implemented null object design pattern

pypatterns/behavioral/null.py

@@ -0,0 +1,75 @@
+class Null(object):
+    """
+    A class for implementing Null objects.
+
+    This class ignores all parameters passed when constructing or
+    calling instances and traps all attribute and method requests.
+    Instances of it always (and reliably) do 'nothing'.
+
+    The code might benefit from implementing some further special
+    Python methods depending on the context in which its instances
+    are used. Especially when comparing and coercing Null objects
+    the respective methods' implementation will depend very much
+    on the environment and, hence, these special methods are not
+    provided here.
+
+    Dinu C. Gherman,
+    August 2001
+    http://code.activestate.com/recipes/68205-null-object-design-pattern/
+    """
+
+    # object constructing
+
+    def __init__(self, *args, **kwargs):
+        """
+        Ignore parameters.
+        """
+        pass
+
+    # object calling
+
+    def __call__(self, *args, **kwargs):
+        """
+        Ignore method calls.
+        """
+        return self
+
+    # attribute handling
+
+    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
+
+    # misc.
+
+    def __repr__(self):
+        """
+        Return a string representation.
+        """
+        return ''
+
+    def __str__(self):
+        """
+        Convert to a string and return it.
+        """
+        return ''
+
+    def __bool__(self):
+        """
+        Boolean of Null object is always False.
+        """
+        return False

tests/behavioral_tests/test_null.py

@@ -0,0 +1,80 @@
+from unittest import TestCase
+from pypatterns.behavioral.null import Null
+
+
+class NullTestCase(TestCase):
+    """
+    Unit testing class for the Null class.
+    """
+    def setUp(self):
+        """
+        Initialize testing data.
+        """
+        self.null = Null()
+
+    def test_constructing(self):
+        """
+        Test constructing a Null object.
+
+        @raise AssertionError: If the test fails.
+        """
+        try:
+            n = Null()
+            n = Null('value')
+            n = Null('value', param='value')
+        except:
+            raise AssertionError()
+
+    def test_calling(self):
+        """
+        Test calling a Null object.
+
+        @raise AssertionError: If the test fails.
+        """
+        self.assertEquals(self.null, self.null())
+        self.assertEquals(self.null, self.null('value'))
+        self.assertEquals(self.null, self.null('value', param='value'))
+
+    def test_attribute_handling(self):
+        """
+        Test attribute handling on a Null object.
+
+        @raise AssertionError: If the test fails.
+        """
+        self.assertEquals(self.null, self.null.attr1)
+        self.assertEquals(self.null, self.null.attr2)
+        self.assertEquals(self.null, self.null.method1())
+        self.assertEquals(self.null, self.null.method1().method2())
+        self.assertEquals(self.null, self.null.method('value'))
+        self.assertEquals(self.null, self.null.method(param='value'))
+        self.assertEquals(self.null, self.null.method('value', param='value'))
+        self.assertEquals(self.null, self.null.attr1.method1())
+        self.assertEquals(self.null, self.null.method1().attr1)
+
+        try:
+            self.null.attr1 = 'value'
+            self.null.attr1.attr2 = 'value'
+            del self.null.attr1
+            del self.null.attr1.attr2.attr3
+        except:
+            raise AssertionError()
+
+    def test_string_representation(self):
+        """
+        Test the string representation of a Null object.
+
+        @raise AssertionError:
+        """
+        self.assertEquals('', repr(self.null))
+        self.assertEquals('', str(self.null))
+
+    def test_truthiness(self):
+        """
+        Test the truthiness of a Null object.
+
+        @raise AssertionError: If the test fails.
+        """
+        self.assertFalse(bool(self.null))
+        self.assertFalse(bool(self.null.attr1))
+        self.assertFalse(bool(self.null.attr1.method1()))
+        self.assertFalse(bool(self.null.method2().attr2.method1().attr1))