They are all mapping functions, though. You have a key 'k', which is accepted by d and will return a unique result. Why should I have to care whether d is a collection, object, or method? In fact in many cases it's pretty easy to implement all three.

    class months(object):
        def __init__(self):
            m='jan feb mar apr may jun jul aug sep oct nov dec'.split(' ')
            n=range(1,13)
            self.__dict__.update(dict(zip(m,n)))
        def __getitem__(self, key):
            return self.__dict__[key]
        def __call__(self, key):
            return self.__dict__[key]
    
    d=months()
    
    print d['jan']  # 1
    print d('jan')  # 1
    print d.jan     # 1