Python Programming for the Absolute Beginner, 3rd Edition

GUI elements are called widgets (short for "window gadgets"). Probably the simplest widget is the Label widget, which is uneditable text or icons (or both). A label widget labels part of a GUI. It's often used to label other widgets. Unlike most other widgets, labels aren't interactive. A user can't click on a label (alright, a user can, but the label won't do anything). Still, labels are important and you'll probably use at least one every time you create a GUI.

Introducing the Labeler Program

The Labeler program creates a root window and adds a label to it. The label widget simply declares that it is a label. Figure 10.7 illustrates the program.

Figure 10.7: A label can provide information about a GUI.

Setting Up the Program

First, I set up the Labeler program by importing Tkinter and creating a root window:

# Labeler # Demonstrates a label # Michael Dawson - 6/5/03 from Tkinter import * # create the root window root = Tk() root.title("Labeler") root.geometry("200x50")

Creating a Frame

A Frame is a widget that can hold other widgets (such as Label widgets). A frame is like the cork in a corkboard; you use it as a base on which to place other things. So, I create a new frame:

# create a frame in the window to hold other widgets app = Frame(root)

Any time you create a new widget, you must pass its master (the thing that will contain the widget) to the constructor of the new object. Here, I pass root to the Frame constructor. As a result, the new frame is placed inside the root window.

Next, I invoke the grid() method of the new object:

app.grid()

grid() is a method that all widgets have. It's associated with a layout manager, which lets you arrange widgets. To keep things simple, I save the discussion of layout managers for a bit later in this chapter.

Creating a Label

I create a Label widget by instantiating an object of the Label class:

# create a label in the frame lbl = Label(app, text = "I'm a label!")

By passing app to the Label object's constructor, I make the frame that app refers to the master of the Label widget. As a result, the label is placed in the frame.

Widgets have options that you can set. Many of these options affect how the widget appears. By passing the string "I'm a label!" to the text parameter, I set the widget's text option to that string. As a result, the text I'm a label! appears when the label is displayed.

Next, I invoke the object's grid() method:

lbl.grid()

This ensures that the label will be visible.

Entering the Root Window's Event Loop

Last, but not least, I invoke the root window's event loop to start up the GUI:

# kick off the window's event loop root.mainloop()