Python Singleton Class: Implementing the Singleton Design Pattern

Learn about Singleton classes in Python and how they ensure only one instance of a class is created. Discover methods for implementing Singleton classes using __init__ and __new__ to optimize memory usage and manage resources efficiently.

Python - Singleton Class

Introduction

In Python, a Singleton class is the implementation of the singleton design pattern, which ensures that a class can have only one object. This helps optimize memory usage for heavy operations like creating a database connection. If multiple objects are created for a singleton class, only the first object is created, and subsequent instances return the same object.

Creating Singleton Classes in Python

Singleton classes in Python can be implemented using the following methods:

Using __init__
Using __new__

Using `init`

The __init__ method is an instance method used for initializing a newly created object. It’s automatically called when an object is created from a class. To create a singleton class using this method, we need to check if an instance of the class already exists and restrict the creation of a new object after the first one.

Example


class Singleton:
    __uniqueInstance = None

    @staticmethod
    def createInstance():
        if Singleton.__uniqueInstance == None:
            Singleton()
        return Singleton.__uniqueInstance

    def __init__(self):
        if Singleton.__uniqueInstance != None:
            raise Exception("Object already exists!")
        else:
            Singleton.__uniqueInstance = self

# Creating instances
obj1 = Singleton.createInstance()
print(obj1)
obj2 = Singleton.createInstance()
print(obj2)

Output


<__main__.Singleton object at 0x7e4da068a910>
<__main__.Singleton object at 0x7e4da068a910>

Using `new`

The __new__ method is a special static method called to create a new instance of a class. It takes the class itself as the first argument and returns a new instance of that class. To implement a Singleton class using __new__, we override this method, check if an instance of the class already exists, and create a new object only if it doesn't.

Example


class SingletonClass:
    _instance = None

    def __new__(cls):
        if cls._instance is None:
            print('Creating the object')
            cls._instance = super(SingletonClass, cls).__new__(cls)
        return cls._instance

# Creating instances
obj1 = SingletonClass()
print(obj1)

obj2 = SingletonClass()
print(obj2)

Output


Creating the object
<__main__.SingletonClass object at 0x000002A5293A6B50>
<__main__.SingletonClass object at 0x000002A5293A6B50>

Python Singleton Class: Implementing the Singleton Design Pattern