traits exception

In [19]: class A(HasTraits):
    foo = Int
    def __init__(self, **kwargs):
        super(A, self).__init__(**kwargs)
        self.on_trait_change(self.raiseerror, 'foo')
    def raiseerror(self, obj, name, old, new):
        raise Exception('testing exception')
   ....:     
   ....:     

In [26]: a = A()

In [27]: a.foo = 2
Exception occurred in traits notification handler.
Please check the log file for details.
Exception occurred in traits notification handler for object: <__main__.A object at 0x10196e950>, trait: foo, old value: 0, new value: 1
Traceback (most recent call last):
  File "/Library/Frameworks/EPD64.framework/Versions/7.0/lib/python2.7/site-packages/Traits-3.6.1-py2.7-macosx-10.5-x86_64.egg/enthought/traits/trait_notifiers.py", line 536, in rebind_call_4
    object, trait_name, old, new )
  File "/Library/Frameworks/EPD64.framework/Versions/7.0/lib/python2.7/site-packages/Traits-3.6.1-py2.7-macosx-10.5-x86_64.egg/enthought/traits/trait_notifiers.py", line 448, in dispatch
    handler( *args )
  File "<ipython console>", line 7, in raiseerror
Exception: testing exception

In [42]: try:
    a.foo=1
except:
    print 'exception_raised'
   ....:     
   ....:     
Exception occurred in traits notification handler for object: <__main__.A object at 0x10196e950>, trait: foo, old value: 2, new value: 1
Traceback (most recent call last):
  File "/Library/Frameworks/EPD64.framework/Versions/7.0/lib/python2.7/site-packages/Traits-3.6.1-py2.7-macosx-10.5-x86_64.egg/enthought/traits/trait_notifiers.py", line 536, in rebind_call_4
    object, trait_name, old, new )
  File "/Library/Frameworks/EPD64.framework/Versions/7.0/lib/python2.7/site-packages/Traits-3.6.1-py2.7-macosx-10.5-x86_64.egg/enthought/traits/trait_notifiers.py", line 448, in dispatch
    handler( *args )
  File "<ipython console>", line 7, in raiseerror
Exception: testing exception