Add an optional `default` argument to trait metadata get, like the Python dict's .get() default argument

After this, trait_metadata('traitname', 'metadata_key', DEFAULT) will return DEFAULT if the metadata key is not set.  DEFAULT defaults to None, like the existing behavior and like the python dict .get() behavior.

IPython/utils/tests/test_traitlets.py

@@ -406,6 +406,13 b' class TestHasTraits(TestCase):'
         a = A()
         self.assertEqual(a.trait_metadata('i','config_key'), 'MY_VALUE')
 
+    def test_trait_metadata_default(self):
+        class A(HasTraits):
+            i = Int()
+        a = A()
+        self.assertEqual(a.trait_metadata('i', 'config_key'), None)
+        self.assertEqual(a.trait_metadata('i', 'config_key', 'default'), 'default')
+
     def test_traits(self):
         class A(HasTraits):
             i = Int

IPython/utils/traitlets.py

@@ -458,8 +458,8 b' class TraitType(object):'
                 % (self.name, self.info(), repr_type(value))
         raise TraitError(e)
 
-    def get_metadata(self, key):
-        return getattr(self, '_metadata', {}).get(key, None)
+    def get_metadata(self, key, default=None):
+        return getattr(self, '_metadata', {}).get(key, default)
 
     def set_metadata(self, key, value):
         getattr(self, '_metadata', {})[key] = value
@@ -728,7 +728,7 b' class HasTraits(py3compat.with_metaclass(MetaHasTraits, object)):'
 
         return result
 
-    def trait_metadata(self, traitname, key):
+    def trait_metadata(self, traitname, key, default=None):
         """Get metadata values for trait by key."""
         try:
             trait = getattr(self.__class__, traitname)
@@ -736,7 +736,7 b' class HasTraits(py3compat.with_metaclass(MetaHasTraits, object)):'
             raise TraitError("Class %s does not have a trait named %s" %
                                 (self.__class__.__name__, traitname))
         else:
-            return trait.get_metadata(key)
+            return trait.get_metadata(key, default)
 
 #-----------------------------------------------------------------------------
 # Actual TraitTypes implementations/subclasses