python-patterns/null.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""http://code.activestate.com/recipes/68205-null-object-design-pattern/"""


class Null:

    def __init__(self, *args, **kwargs):
        """Ignore parameters."""
        return None

    def __call__(self, *args, **kwargs):
        """Ignore method calls."""
        return self

    def __getattr__(self, mname):
        """Ignore attribute requests."""
        return self

    def __setattr__(self, name, value):
        """Ignore attribute setting."""
        return self

    def __delattr__(self, name):
        """Ignore deleting attributes."""
        return self

    def __repr__(self):
        """Return a string representation."""
        return "<Null>"

    def __str__(self):
        """Convert to a string and return it."""
        return "Null"


def test():
    """
    Perform some decent tests, or rather: demos.
    >>> print(Null())
    Null
    >>> print(Null('value'))
    Null
    >>> n= Null('vale',param='value')
    >>> print(n)
    Null
    >>> n()
    <Null>
    >>> n('value')
    <Null>
    >>> n('value', param='value')
    <Null>
    >>> print(n)
    Null
    """

    # constructing and calling

    #n = Null()
    #print(n)

    #n = Null('value')
    #print(n)

    n = Null('value', param='value')
    #print(n)

    n()
    #n('value')
    #n('value', param='value')
    #print(n)

    # attribute handling

    n.attr1
    print('attr1', n.attr1)
    n.attr1.attr2
    n.method1()
    n.method1().method2()
    n.method('value')
    n.method(param='value')
    n.method('value', param='value')
    n.attr1.method1()
    n.method1().attr1

    n.attr1 = 'value'
    n.attr1.attr2 = 'value'

    del n.attr1
    del n.attr1.attr2.attr3

    # representation and conversion to a string

    assert repr(n) == '<Null>'
    assert str(n) == 'Null'


if __name__ == '__main__':
    test()
    import doctest
    doctest.testmod()
### OUTPUT ###
# Null
# Null
# Null
# Null
# attr1 Null