Suppress Redundant Configurable Validation

traitlets/config/configurable.py

@@ -79,6 +79,17 @@ def __init__(self, config=None):
 
         # load kwarg traits, other than config
         super(Configurable, self).__init__(**kwargs)
+
+        # record traits set by config
+        config_override_names = set()
+        def notice_config_override(change):
+            """Record traits set by both config and kwargs.
+
+            They will need to be overridden again after loading config.
+            """
+            if change.name in kwargs:
+                config_override_names.add(change.name)
+        self.observe(notice_config_override)
 
         # load config
         if config is not None:
@@ -93,12 +104,11 @@ def __init__(self, config=None):
         else:
             # allow _config_default to return something
             self._load_config(self.config)
+        self.unobserve(notice_config_override)
+
+        for name in config_override_names:
+            setattr(self, name, kwargs[name])
 
-        # Ensure explicit kwargs are applied after loading config.
-        # This is usually redundant, but ensures config doesn't override
-        # explicitly assigned values.
-        for key, value in kwargs.items():
-            setattr(self, key, value)
 
     #-------------------------------------------------------------------------
     # Static trait notifiations

traitlets/config/tests/test_configurable.py

@@ -6,6 +6,7 @@
 
 import logging
 import sys
+import warnings
 from unittest import TestCase
 
 from pytest import mark
@@ -18,7 +19,7 @@
 
 from traitlets.traitlets import (
     Integer, Float, Unicode, List, Dict, Set, Enum,
-    _deprecations_shown,
+    _deprecations_shown, validate,
 )
 
 from traitlets.config.loader import Config
@@ -498,6 +499,32 @@ def _config_default(self):
         d2 = DefaultConfigurable()
         self.assertIs(d2.config, single.config)
         self.assertEqual(d2.a, 5)
+
+    def test_kwarg_config_priority(self):
+        # a, c set in kwargs
+        # a, b set in config
+        # verify that:
+        # - kwargs are set before config
+        # - kwargs have priority over config
+        class A(Configurable):
+            a = Unicode('default', config=True)
+            b = Unicode('default', config=True)
+            c = Unicode('default', config=True)
+            c_during_config = Unicode('never')
+            @validate('b')
+            def _record_c(self, proposal):
+                # setting b from config records c's value at the time
+                self.c_during_config = self.c
+                return proposal.value
+
+        cfg = Config()
+        cfg.A.a = 'a-config'
+        cfg.A.b = 'b-config'
+        obj = A(a='a-kwarg', c='c-kwarg', config=cfg)
+        assert obj.a == 'a-kwarg'
+        assert obj.b == 'b-config'
+        assert obj.c == 'c-kwarg'
+        assert obj.c_during_config == 'c-kwarg'
 
 
 class TestLogger(TestCase):