Add instance filter.

Filter.py

@@ -73,3 +73,25 @@ class DependencyFilter(Filter):
         uncompleted = [todo for todo in children if not todo.is_completed()]
 
         return not uncompleted
+
+class InstanceFilter(Filter):
+    def __init__(self, p_todos):
+        """
+        Constructor.
+
+        A filter which selects a number of Todo instances from a TodoList
+        instance.
+
+        This is handy for constructing a view given a plain list of Todo items.
+        """
+        self.todos = p_todos
+
+    def match(self, p_todo):
+        """
+        Returns True when p_todo appears in the list of given todos.
+        """
+        try:
+            self.todos.index(p_todo)
+            return True
+        except ValueError:
+            return False

test/FilterTest.py

@@ -83,3 +83,30 @@ class FilterTest(unittest.TestCase):
 
         self.assertEquals(todolist_to_string(filtered_todos), \
             todolist_to_string(reference))
+
+    def test_filter9(self):
+        """ Test instance filter """
+        todos = load_file('data/FilterTest1.txt')
+        instance_filter = Filter.InstanceFilter(todos[2:])
+
+        filtered_todos = instance_filter.filter(todos)
+
+        self.assertEquals(todos[2:], filtered_todos)
+
+    def test_filter10(self):
+        """ Test instance filter """
+        todos = load_file('data/FilterTest1.txt')
+        instance_filter = Filter.InstanceFilter([])
+
+        filtered_todos = instance_filter.filter(todos)
+
+        self.assertEquals([], filtered_todos)
+
+    def test_filter11(self):
+        """ Test instance filter """
+        todos = load_file('data/FilterTest1.txt')
+        instance_filter = Filter.InstanceFilter(todos[2:])
+
+        filtered_todos = instance_filter.filter([])
+
+        self.assertEquals([], filtered_todos)