macleod2.py

class Highlander(type):
    born = False

    def __call__(cls, *args, **kwargs):
        if cls.born:
            raise RuntimeError("THERE CAN BE ONLY ONE !") # You cannot make more than 1 instance of a Highlander class, it's too dangerous to have 2 !
        cls.born = man = super(Highlander, cls).__call__(*args, **kwargs)
        return man

class MacLeod(object):
    __metaclass__ = Highlander

man = MacLeod()

man2 = MacLeod() # raises epic RuntimeError