copy self.config in update_config

traitlets/config/configurable.py

@@ -187,6 +187,13 @@ def _config_changed(self, change):
 
     def update_config(self, config):
         """Update config and load the new values"""
+        # traitlets prior to 4.2 created a copy of self.config in order to trigger change events.
+        # Some projects (IPython < 5) relied upon one side effect of this,
+        # that self.config prior to update_config was not modified in-place.
+        # For backward-compatibility, we must ensure that self.config
+        # is a new object and not modified in-place,
+        # but config consumers should not rely on this behavior.
+        self.config = deepcopy(self.config)
         # load config
         self._load_config(config)
         # merge it into self.config

traitlets/config/tests/test_application.py

@@ -115,6 +115,32 @@ def test_config_propagation(self):
         self.assertEqual(app.foo.i, 10)
         self.assertEqual(app.foo.j, 10)
         self.assertEqual(app.bar.enabled, False)
+
+    def test_ipython_cli_priority(self):
+        name = 'config.py'
+        class TestApp(Application):
+            value = Unicode().tag(config=True)
+            aliases = {'v': 'TestApp.value'}
+        app = TestApp()
+        with TemporaryDirectory() as td:
+            config_file = pjoin(td, name)
+            with open(config_file, 'w') as f:
+                f.write("c.TestApp.value = 'config file'")
+            # follow IPython's config-loading sequence to ensure CLI priority is preserved
+            app.parse_command_line(['--v=cli'])
+            # this is where IPython makes a mistake:
+            # it assumes app.config will not be modified,
+            # and storing a reference is storing a copy
+            cli_config = app.config
+            assert 'value' in app.config.TestApp
+            assert app.config.TestApp.value == 'cli'
+            app.load_config_file(name, path=[td])
+            assert app.config.TestApp.value == 'config file'
+            # enforce cl-opts override config file opts:
+            # this is where IPython makes a mistake: it assumes
+            # that cl_config is a different object, but it isn't.
+            app.update_config(cli_config)
+            assert app.config.TestApp.value == 'cli'
 
     def test_flags(self):
         app = MyApp()