Merge pull request #43 from SylvainCorlay/on_trait_change_signature

Add HasTraits instance to the signature of trait changed handler

traitlets/tests/test_traitlets.py

@@ -299,6 +299,8 @@ def callback2(name, new):
             self.cb = (name, new)
         def callback3(name, old, new):
             self.cb = (name, old, new)
+        def callback4(name, old, new, obj):
+            self.cb = (name, old, new, obj)
 
         class A(HasTraits):
             a = Int
@@ -324,6 +326,11 @@ class A(HasTraits):
         self.assertEqual(self.cb,('a',1000,10000))
         a.on_trait_change(callback3, 'a', remove=True)
 
+        a.on_trait_change(callback4, 'a')
+        a.a = 100000
+        self.assertEqual(self.cb,('a',10000,100000,a))
+        a.on_trait_change(callback4, 'a', remove=True)
+
         self.assertEqual(len(a._trait_notifiers['a']),0)
 
     def test_notify_only_once(self):

traitlets/traitlets.py

@@ -681,9 +681,11 @@ def _notify_trait(self, name, old_value, new_value):
                     c(name, new_value)
                 elif nargs + offset == 3:
                     c(name, old_value, new_value)
+                elif nargs + offset == 4:
+                    c(name, old_value, new_value, self)
                 else:
                     raise TraitError('a trait changed callback '
-                                        'must have 0-3 arguments.')
+                                        'must have 0-4 arguments.')
             else:
                 raise TraitError('a trait changed callback '
                                     'must be callable.')
@@ -719,8 +721,8 @@ def on_trait_change(self, handler, name=None, remove=False):
         ----------
         handler : callable
             A callable that is called when a trait changes.  Its
-            signature can be handler(), handler(name), handler(name, new)
-            or handler(name, old, new).
+            signature can be handler(), handler(name), handler(name, new),
+            handler(name, old, new), or handler(name, old, new, self).
         name : list, str, None
             If None, the handler will apply to all traits.  If a list
             of str, handler will apply to all names in the list.  If a