Fixed button on_click handler so it's initiated on instanciation

IPython/html/widgets/widget_button.py

@@ -1,72 +1,75 b''
 """ButtonWidget class.  
 
 Represents a button in the frontend using a widget.  Allows user to listen for
 click events on the button and trigger backend code when the clicks are fired.
 """
 #-----------------------------------------------------------------------------
 # Copyright (c) 2013, the IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 import inspect
 import types
 
 from .widget import Widget
 from IPython.utils.traitlets import Unicode, Bool, Int
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
 class ButtonWidget(Widget):
     target_name = Unicode('ButtonWidgetModel')
     default_view_name = Unicode('ButtonView')
 
     # Keys
     _keys = ['clicks', 'description', 'disabled']
     clicks = Int(0, help="Number of times the button has been clicked.")
     description = Unicode('', help="Description of the button (label).")
     disabled = Bool(False, help="Enable or disable user changes.")
     
-    _click_handlers = []
+
+    def __init__(self, **kwargs):
+        self._click_handlers = []
+        super(ButtonWidget, self).__init__(**kwargs)
 
 
     def on_click(self, callback, remove=False):
         """Register a callback to execute when the button is clicked.
 
         Parameters
         ----------
         remove : bool (optional)
             Set to tru to remove the callback from the list of callbacks."""
         if remove:
             self._click_handlers.remove(callback)
         else:
             self._click_handlers.append(callback)
 
 
     def _clicks_changed(self, name, old, new):
         """Handles when the clicks property has been changed.  Fires on_click
         callbacks when appropriate."""
         if new > old:
             for handler in self._click_handlers:
                 if callable(handler):
                     argspec = inspect.getargspec(handler)
                     nargs = len(argspec[0])
 
                     # Bound methods have an additional 'self' argument
                     if isinstance(handler, types.MethodType):
                         nargs -= 1
 
                     # Call the callback
                     if nargs == 0:
                         handler()
                     elif nargs == 1:
                         handler(self)
                     else:
                         raise TypeError('ButtonWidget click callback must ' \
                             'accept 0 or 1 arguments.')