Design Patterns Overview (Factory, Singleton, Observer, Strategy)

4 minute read

Design patterns are essential tools in software development, offering reusable solutions to common problems faced by developers. They encapsulate best practices, making code more efficient, maintainable, and scalable. In this tutorial, we’ll delve into four fundamental design patterns: Factory, Singleton, Observer, and Strategy, understanding their concepts, exploring real-world examples, and learning how to implement them in code.

Introduction to Design Patterns

Design patterns are recurring solutions to common problems encountered in software design. They provide a template for solving issues in a particular context, ensuring code quality and maintainability. Understanding design patterns is crucial for every software developer as they streamline the development process and promote code reusability.

Factory Design Pattern

The Factory Design Pattern is a creational pattern that provides an interface for creating objects in a superclass but allows subclasses to alter the type of objects that will be created. This pattern is beneficial when the exact type of object to be created is not known until runtime.

For instance, consider a scenario where an application needs to generate different types of documents such as PDFs, spreadsheets, and text files. By using the Factory pattern, we can create a DocumentFactory interface with methods for creating each type of document. Subclasses like PDFDocumentFactory and SpreadsheetDocumentFactory can then implement these methods to produce specific document types.

class DocumentFactory:
    def create_document(self):

class PDFDocumentFactory(DocumentFactory):
    def create_document(self):
        return PDFDocument()

class SpreadsheetDocumentFactory(DocumentFactory):
    def create_document(self):
        return SpreadsheetDocument()

The Factory pattern simplifies object creation, promotes loose coupling, and enhances code scalability. However, it may lead to a proliferation of subclasses if not used judiciously.

Singleton Design Pattern

The Singleton Design Pattern ensures that a class has only one instance and provides a global point of access to it. This pattern is useful when exactly one object is needed to coordinate actions across the system.

A typical example of Singleton is a logging class that maintains a single log file throughout the application’s lifecycle. Regardless of where logging is required, the Singleton pattern ensures that all components share the same logging instance.

class Logger:
    __instance = None

    def get_instance():
        if Logger.__instance is None:
        return Logger.__instance

    def __init__(self):
        if Logger.__instance is not None:
            raise Exception("This class is a singleton!")
            Logger.__instance = self

Singletons provide a global point of access, conserve memory, and ensure thread safety. However, they can hinder unit testing and introduce tight coupling if overused.

Observer Design Pattern

The Observer Design Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. This pattern is prevalent in event handling systems.

Suppose we have a weather station that notifies various display devices whenever the weather conditions change. We can implement the Observer pattern by defining a subject (the weather station) and multiple observers (display devices) that register themselves with the subject.

class Subject:
    def __init__(self):
        self._observers = []

    def attach(self, observer):

    def detach(self, observer):

    def notify(self):
        for observer in self._observers:

class Observer:
    def update(self, subject):

class WeatherStation(Subject):
    def set_temperature(self, temperature):
        self.temperature = temperature

class DisplayDevice(Observer):
    def update(self, subject):
        if isinstance(subject, WeatherStation):
            print(f"Current temperature: {subject.temperature}")

The Observer pattern facilitates loose coupling between objects, allowing for easy modification and extension. However, it can result in unexpected updates if not carefully implemented.

Strategy Design Pattern

The Strategy Design Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from clients that use it.

Consider a scenario where an application needs to perform sorting based on different criteria such as alphabetical order or numerical order. By employing the Strategy pattern, we can define sorting algorithms as separate classes and switch between them dynamically.

class SortStrategy:
    def sort(self, data):

class BubbleSortStrategy(SortStrategy):
    def sort(self, data):
        # Implementation of bubble sort

class QuickSortStrategy(SortStrategy):
    def sort(self, data):
        # Implementation of quick sort

class SortingContext:
    def __init__(self, strategy):
        self._strategy = strategy

    def set_strategy(self, strategy):
        self._strategy = strategy

    def sort_data(self, data):

The Strategy pattern promotes code reuse, enhances flexibility, and simplifies testing. However, it may increase the number of objects in the system, leading to higher memory consumption.

Comparison of Design Patterns

Each design pattern serves a specific purpose and has its strengths and weaknesses. The Factory pattern excels in creating objects without specifying their exact class, while the Singleton pattern ensures a single instance of a class. The Observer pattern facilitates communication between objects, whereas the Strategy pattern enables dynamic algorithm selection.

Choosing the right design pattern depends on the problem at hand and the desired outcomes. By understanding the principles and characteristics of each pattern, developers can make informed decisions to design robust and scalable software systems.


In conclusion, design patterns are invaluable tools for software developers, offering proven solutions to recurring problems in software design. The Factory, Singleton, Observer, and Strategy patterns discussed in this tutorial provide effective ways to enhance code maintainability, flexibility, and scalability. By incorporating these patterns into their projects, developers can write cleaner, more efficient code and build robust software systems that meet evolving requirements.


  1. Why are design patterns important in software development? Design patterns promote code reuse, maintainability, and scalability, reducing development time and


  1. How do design patterns improve code quality? Design patterns encapsulate best practices, making code more modular, flexible, and easier to understand and maintain.

  2. When should I use the Singleton pattern? The Singleton pattern is suitable when exactly one instance of a class is needed globally throughout the application.

  3. What is the difference between the Observer and Strategy patterns? The Observer pattern facilitates communication between objects, while the Strategy pattern enables dynamic algorithm selection.

  4. Can design patterns be overused? Yes, using design patterns excessively can lead to overly complex code and hinder readability. It’s essential to apply patterns judiciously based on the specific requirements of the project.