Getting a callback when a Tkinter Listbox selection is changed?

Question

There are a number of ways of getting callbacks when Text or Entry widgets are changed in Tkinter, but I haven't found one for Listbox's (it doesn't help that much of the event documentation I can find is old or incomplete). Is there some way of generating an event for this?

Answer 1

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) 

Answer 2

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.

Answer 3

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()