# a singleton metaclass, shamelessly lifted off agf's answer at
# https://stackoverflow.com/questions/6760685/creating-a-singleton-in-python

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]
        
