Null object pattern (Python)

null.py

"""
This module provides the Null class for the null object design pattern.
"""

from collections import MutableMapping, MutableSequence

class _MetaNull(type): pass

class Null(type):
    """Null object design pattern.
    
    This class traps and ignores everything. Instances of it always and
    reliably do nothing, or return 0, '', False, or other logical,
    contextually-appropriate emptiness or negation.
    
    >>> Null()(1, kwarg=2)()[3]['4'] is Null
    True
    >>> Null.method().attr is Null
    True
    >>> Null * 1 / 2 + 3 == Null
    <Null>
    >>> bool(Null), str(Null), repr(Null), int(Null), len(Null)
    (False, '', '<Null>', 0, 0)
    >>> [i for i in Null]
    []

    Roughly, the goal with Null objects is to provide an 'intelligent'
    replacement for the often used primitive data type None in Python
    or Null (or Null pointers) in other languages. These are used for
    many purposes including the important case where one member of some
    group of otherwise similar elements is special for whatever reason.
    Most often this results in conditional statements to distinguish
    between ordinary elements and the primitive Null value.

    Among the advantages of using Null objects are the following:

      * Superfluous conditional statements can be avoided 
        by providing a first class object alternative for 
        the primitive value None.

      * Code readability is improved.

      * Null objects can act as a placeholder for objects 
        with behaviour that is not yet implemented.

      * Null objects can be replaced for any other class.

      * Null objects are very predictable at what they do.

    To cope with the disadvantage of creating large numbers of passive 
    objects that do nothing but occupy memory space Null objects are 
    often combined with the Singleton pattern.

    For more information use any internet search engine and look for 
    combinations of these words: Null, object, design and pattern.

    Dinu C. Gherman,
    August 2001
    """
    __metaclass__ = _MetaNull
    __hash__ = type.__hash__
    __slots__ = ()
    def __next__(_): raise StopIteration
    next = __next__  # Python 2 compatibility
    __dir__ = lambda _: []
    __str__ =   lambda _: ''
    __repr__ =  lambda _: '<Null>'
    __bytes__ = lambda _: b''
    __int__ =   \
    __len__ =   \
    __index__ = \
    __round__ = \
    __complex__ = lambda *_: 0
    __float__ =   lambda *_: 0.
    __eq__ = \
    __ge__ = \
    __gt__ = \
    __le__ = \
    __lt__ = \
    __ne__ = \
    __or__ = \
    __and__ = \
    __del__ = \
    __get__ = \
    __set__ = \
    __abs__ = \
    __add__ = \
    __mod__ = \
    __mul__ = \
    __new__ = \
    __neg__ = \
    __pos__ = \
    __pow__ = \
    __ror__ = \
    __sub__ = \
    __xor__ = \
    __call__ = \
    __exit__ = \
    __init__ = \
    __iter__ = \
    __radd__ = \
    __rand__ = \
    __rmod__ = \
    __rmul__ = \
    __rpow__ = \
    __rsub__ = \
    __rxor__ = \
    __enter__ =  \
    __delete__ = \
    __divmod__ = \
    __invert__ = \
    __lshift__ = \
    __rshift__ = \
    __delattr__ = \
    __delitem__ = \
    __getattr__ = \
    __getitem__ = \
    __rdivmod__ = \
    __rlshift__ = \
    __rrshift__ = \
    __setattr__ = \
    __setitem__ = \
    __truediv__ = \
    __floordiv__ = \
    __rtruediv__ = \
    __rfloordiv__ = \
    __getattribute__ = lambda self, *_, **__: self

Null.__class__ = Null


class nulldict(dict):
    def __missing__(self, key):
        return Null


class nulllist(list):
    def __getitem__(self, i):
        return list.__getitem__(self, i) if i < len(self) else Null

        
class nulltuple(tuple):
    def __getitem__(self, i):
        return tuple.__getitem__(self, i) if i < len(self) else Null


def nullify(obj):
    if isinstance(obj, MutableMapping):
        return nulldict(obj)
    if isinstance(obj, tuple):
        return nulltuple(obj)
    if isinstance(obj, MutableSequence):
        return nulllist(obj):
    return obj or Null