Source code for Creational.Singleton.python.singleton

"""
singleton.py
Implementation of the Singleton design pattern in Python.

This module defines a Singleton class to ensure that only one instance
of the class is created during the program's lifetime.
"""



class SingletonMeta(type):
	"""
	A metaclass for creating Singleton classes.

	Ensures that only one instance of the class exists.
	"""
	_instances = {}
	
	def __call__(cls, *args, **kwargs):
		"""
		Creates and retrieves the Singleton instance for the given class.

		The method first checks if the class `cls` is not already in the
		`_instances` dictionary of the metaclass. If it's not, it creates a new
		instance of the class using the `super().__call__(*args, **kwargs)`
		method, which calls the `__call__` method of the parent class. The new
		instance is then stored in the `_instances` dictionary with the class
		`cls` as the key.

		Finally, the method returns the instance stored in the `_instances`
		dictionary for the class `cls`. This ensures that only one instance
		of the class is created and allows for easy retrieval of that instance
		in the future.
		"""
		if cls not in cls._instances:
			instance = super().__call__(*args, **kwargs)
			cls._instances[cls] = instance
		return cls._instances[cls]





class Singleton(metaclass=SingletonMeta):
	"""
	A Singleton class that uses the SingletonMeta metaclass.
	"""


	def do_something(self):
		"""
		An example method demonstrating Singleton functionality.

		This method performs an example action to showcase how to use
		the Singleton instance.

		:returns: A message indicating that the Singleton instance is working.
		:rtype: str
		"""
		return "Singleton instance is working!"




# Example usage
if __name__ == "__main__":
	s1 = Singleton()
	s2 = Singleton()

	print(s1 is s2)  # True
	print(s1.do_something())  # "Singleton instance is working!"