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 Status Bars

Finally, most applications sport a status bar at the bottom of each application window. Implementing a status bar with Tkinter is trivial: you can simply use a suitably configured Label widget, and reconfigure the text option now and then. Here's one way to do it:

    status = Label(master, text="", bd=1, relief=SUNKEN, anchor=W)
    status.pack(side=BOTTOM, fill=X)

If you wish to be fancy, you can use the following class instead. It wraps a label widget in a convenience class, and provides set and clear methods to modify the contents.

# File: tkSimpleStatusBar.py

class StatusBar(Frame):

    def __init__(self, master):
        Frame.__init__(self, master)
        self.label = Label(self, bd=1, relief=SUNKEN, anchor=W)
        self.label.pack(fill=X)

    def set(self, format, *args):
        self.label.config(text=format % args)
        self.label.update_idletasks()

    def clear(self):
        self.label.config(text="")
        self.label.update_idletasks()

The set method works like C's printf function; it takes a format string, possibly followed by a set of arguments (a drawback is that if you wish to print an arbitrary string, you must do that as set("%s", string)). Also note that this method calls the update_idletasks method, to make sure pending draw operations (like the status bar update) are carried out immediately.

But the real trick here is that we've inherited from the Frame widget. At the cost of a somewhat awkward call to the frame widget's constructor, we've created a new kind of custom widget that can be treated as any other widget. You can create and display the status bar using the usual widget syntax:

    status = StatusBar(root)
    status.pack(side=BOTTOM, fill=X)

We could have inherited from the Label widget itself, and just extended it with set and clear methods. This approach have a few drawbacks, though:

• It makes it harder to maintain the status bar's integrity. Some team members may cheat, and use config instead of set. That's not a big deal, until the day you decide to do some extra processing in the set method. Or the day you decide to use a Canvas widget to implement a fancier status bar.

• It increases the risk that your additional methods conflict with attributes or methods used by Tkinter. While the Frame and Toplevel widgets have relatively few methods, other widgets can have several dozens of widget specific attributes and methods.

• Future versions of Tkinter may use factory functions rather than class constructors for most widgets. However, it's more or less guaranteed that such versions will still provide Frame and Toplevel classes. Better safe than sorry, in other words.

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