singleton_meta.py

"""
Singleton: Inheritable version.

This is an example of the Singleton design pattern implemented in Python.
The Singleton pattern is used to ensure that only one instance of a class is created.
All further references to objects of the singleton class refer to the same underlying instance.
The Singleton pattern is also referred to as the Singleton Object Creational Pattern.

In this example, the Singleton class is meant to be inherited.
Classes that inherit from the Singleton class will only be instantiated once.

The instance of the child classes is stored in a dictionary in the metaclass.

For the non-inheritable version, see design_patterns/singleton.py.
"""

class SingletonMeta(type):
    """
    Metaclass for Singleton class.
    A metaclass is a blueprint for a class.
    """
    _instances = dict() # Dictionary to store instances of the class.

    def __call__(cls, *args, **kwargs): # __call__ is called when an instance of the class is created.
        """
        This method is called when an instance of the class is created.
        """
        if cls not in cls._instances: # If the class is not in the dictionary, create an instance of the class and store it in the dictionary.
            instance = super().__call__(*args, **kwargs) # Create an instance of the class.
            cls._instances[cls] = instance # Store the instance in the dictionary.
        return cls._instances[cls] # Return the instance of the class.

class Singleton(metaclass=SingletonMeta):
    """
    Singleton class.
        - Inheritable version.
        - e.g. class MyClass(Singleton):
    """
    pass