[Level 2] Singleton in Python.

#!/usr/bin/env python
class Singleton(object):
    _instance = None
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
        return cls._instance

print me
print you 

print me == you        
print id(me) == id(you)

$ ./testSingleton.py
True
True
$ 

You could also use metaclass to implement it.

#!/usr/bin/env python
class Singleton(type):
    def __init__(self, *args, **kwargs):
        super(Singleton, self).__init__(*args, **kwargs)
        self.__instance = None

    def __call__(self, *args, **kwargs):
        if self.__instance is None:
            self.__instance = super(Singleton, self).__call__(*args, **kwargs)
        return self.__instance

class s(object):
    __metaclass__ = Singleton
    def __init__(self, size):
        self.__size = size

    def getSize(self):
        return self.__size

class n(object):
    def __init__(self, size):
        self.__size = size

    def getSize(self):
        return self.__size

a = s(1)
print a.getSize()
b = s(2)
print b.getSize()

c = n(3)
print c.getSize()
d = n(4)
print d.getSize()

print a == b
print c == d

$ ./testMetaclassSingleton.py
1
1
3
4
True
False
$ 

in Python 2.6 or later, you could use decorator to implement Singleton:

#!/bin/env python
def singleton(cls):
    instances = {}
    def getInstance(*args, **kwargs):
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return getInstance

@singleton
class s(object):
    def __init__(self, size):
        self.__size = size

    def getSize(self):
        return self.__size

class n(object):
    def __init__(self, size):
        self.__size = size

    def getSize(self):
        return self.__size

a = s(1)
print a.getSize()
b = s(2)
print b.getSize()

c = n(3)
print c.getSize()
d = n(4)
print d.getSize()

print a == b
print c == d

Wish this helps.
regards,
Stanley Huang