Getting a callback when a Tkinter Listbox selection is changed?

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)

You can bind to the <<ListboxSelect>> event. This event will be generated whenever the selection changes, whether it changes from a button click, via the keyboard, or any other method.

Here's a simple example which updates a label whenever you select something from the listbox:

import tkinter as tk

root = tk.Tk()
label = tk.Label(root)
listbox = tk.Listbox(root)
label.pack(side="bottom", fill="x")
listbox.pack(side="top", fill="both", expand=True)

listbox.insert("end", "one", "two", "three", "four", "five")

def callback(event):
    selection = event.widget.curselection()
    if selection:
        index = selection[0]
        data = event.widget.get(index)
        label.configure(text=data)
    else:
        label.configure(text="")

listbox.bind("<<ListboxSelect>>", callback)

root.mainloop()

This event is mentioned in the canonical man page for listbox. All predefined virtual events can be found on the bind man page.