# class Singleton(type):
#     _instances = {}
#    
#     def __call__(cls, *args, **kwargs):
#         if cls not in cls._instances:
#             cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
#         return cls._instances[cls]
 
class Singleton(type):
     _instances = {}
 
     def __call__(cls, *args, **kwargs):
         cls._instances.setdefault(cls, super().__call__(*args, **kwargs))
         return cls._instances[cls]
 
 
# Python2
class MyClass(BaseClass):
    __metaclass__ = Singleton
    pass
 
 
# Python3
class MyClass(BaseClass, metaclass=Singleton):
    pass
 
 
# Test
for x in range(5):
    MyClass().x = x
    print(MyClass().x)

* http://stackoverflow.com/questions/6760685/creating-a-singleton-in-python