__copy__ configuration directive now accepts a __default__

docs/user/configuration.rst

@@ -113,3 +113,15 @@ With ``__copy__``, you can reduce this down to:
         }
 
 Reducing duplication in your configuration can help avoid errors.
+
+If the target of the ``__copy__`` directive does not exist, we can
+fall back to a default using the ``__default__`` special keyword.  An
+example that defaults to an empty ``param_grid`` for cross
+validation:
+
+.. code-block:: python
+
+    'grid_search': {
+        'param_grid': {'__copy__': 'param_grid', '__default__': {}},
+        # ... some involved grid search configuration
+    }

palladium/config.py

@@ -94,10 +94,20 @@ def __call__(self, name, props):
         if self_reference:
             value = self._resolve(self.configs[:-1], dotted_path)
         else:
-            value = self._resolve(self.configs, dotted_path)
+            try:
+                value = self._resolve(self.configs, dotted_path)
+            except KeyError:
+                if '__default__' in props:
+                    return props['__default__']
+                else:
+                    raise
 
         value = deepcopy(value)
-        if len(props) > 1:
+        nonmagicprops = [
+            prop for prop in props
+            if not (prop.startswith('__') and prop.endswith('__'))
+            ]
+        if nonmagicprops:
             recursive_copy = self.key in value
             value.update(props)
             if not recursive_copy:

palladium/tests/test_config.py

@@ -235,6 +235,11 @@ def config2(self):
                 },
             'mycopiedconstant': {
                 '__copy__': 'mycopiedconstant',
+                '__default__': 42,
+                },
+            'mycopywithdefault': {
+                '__copy__': 'nonexistant',
+                '__default__': 42,
                 },
             }
 
@@ -301,6 +306,7 @@ def test_config1_and_2(self, process_config, config1, config2):
             config['mysupernewdict']['mycopiedcomponent'], MyDummyComponent)
 
         assert config['mycopiedconstant'] == 3
+        assert config['mycopywithdefault'] == 42
 
     def test_initialize_config_logging(self, process_config):
         with patch('palladium.config.dictConfig') as dictConfig: