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

Thanks, so helpful!

Thanks bro, after Math-like Java Python is very unstable.

Can you please look into this and let me know which one is the correct approach. Singleton Class in Python

thanks, very good.

Super! Thank you!

FYI to anyone using this in the future, this is NOT thread safe.

@ryandgoulding

FYI to anyone using this in the future, this is NOT thread safe.

Any solution to this?

Nice..

Well designed.

== 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.