Decorator Recipe: Event Binding in Tkinter
==========================================

Step 3
------
The "command" callback for the Button class is a special case of event handling in Tkinter. All other event handling goes through the ``Widget.bind`` method, which is used like this:

.. sourcecode:: python

    def onselect(evt):
        # Note here that Tkinter passes an event object to onselect()
        w = evt.widget
        index = int(w.curselection()[0])
        value = w.get(index)
        print 'You selected item %d: "%s"' % (index, value)

    lb = Listbox(frame, name='lb')
    lb.bind('<<ListboxSelect>>', onselect)

Create a decorator function called ``bind``, which accepts two arguments: the first argument is a widget object, the second is an event descriptor string (e.g. '<<ListboxSelect>>').

.. sourcecode:: python 

    from Tkinter import *

    def bind(widget, event):
        """Write the body of this function"""

    if __name__ == '__main__':
        frame = Frame()                    
        frame.master.title("Event binding with decorators")
        frame.pack()

        lb = Listbox(frame, name='lb')
        for s in ['One', 'Two', 'Three', 'Four']:
            lb.insert(END, s)
        lb.pack()
        
        @bind(lb, '<<ListboxSelect>>')
        def onselect(evt):
            w = evt.widget
            index = int(w.curselection()[0])
            value = w.get(index)
            print 'You selected item %d: "%s"' % (index, value)

        frame.mainloop()                  

Expected output:

.. image:: tkinter-screenshot3.png        

Hints:

.. class:: hintlist

#. The ``bind`` function needs to return a function.
#. The function returned by ``bind`` also returns a function.
#. The function returned by ``bind`` is a decorator.
#. You will need to use a closure.
#. Your solution might contain the following statement: ``widget.bind(event, func)``.

Solution: `tkinter3.py <solutions/tkinter3.py>`_

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