nicer repr of LazyConfigValues

traitlets/config/loader.py

@@ -146,6 +146,12 @@ def to_dict(self):
             d['inserts'] = self._inserts
         return d
 
+    def __repr__(self):
+        if self._value is not None:
+            return "<%s value=%r>" % (self.__class__.__name__, self._value)
+        else:
+            return "<%s %r>" % (self.__class__.__name__, self.to_dict())
+
 
 def _is_section_key(key):
     """Is a Config key a section name (does it start with a capital)?"""

traitlets/config/tests/test_loader.py

@@ -483,6 +483,20 @@ def test_getattr_private_missing(self):
             _ = cfg._repr_html_
         self.assertNotIn('_repr_html_', cfg)
         self.assertEqual(len(cfg), 0)
+
+    def test_lazy_config_repr(self):
+        cfg = Config()
+        cfg.Class.lazy.append(1)
+        cfg_repr = repr(cfg)
+        assert '<LazyConfigValue' in cfg_repr
+        assert "extend" in cfg_repr
+        assert " [1]}>" in cfg_repr
+        assert 'value=' not in cfg_repr
+        cfg.Class.lazy.get_value([0])
+        repr2 = repr(cfg)
+        assert repr([0,1]) in repr2
+        assert 'value=' in repr2
+
 
     def test_getitem_not_section(self):
         cfg = Config()