the methods and attributes of the base class.
Is a Relationship
Is a relationship, which is also referred to as an inheritance relationship, is the term used to describe a derived class’s relationship to a base class. The term “is a” represents the relationship of the derived class and base class.
For example, if we have a class called Animal, and we create a new class called Dog, we can say that a Dog “is a” Animal.
Derived Class
A derived class is a type of class that inherits data and behavior from another class known as a base class. A derived class is committed to using the properties and methods inherited from its base class.
In OOP, a derived class can also modify the derived class of its base class to add its own behavior and data.
Base Class
A base class is a class used as a foundation for creating derived classes. Base classes provide a common interface that allows derived classes to communicate with the outside world consistently.
Base classes allow developers to provide an empty shell that other classes can inherit from and modify.
Interface and Implementation
An interface is a set of behavior that objects can use even if the object’s class does not implement all the necessary methods. An interface class is used to describe a set of methods that should be implemented by any class that implements the interface.
On the other hand, implementation provides the specific behavior of a class. A class that implements an interface class provides the specific behavior, while the interface class indicates the expected properties available to the user.
Liskov Substitution Principle
The Liskov substitution principle (LSP) is a crucial concept in the field of Object-Oriented Programming. It is a principle that identifies what should and should not be a working substitute for an object.
It states that if a program is designed using a base class, it should be interchangeable with objects of any derived class. Put simply, any derived class should be able to be replaced by its base class without changing the program’s functionality.
What is Composition?
Composition, in OOP, is a technique that allows the creation of a complex object from simpler objects.
It is a way of combining simpler objects to create a whole, more complex object. Composition is essential for creating reusable code because it enables the utilization of existing objects as modules.
Has a Relationship
Has a relationship describes the relationship between two objects where one object is a composite of the other. In this situation, the composite object holds a reference to the other object, which holds the data that it needs to operate.
Composition is an essential design pattern in Object-Oriented Programming. It allows the creation of objects and allows these objects to call the methods of the objects they are composed of.
By breaking down complex objects into simpler objects, it is possible to manage each of those object types effectively.
Conclusion:
Inheritance and composition are crucial components of Object-Oriented Programming. They allow developers to use existing code effectively and create new code from existing code.
Understanding the difference between the two techniques is critical to optimizing code reusability. Inheritance creates a hierarchical relationship between classes, while composition creates relationships between objects by combining simpler objects to form a complex one.
Mastering these two techniques can aid developers in creating solutions that solve programming problems and provide efficient software design.
An Overview of Inheritance in Python
Inheritance is a powerful tool in object-oriented programming that enables code reuse and creates relationships between different classes. In Python, every class is derived from an object type called the Object class.
In this article, we will explore the Object super class and creating class hierarchies in Python.
The Object Super Class
In Python, every object is an instance of the Object class. The Object class is a built-in super class that is automatically inherited by every Python class created.
Every new class created is inherited from the Object class at the top of the inheritance hierarchy. Thus, Python follows a single inheritance model, where a class can only inherit from a single base class.
The Object class makes it easy to create new classes by providing a set of pre-defined methods and attributes that are inherited by all classes derived from it. Additionally, the methods and attributes defined in the Object class can be overridden or extended by the derived class.
Creating Class Hierarchies
Class hierarchies are the relationships between classes that Python uses to provide code reuse and inheritance. In Python, class hierarchies are implemented using a base class, which is then inherited by other classes to create a hierarchy.
The base class is the highest level in the hierarchy and is used to provide default functionality that is inherited by the derived classes.
Base Class Employee
To illustrate creating class hierarchies, let’s assume that we are building an HR system to calculate employee payrolls. We can start by creating a base class called Employee, which will be inherited by other classes in the hierarchy.
The Employee class will include an initialization method with the employee’s name, and an empty method called calculate_payroll(), which will be implemented by the derived classes.
class Employee:
def __init__(self, name):
self.name = name
def calculate_payroll(self):
pass
Derived Classes
To create derived classes, we would inherit from the base class Employee and add unique functionality to it. We would start by creating an IPayrollCalculator interface that sets the blueprint for classes that calculate payroll and implements a calculate_payroll() method.
from abc import ABC, abstractmethod
class IPayrollCalculator(ABC):
@abstractmethod
def calculate_payroll(self):
pass
Next, we would create three classes that derive from Employee – SalaryEmployee, HourlyEmployee, and CommissionEmployee. The SalaryEmployee class would include an annual salary in the initialization and implement the calculate_payroll method by returning the annual salary divided by the number of pay periods in a year.
class SalaryEmployee(Employee, IPayrollCalculator):
def __init__(self, name, annual_salary):
super().__init__(name)
self.annual_salary = annual_salary
def calculate_payroll(self):
return self.annual_salary / 12
The HourlyEmployee class would include an hourly wage and the number of hours worked in a pay period in the initialization. The calculate_payroll method would multiply the hourly wage by the number of hours worked in a pay period.
class HourlyEmployee(Employee, IPayrollCalculator):
def __init__(self, name, hours_worked, hourly_wage):
super().__init__(name)
self.hours_worked = hours_worked
self.hourly_wage = hourly_wage
def calculate_payroll(self):
return self.hours_worked * self.hourly_wage
The CommissionEmployee class would include a commission percentage and sales made during a pay period in the initialization. The calculate_payroll method would calculate the employee’s commission by multiplying the commission percentage by the sales made during the pay period.
class CommissionEmployee(Employee, IPayrollCalculator):
def __init__(self, name, commission_percentage, sales):
super().__init__(name)
self.commission_percentage = commission_percentage
self.sales = sales
def calculate_payroll(self):
return self.commission_percentage * self.sales
UML Diagram
To visualize the inheritance hierarchy, we can use a Unified Modeling Language (UML) diagram. In the diagram below, we can see that the Employee class is the base class and serves as the template for the derived classes.
The IPayrollCalculator interface ensures that each derived class has a calculate_payroll() method.
_______________
| Employee |
|-------+-------|
| name:string |
+---------------+
| calculate_payroll()
+---------------+
|
|
________|_______
| IPayroll |
| Calculator |
|_______+_______|
|calculate_payroll()|
+-------------------+
|
|
+-------------------+
| SalaryEmployee |
|---------+---------|
| annual_salary: int |
+-------------------+
| calculate_payroll()|
+-------------------+
|
|
+-------------------+
| HourlyEmployee |
|---------+---------|
| hours_worked: int |
| hourly_wage: int |
+-------------------+
| calculate_payroll()|
+-------------------+
|
|
+--------------------+
| CommissionEmployee |
|----------+---------|
| commission: int |
| sales: int |
+--------------------+
| calculate_payroll() |
+--------------------+
Conclusion
Python's inheritance hierarchy enables code reuse and promotes object-oriented programming best practices. By creating a base class with default functionality that is inherited by derived classes, we can reuse code and make development more efficient.
The ability to override or extend base class methods and attributes provides flexibility without sacrificing efficiency. Finally, the use of UML diagrams can help visualize the inheritance hierarchy, making it easier to understand for other developers working on the project.
Abstract Base Classes in Python
In Python, Abstract Base Classes (ABCs) promote polymorphism by allowing classes to declare the methods and properties they need to be considered compatible with their interfaces. In this article, we will explore Abstract Base Classes in Python and their implementation in the Employee class.
Abstract Base Classes
Abstract Base Classes provide a way to define a set of abstract methods and attribute requirements that an object must satisfy to be considered a 'conforming' implementation of the class.
In Python, the abc module provides the ABC class that can be inherited to create an abstract base class. For example, we can define an abstract method in the ABC by using the '@abstractmethod' decorator.
This decorator specifies that the method must be implemented in derived classes which inherit from this abstract base class.
from abc import ABC, abstractmethod
class MyABC(ABC):
@abstractmethod
def foo(self):
pass
@abstractmethod
def bar(self):
pass
The above example shows how we can create an abstract class 'MyABC' by inheriting from the 'ABC' base class provided by the 'abc' module.
An abstract method is a method that is declared but contains no implementation. In this case, 'foo' and 'bar' are abstract methods and must be implemented by any class that inherits from the MyABC class.
Implementation of Abstract Base Class in Employee
To illustrate the implementation of an abstract base class in Python, let's take a look at the Employee class from the previous section. In the inheritance hierarchy example, we defined an interface class 'IPayrollCalculator.' We can use this interface class to create an Abstract Base Class.
from abc import ABC, abstractmethod
class Employee(ABC):
def __init__(self, name):
self.name = name
@abstractmethod
def calculate_payroll(self):
pass
In this implementation, the Employee class is derived from the ABC class in the abc module, making it an abstract base class. The 'calculate_payroll' method is defined as an abstract method, with the '@abstractmethod' decorator.
This method must be implemented in any derived class that inherits from the Employee class.
Composition
Composition is a technique in OOP used to build complex objects out of simpler objects or components. Composition is achieved by creating a class that is composed of one or more objects of other classes.
Has a Relationship
Has a relationship, which is also known as composition or containment, is a relationship where a composite object (the container) holds a reference to another object or set of objects (the components). In UML diagrams, the has a relationship is depicted as a diamond pointing to the component class.
The cardinality notation determines the allowed number of instances of the component class that are contained in the composite class. For example, a zero-to-many relationship could have a cardinality notation of "0..*", indicating zero or many instances of the component class are allowed.
Comparison with Inheritance
Inheritance and Composition are both essential techniques in Object-Oriented Programming. They allow developers to reduce code redundancy and promote code reuse.
Inheritance creates a hierarchy of objects, where derived classes can reuse functions, methods, and attributes from their base class while still being able to add their own unique functionality. Inheritance aims to reduce the amount of duplicated code by isolating common functionality in a base class.
On the other hand, Composition allows objects to be created out of simpler objects or components. In Composition, when a composite object changes, it's the components that are changed, not the composite class itself.
Composition allows developers to create complex objects from simpler ones by combining their individual states and functionality.
A significant difference between inheritance and composition is that inheritance represents an 'Is-a' relationship, where the derived class is a specialized version of the base class.
While in Composition, the has-a relationship refers to the composite class containing the components.
Conclusion
Abstract Base Classes in Python help developers define abstract methods and attribute requirements that a class implementing the ABC must satisfy. Through them, we can achieve a more uniform interface to make codes much more modular and easier to maintain.
Composition also provides flexible development and great control over the complexity of a given system and can also be used in conjunction with other OOP techniques like inheritance. Choosing between inheritance and composition, or combining them, requires careful consideration and must be tailored to the problem being solved.
Composition in Python
Composition is a technique in object-oriented programming that promotes code reusability and allows developers to create more flexible designs. In this article, we will explore how composition enables flexible designs and customizes behavior.
We will also examine how to choose between inheritance and composition in Python.
Flexible Designs with Composition
Composition is a powerful tool that allows developers to create flexible designs by combining simpler classes to create more complex ones. It enables the development of highly specialized classes by reusing the parts of existing classes instead of using inheritance, which can quickly become complicated.
The composite class can hold a reference to variables and methods defined in its component classes.
Customizing Behavior with Composition
Using composition, we can customize the behavior of a class by composing it of different components.
By breaking down any given system into its components, we create flexibility in how we change its behavior. Let's take an example of a car system having a Brake System, Fuel System, and Engine System.
We can use composition to change the behavior of our car at runtime. This means that we can swap any component like changing the engine of our car, or swapping the brake system as needed.
Choosing Between Inheritance and Composition in Python
Choosing between inheritance and composition can be a challenging task when trying to design new classes or refactor existing code. Here are some guidelines to help decide which technique to use.
Inheritance to Model "Is A" Relationship
Inheritance should be used to model an 'is a' relationship.