class NULLType(type):

    def __new__(cls, name, bases, dct):
        return type.__new__(cls, name, bases, dct)

def __init__(cls, name, bases, dct):
    super(NULLType, cls).__init__(name, bases, dct)

def __str__(self):
    return ''

def __repr__(self):
    return 'NULL'

def __nonzero__(self):
    return False

def __len__(self):
    return 0

def __call__(self, *args, **kwargs):
    return None

def __contains__(self, item):
    return False

def __iter__(self):
    return self

def next(*args):
    raise StopIteration
NULL = NULLType('NULL', (type,), {})
Пользовательский заполнитель, например None в python