Python singleton

singleton.py

# -*- coding: utf-8 -*-

"""Trying to create a singleton."""


class Singleton(object):
    """The public class."""

    instance = None

    class _InnerSingleton(object):
        """The actual object that is instantiated. Define your class here."""

        def __init__(self, instance_property=""):
            self.instance_property = instance_property

    def __init__(self):
        """
        Do not instantiate this class directly.

        Raises:
            NotImplementedError: This class is not supposed to be instantiated
                directly. Trying to do so raises this exception.

        """
        err_msg = (
            "This class can not be instantiated directly."
            + " There can only be one instance of this class."
            + " Retrieve the existing instance with"
            + " ``Singleton.get_instance()``."
        )
        raise NotImplementedError(err_msg)

    @classmethod
    def get_instance(cls):
        """
        Get instance of the singleton class.

        Returns:
            Instance of the (inner) singleton class.

        """
        if cls.instance is None:
            print("No instance exists yet. Instantiating...")
            cls.instance = cls._InnerSingleton()
        else:
            print("Singleton is already instantiated. Returning instance...")
        return cls.instance


A = Singleton.get_instance()
# No instance exists yet. Instantiating...
print(A)
# <__main__.Singleton._InnerSingleton object at 0x102476b38>

B = Singleton.get_instance()
# Singleton is already instantiated. Returning instance...
print(B)
# <__main__.Singleton._InnerSingleton object at 0x102476b38>

A.instance_property = "some value"
print(A.instance_property)
# some value
print(B.instance_property)
# some value


# direct = Singleton()
# >>> direct = Singleton()
# Traceback (most recent call last):
#   File "singleton.py", line 18, in <module>
#     direct = Singleton()
#   File "singleton.py", line 15, in __init__
#     raise NotImplementedError(err_msg)
# NotImplementedError: This class can not be instantiated directly. There can only be one instance of this class. Retrieve the existing instance with `Singleton.get_instance()`.