Expose loaded config files, make load idempotent

traitlets/config/application.py

@@ -754,6 +754,11 @@ def _load_config_files(cls, basefilename, path=None, log=None, raise_config_file
                     loaded.append(config)
                     filenames.append(loader.full_filename)
 
+    @property
+    def loaded_config_files(self):
+        """Currently loaded configuration files"""
+        return self._loaded_config_files[:]
+
     @catch_config_error
     def load_config_file(self, filename, path=None):
         """Load config files by filename and path."""
@@ -763,7 +768,8 @@ def load_config_file(self, filename, path=None):
             raise_config_file_errors=self.raise_config_file_errors,
         ):
             new_config.merge(config)
-            self._loaded_config_files.append(filename)
+            if filename not in self._loaded_config_files:  # only add if not present (support reloads)
+                self._loaded_config_files.append(filename)
         # add self.cli_config to preserve CLI config priority
         new_config.merge(self.cli_config)
         self.update_config(new_config)

traitlets/config/tests/test_application.py

@@ -547,6 +547,48 @@ def test_subcommands_instanciation(self):
         self.assertIs(app.subapp.parent, app)
         self.assertIs(app.subapp.subapp.parent, app.subapp)     # Set by factory.
 
+    def test_loaded_config_files(self):
+        app = MyApp()
+        app.log = logging.getLogger()
+        name = 'config.py'
+        with TemporaryDirectory('_1') as td1:
+            config_file = pjoin(td1, name)
+            with open(config_file, 'w') as f:
+                f.writelines([
+                    "c.MyApp.running = True\n"
+                ])
+
+            app.load_config_file(name, path=[td1])
+            self.assertEqual(len(app.loaded_config_files), 1)
+            self.assertEquals(app.loaded_config_files[0], config_file)
+
+            app.start()
+            self.assertEqual(app.running, True)
+
+            # emulate an app that allows dynamic updates and update config file
+            with open(config_file, 'w') as f:
+                f.writelines([
+                    "c.MyApp.running = False\n"
+                ])
+
+            # reload and verify update, and that loaded_configs was not increased
+            app.load_config_file(name, path=[td1])
+            self.assertEqual(len(app.loaded_config_files), 1)
+            self.assertEqual(app.running, False)
+
+            # Attempt to update, ensure error...
+            with self.assertRaises(AttributeError):
+                app.loaded_config_files = "/foo"
+
+            # ensure it can't be udpated via append
+            app.loaded_config_files.append("/bar")
+            self.assertEqual(len(app.loaded_config_files), 1)
+
+            # repeat to ensure no unexpected changes occurred
+            app.load_config_file(name, path=[td1])
+            self.assertEqual(len(app.loaded_config_files), 1)
+            self.assertEqual(app.running, False)
+
 
 class Root(Application):
     subcommands = {