Programming Python, 3rd Edition

Reusable GUI Components with Classes

Larger GUI interfaces are often built up as subclasses of Frame, with callback handlers implemented as methods. This structure gives us a natural place to store information between events: instance attributes record state. It also allows us to both specialize GUIs by overriding their methods in new subclasses, and attach them to larger GUI structures to reuse them as general components. For instance, a GUI text editor implemented as a Frame subclass can be attached to and configured by any number of other GUIs; if done well, we can plug such a text editor into any user interface that needs text editing tools.

We’ll meet such a text editor component in Chapter 12. For now, Example 8-20 illustrates the concept in a simple way. The script gui6.py produces the window in Figure 8-22.

Figure 8-22. A custom Frame in action

Example 8-20. PP3E\Gui\Intro\gui6.py

from Tkinter import *

class Hello(Frame):                             # an extended Frame
    def _ _init_ _(self, parent=None):
        Frame._ _init_ _(self, parent)             # do superclass init
        self.pack( )
        self.data = 42
        self.make_widgets( )                       # attach widgets to self
    def make_widgets(self):
        widget = Button(self, text='Hello frame world!', command=self.message)
        widget.pack(side=LEFT)
    def message(self):
        self.data += 1
        print 'Hello frame world %s!' % self.data

if _ _name_ _ == '_ _main_ _': Hello().mainloop( )

This example pops up a single-button ...