First version of the internal filtering capability

The Manager is already capable of receiving a filter callback function
to filter the returned implementors list.

ref #11

plugnplay/manager.py

@@ -11,5 +11,6 @@ def add_implementor(self, interface, implementor_instance):
         self.iface_implementors.setdefault(interface, [])
         self.iface_implementors[interface].append(implementor_instance)
 
-    def implementors(self, interface):
-        return self.iface_implementors.get(interface, [])
+    def implementors(self, interface, filter_callback=None):
+        all_implementors = self.iface_implementors.get(interface, [])
+        return filter(filter_callback, all_implementors)

tests/manager.py

@@ -40,3 +40,20 @@ def test_add_second_implementor_of_an_interface(self):
     '''
     def test_return_empty_list_of_implementors(self):
         self.assertEquals(0, len(self.man.implementors(SomeInterface)))
+
+
+class FilteredImplementorsTest(unittest.TestCase):
+
+    def setUp(self):
+        self.man = Manager()
+
+    def test_simple_call_back_no_parameters(self):
+        def _simple_callback_filter(implementor):
+            return implementor.__class__.__name__.endswith("A")
+        implementor_a = PlugA()
+        self.man.add_implementor(SomeInterface, implementor_a)
+        self.man.add_implementor(SomeInterface, PlugB())
+        assert 2 == len(self.man.implementors(SomeInterface))
+        filtered_implementors = self.man.implementors(SomeInterface, _simple_callback_filter)
+        assert 1 == len(filtered_implementors)
+        assert [implementor_a] == filtered_implementors