Python in a Nutshell

Name

after_idle

Synopsis

                        w.after_idle(callable,*args)

Registers a callback to callable (* args ) to be performed when the event loop is idle (i.e., when all pending events have been processed).

The following example shows how to use after to implement a simple digital clock:

import Tkinter
import time

curtime = ''
clock = Tkinter.Label( )
clock.pack( )

def tick( ):
    global curtime
    newtime = time.strftime('%H:%M:%S')
    if newtime != curtime:
        curtime = newtime
        clock.config(text=curtime)
    clock.after(200, tick)

tick( )
clock.mainloop( )

The kind of polling that method after lets you establish is an important Tkinter technique. Several Tkinter widgets have no callbacks to let you know about user actions on them, so if you want to track such actions in real-time, polling may be your only option. For example, here’s how to use polling established with after to track a Listbox selection in real time:

import Tkinter

F1 = Tkinter.Frame( )
s = Tkinter.Scrollbar(F1)
L = Tkinter.Listbox(F1)
s.pack(side=Tkinter.RIGHT, fill=Tkinter.Y)
L.pack(side=Tkinter.LEFT, fill=Tkinter.Y)
s['command'] = L.yview
L['yscrollcommand'] = s.set
for i in range(30): L.insert(Tkinter.END, str(i))
F1.pack(side=Tkinter.TOP)

F2 = Tkinter.Frame( )
lab = Tkinter.Label(F2)
def poll( ):
    lab.after(200, poll)
    sel = L.curselection( )
    lab.config(text=str(sel))
lab.pack( )
F2.pack(side=Tkinter.TOP)

poll( )
Tkinter.mainloop( )