A Comprehensive Introduction to Python Programming and ... - MSDL

5.3 Tkinter Variables 45 / 75
from Tkinter import *
import tkFont
root = Tk()
myfont = tkFont.Font(family="arial", size=20, weight=tkFont.BOLD) 5
b1 = Button(root, text="Button 1", font=myfont)
b1.pack()
b2 = Button(root, text="Button 2", font=myfont)
b2.pack()
myfont.config(slant=tkFont.ITALIC) 10
root.mainloop()
This example is much more interesting when coded interactively, so that it is possible
to see both buttons reflect the change in the font.
5.3 Tkinter Variables
Tkinter has a special Variable class that all other variable subclasses inherit from.
Various Tkinter widgets have options that rely on the Variable class, such as the
“variable” option for Radiobuttons, the “textvariable” option for the Label, etc.
5.3.1 The Need for New Variables
When the Tkinter variables are used for simple tasks, they do not behave differently
than the regular Python variables, apart from the fact that their values have to be handled
through method calls, such as get() and set(). The choice behind the implementation
of this master variable class is therefore not obvious. However, by taking a
closer look at what the Variable class offers, the motivation behind it becomes much
more apparent. Tkinter variables provide, in true Tkinter style, the possibility to add
a callback which will be executed when read and write operations are performed on
the variable, and also when the value associated with the variable is undefined. This
allows, for example, to bind a StringVar() instance to a Label widget, which then
automatically updates its caption when the value of the variable changes.
5.3.2 Using Tkinter Variables
Tkinter provides four descendants of the Variable class:
• StringVar
• IntVar
• DoubleVar
• BooleanVar