Singleton with Decorator

In Python, a singleton is a design pattern that restricts the instantiation of a class to a single object. A singleton class ensures that only one instance of the class is created and provides a global point of access to that instance.

One way to implement a singleton in Python is to use a decorator. Here’s an example:

def singleton(cls):
    instances = {}

    def get_instance(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]

    return get_instance

@singleton
class MyClass:
    def __init__(self, x):
        self.x = x

my_obj1 = MyClass(42)
my_obj2 = MyClass(100)

print(my_obj1.x)  # Output: 42
print(my_obj2.x)  # Output: 42
print(my_obj1 is my_obj2)  # Output: True

In this example, the singleton decorator takes a class as an argument and returns a new function that acts as a factory for creating instances of the class. The instances dictionary is used to keep track of the instances created so far.

The get_instance function checks if an instance of the class already exists in the instances dictionary. If it does, it returns that instance. If not, it creates a new instance of the class and adds it to the instances dictionary before returning it.

Finally, the @singleton decorator is applied to the MyClass class, which wraps it with the get_instance function. This ensures that only one instance of MyClass is created and returned by the get_instance function.

When the MyClass constructor is called with different arguments to create my_obj1 and my_obj2 objects, the constructor initializes the x attribute of each object with the respective argument value. However, when we access the x attribute of my_obj2, we get the same value as my_obj1, because they are both instances of the same MyClass object.