Python in a Nutshell

Tkinter Events

So far, we’ve seen only the most elementary kind of event handling: the callbacks performed on callables installed with the command= option of buttons and menu entries of various kinds. Tkinter also lets you install callables to call back when needed to handle a variety of events. However, Tkinter does not let you create your own custom events; you are limited to working with events predefined by Tkinter itself.

The Event Object

General event callbacks must accept one argument event that is a Tkinter event object. Such an event object has several attributes describing the event:

char

A single-character string that is the key’s code (only for keyboard events)

keysym

A string that is the key’s symbolic name (only for keyboard events)

num

Button number (only for mouse-button events); 1 and up

x, y

Mouse position, in pixels, relative to the upper left corner of the widget

x_root , y_root

Mouse position, in pixels, relative to the upper left corner of the screen

widget

The widget in which the event has occurred

Binding Callbacks to Events

To bind a callback to an event in a widget w, call w .bind, describing the event with a string, usually enclosed in angle brackets ('<...>'). The following example prints 'Hello World' each time the user presses the Enter key:

from Tkinter import *

root = Tk( )
def greet(*ignore): print 'Hello World'
root.bind('<Return>', greet)
root.mainloop( )

Method tag_bind of classes Canvas and Text, covered earlier in this chapter, ...