Add null object pattern

README.md

@@ -29,6 +29,7 @@ Current Patterns:
 | [mediator](mediator.py) | an object that knows how to connect other objects and act as a proxy |
 | [memento](memento.py) | generate an opaque token that can be used to go back to a previous state |
 | [mvc](mvc.py) | model<->view<->controller (non-strict relationships) |
+| [null object](null_object.py) | Avoid null references by providing a default object |
 | [observer](observer.py) | provide a callback for notification of events/changes to data |
 | [pool](pool.py) | preinstantiate and maintain a group of instances of the same type |
 | [prototype](prototype.py) | use a factory and clones of a prototype for new instances (if instantiation is expensive) |

null_object.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+"""
+Author: https://github.com/fousteris-dim
+Reference: http://en.wikipedia.org/wiki/Null_Object_pattern
+"""
+
+class Item:
+	"""Represents an abstract item"""
+	def __init__(self):
+		self._value = None
+
+	def setValue(self, value):
+		self._value = value
+
+class RealItem(Item):
+	"""Represent a valid real item"""
+	def __init__(self, value):
+		self._value = value
+
+	def printValue(self):
+		print 'The value is: ' + str(self._value)
+
+class NullItem(Item):
+	"""Represents a null item. This is treated like real items except having no value"""
+	def printValue(self):
+		print '[Null object. No value]'
+
+class ItemStore:
+	"""Stores key/value pairs of items"""
+	nullItem = NullItem()
+
+	def __init__(self):
+		self._items = {};
+
+	def getItem(self, name):
+		return self._items.setdefault(name, ItemStore.nullItem)
+
+	def setItem(self, name, value):
+		item = self._items.setdefault(name, None)
+		if item != None:
+			item.setValue(value)
+		else:
+			self._items[name] = RealItem(value)
+
+itemstore = ItemStore()
+itemstore.setItem('item2', 'test item2')
+
+itemstore.getItem('item2').printValue()
+itemstore.getItem('item3').printValue()
+
+### OUTPUT ###
+# The value is: test item2
+# [Null object. No value]