Singleton example in Python

gistfile1.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# Example of Singleton design pattern
# Copyright (C) 2011 Radek Pazdera

# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.

# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.

# You should have received a copy of the GNU General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.


class Singleton:
    # Here will be the instance stored.
    __instance = None

    @staticmethod
    def getInstance():
        """ Static access method. """
        if Singleton.__instance is None:
            Singleton()
        return Singleton.__instance

    def __init__(self):
        """ Virtually private constructor. """
        if Singleton.__instance is not None:
            raise Exception("This class is a singleton!")
        else:
            Singleton.__instance = self


# A little testing

s = Singleton()  # Ok
# Singleton() # will raise exception
print(s)

s = Singleton.getInstance()
print(s)

s = Singleton.getInstance()
print(s)  # will print the same instance as before