Refactor NotifierBuilder into its own mixin

This allows for easier unit testing

flake8/plugins/manager.py

@@ -253,16 +253,8 @@ class PluginTypeManager(object):
         list(self.manager.map(call_provide_options))
 
 
-class Checkers(PluginTypeManager):
-    """All of the checkers registered through entry-ponits."""
-
-    namespace = 'flake8.extension'
-
-
-class Listeners(PluginTypeManager):
-    """All of the listeners registered through entry-points."""
-
-    namespace = 'flake8.listen'
+class NotifierBuilder(object):
+    """Mixin class that builds a Notifier from a PluginManager."""
 
     def build_notifier(self):
         """Build a Notifier for our Listeners.
@@ -279,6 +271,18 @@ class Listeners(PluginTypeManager):
         return notifier_trie
 
 
+class Checkers(PluginTypeManager):
+    """All of the checkers registered through entry-ponits."""
+
+    namespace = 'flake8.extension'
+
+
+class Listeners(PluginTypeManager, NotifierBuilder):
+    """All of the listeners registered through entry-points."""
+
+    namespace = 'flake8.listen'
+
+
 class ReportFormatters(PluginTypeManager):
     """All of the report formatters registered through entry-points."""
 

tests/unit/test_plugin_type_manager.py

@@ -165,3 +165,31 @@ def test_provide_options(PluginManager):
         plugin.provide_options.assert_called_with(optmanager,
                                                   options,
                                                   extra_args)
+
+
+class FakePluginTypeManager(manager.NotifierBuilder):
+    """Provide an easy way to test the NotifierBuilder."""
+
+    def __init__(self, manager):
+        """Initialize with our fake manager."""
+        self.names = sorted(manager.keys())
+        self.manager = manager
+
+
+@pytest.fixture
+def notifier_builder():
+    """Create a fake plugin type manager."""
+    return FakePluginTypeManager(manager={
+        'T100': object(),
+        'T101': object(),
+        'T110': object(),
+    })
+
+
+def test_build_notifier(notifier_builder):
+    """Verify we properly build a Notifier object."""
+    notifier = notifier_builder.build_notifier()
+    for name in ('T100', 'T101', 'T110'):
+        assert list(notifier.listeners_for(name)) == [
+            notifier_builder.manager[name]
+        ]