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 == None:
            Singleton()
        return Singleton.__instance 

    def __init__(self):
        """ Virtually private constructor. """
        if Singleton.__instance != 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

== and != None should really be is and is not, but otherwise this is quite good.

Thanks!!

I made a slight variation of this that uses the functools @lru_cache decorator instead of a class property to hold the instance.