python-design-pattern-rag

The Singleton Pattern (Creational)

Purpose

The Singleton pattern ensures that a class has only one instance and provides a global point of access to it. It’s useful for managing shared resources like configuration settings, logging systems, or database connections.

Problem It Solves

In many programs, certain components—such as a settings manager or logger—should only exist once. Creating multiple instances can cause conflicts or waste resources. The Singleton pattern gives you a structured way to enforce a single instance and provide controlled access to it.

When to Use It

Use the Singleton pattern when:

• You need only one instance of a class in your program.
• You want to manage a shared resource (e.g., config, logger) globally.
• Creating multiple instances could cause issues or be inefficient.

When Not to Use It

Avoid this pattern if:

• You need multiple independent instances (use Factory or Prototype patterns instead).
• The Singleton class starts to do too much and becomes hard to maintain.
• You need to write unit tests—Singletons can make testing harder due to hidden state.

How It Works

The class holds a reference to a single instance, typically using a private variable. Access is controlled through a static method like getInstance(), which either returns the existing instance or creates it if it doesn’t exist yet. Some implementations use metaclasses for a cleaner approach.

Real-World Analogy

Imagine a passport office. No matter how many people apply, there’s only one official place that issues passports. That office is a Singleton—everyone interacts with the same one.

Simplified Example

Here’s a basic implementation in Python:

class Singleton:
    _instance = None

    @staticmethod
    def getInstance():
        if Singleton._instance is None:
            Singleton()
        return Singleton._instance

    def __init__(self):
        if Singleton._instance is not None:
            raise Exception("This class is a singleton and cannot be instantiated more than once.")
        Singleton._instance = self

Usage

s1 = Singleton.getInstance()
s2 = Singleton.getInstance()
print(s1 is s2)  # True — both refer to the same instance

This guarantees that only one instance of the Singleton class exists, even if getInstance() is called multiple times.

Learn More

See the full implementation in Python here: Singleton Pattern on GitHub

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