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 tkroot = 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.

I had the problem that I needed to get the last selected item in a listbox with selectmode=MULTIPLE. In case someone else has the same problem, here is what I did:

lastselectionList = []def onselect(evt):    # Note here that Tkinter passes an event object to onselect()    global lastselectionList    w = evt.widget    if lastselectionList: #if not empty    #compare last selectionlist with new list and extract the difference        changedSelection = set(lastselectionList).symmetric_difference(set(w.curselection()))        lastselectionList = w.curselection()    else:    #if empty, assign current selection        lastselectionList = w.curselection()        changedSelection = w.curselection()    #changedSelection should always be a set with only one entry, therefore we can convert it to a lst and extract first entry    index = int(list(changedSelection)[0])    value = w.get(index)    tkinter.messagebox.showinfo("You selected ", value)listbox = tk.Listbox(frame,selectmode=tk.MULTIPLE)listbox.bind('<<ListboxSelect>>', onselect)listbox.pack()