top.mainloop()

The call to the top.mainloop method starts the Tkinter event loop generating events. In this case the only event caught will be the button press event which is connected to the F.quit method. F.quit in turn will terminate the application. Try it, it should look like this:

Exploring Layout

Note: Moving away from the >>> prompt, from now on I'll provide examples within Python script files.

In this section I want to look at how Tkinter positions widgets within a window. We already have seen Frame, Label and Button widgets and those are all we need for this section. In the previous example we used the pack method of the widget to locate it within its parent widget. Technically what we are doing is invoking Tk's packer Layout Manager. The Layout Manager's job is to determine the best layout for the widgets based on hints that the programmer provides plus constraints such as the size of the window as controlled by the user. Some Layout managers use exact locations within the window, specified in pixels normally, and this is very common in Microsoft Windows environments such as Visual Basic. Tkinter includes a Placer Layout Manager which can do this too via a place method. I won't look at that in this tutor because usually one of the other, more intelligent managers is a better choice since they take the need to worry about what happens when a window is resized away from us as programmers.

The simplest Layout Manager in Tkinter is the packer which we've been using. The packer, by defauilt, just stacks widgets one on top of the other. That is very rarely what we want for normal widgets, but if we build our applications from Frames then stacking Frames on top of each other is quite a reasonable approach. We can then put out other widgets into the Frames using either the packer or other Layout Manager within each Frame as appropriate. You can see an example of this in action in the Case Study topic.

Even the simple packer provides a multitude of options, however. For example we can arrange our widgets horizontally instead of vertically by providing a side argument, like so:

lHello.pack(side="left")

bQuit.pack(side="left")

That will force the widgets to go to the left thus the first widget (the label) will appear at the extreme left hand side, followed by the next widget (the Button). If you modify the lines in the example above it will look like this:

And if you change the "left" to "right" then the Label appears on the extreme right and the Button to the left of it, like so:

One thing you notice is that it doesn't look very nice because the widgets are squashed together. The packer also provides us with some parameters to deal with that. The easiest to use is Padding and is specified in

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terms of horizontal padding (padx), and vertical padding(pady). These values are specified in pixels. Lets try adding some horizontal padding to our example:

lHello.pack(side="left", padx=10) bQuit.pack(side='left', padx=10)

It should look like this:

If you try resizing the window you'll see that the widgets retain their positions relative to one another but stay centered in the window. Why is that, if we packed them to the left? The answer is that we packed them into a Frame but the Frame was packed without a side, so it is positioned top, centre - the packers default. If you want the widgets to stay at the correct side of the window you will need to pack the Frame to the appropriate side too:

F.pack(side='left')

Also note that the widgets stay centred if you resize the window vertically - again that's the packers default behaviour.

I'll leave you to play with padx and pady for yourself to see the effect of different values and combinations etc. Between them, side and padx/pady allow quite a lot of flexibility in the positioning of widgets using the packer. There are several other options, each adding another subtle form of control, please check the Tkinter reference pages for details.

There are a couple of other layout managers in Tkinter, known as the grid and the placer. To use the grid manager you use grid() instead of pack() and for the placer you call place() instead of pack(). Each has its own set of options and since I'll only cover the packer in this intro you'll need to look up the Tkinter tutorial and reference for the details. The main points to note are that the grid arranges components in a grid (surprise!) within the window - this can often be useful for dialog boxes with lined up text entry boxes, for example. The placer user either fixed coordinates in pixels or relative coordinates within a window. The latter allow the component to resize along with the window - always occupying 75% of the vertical space say. This can be useful for intricate window designs but does require a lot of pre planning - I strongly recommend a pad of squared paper, a pencil and eraser!

Controlling Appearance using Frames and the Packer

The Frame widget actually has a few useful properties that we can use. After all, it's very well having a logical frame around components but sometimes we want something we can see too. This is especially useful for grouped controls like radio buttons or check boxes. The Frame solves this problem by providing, in common with many other Tk widgets, a Relief property. Relief can have any one of several values: sunken, raised, groove, ridge or flat. Let's use the sunken value on our simple dialogue box. Simply change the Frame creationline to:

F = Frame(top,relief="sunken", border=1)

Note 1:You need to provide a border too. If you don't the Frame will be sunken but with an invisible border - you don't see any difference!

Note 2: that you don't put the border size in quotes. This is one of the confusing aspects of Tk programming is knowing when to use quotes around an option and when to leave them out. In general if it's a numeric or single character value you can leave the quotes off. If it's a mixture of digits and letters or a string then you need the quotes. Likewise with which letter case to use. Unfortunately there is no easy solution, you just learn from experience - Python often gives a list of the valid options in it's error messages!

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One other thing to notice is that the Frame doesn't fill the window. We can fix that with another packer option called, unsurprisingly, fill. When you pack the frame do it thusly:

F.pack(fill="x")

This fills horizontally, if you want the frame to fill the entire window just use fill='y' too.

The end result of running the script now looks like:

Adding more widgets

Let's now look at a text Entry widget. This is the familiar single line of text input box. It shares a lot of the methods of the more sophisticated Text widget which we won't look at here. Hopefully using the methods of the Entry widget will provide a sound basis for experimenting with the Text box later.

Going back to our "Hello World" program we'll add a text entry widget inside a Frame of its own and a button that can clear the text that we type into it. This will demonstrate not only how to create and use the Entry widget but also how to define our own event handling functions and connect them to widgets.

from Tkinter import *

#create the event handler first def evClear():

eHello.delete(0,END)

#create the top level window/frame top = Tk()

F = Frame(top) F.pack(expand="true")

#Now the frame with text entry fEntry = Frame(F, border="1") eHello = Entry(fEntry)

fEntry.pack(side="top", expand="true") eHello.pack(side="left", expand="true")

#Finally the frame with the buttons.

#We'll sink this one for emphasis

fButtons = Frame(F, relief="sunken", border=1)

bClear = Button(fButtons, text="Clear Text", command=evClear) bClear.pack(side="left", padx=5, pady=2)

bQuit = Button(fButtons, text="Quit", command=F.quit) bQuit.pack(side="left", padx=5, pady=2) fButtons.pack(side="top", expand="true")

# Now run the eventloop F.mainloop()

Note that once more we pass the name of the event handler (evClear) as the command argument to the bClear button. Note also the use of a naming convention, evXXX to link the event handler with the corresponding widget.

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Running the program yields this:

And if you type something in the text entry box then hit the "Clear Text" button it removes it again.

Binding events - from widgets to code

Up till now we have used the command property of buttons to associate Python functions with GUI events. Sometimes we want more explicit control, for example to catch a particular key combination. The way to do that is use the bind function to explicitly tie together (or bind) an event and a Python function.

We'll now define a hot key - let's say CTRL-c - to delete the text in the above example. To do that we need to bind the CTRL-C key combination to the same event handler as the Clear button. Unfortunately there's an unexpected snag. When we use the command option the functuion specified must take no arguments. When we use the bind function to do the same job the bound function must take one argument. This we need to create a new function with a single parameter which calls evClear. Add the following after the evClear definition:

def evHotKey(event): evClear()

And add the following line following the definition of the Entry widget:

eHello.bind("<Control-c>",evHotKey) # the key definition is case sensitive

Run the program again and you can now clear the text by either hitting the button or typing Ctrl-c. We could also use bind to capture things like mouse clicks or capturing or losing Focus or even the windows becoming visible. See the Tkinter documentation for more information on this. The hardest part is usually figuring out the format of the event description!

A Short Message

You can report short messages to your users using a MessageBox. This is very easy in Tk and is accomplished using the tkMessageBox module functions as shown:

import tkMessageBox

tkMessageBox.showinfo("Window Title", "A short message")

There are also error, warning, Yes/No anfd OK/Cancel boxes available via different showXXX functions. They are distinguished by different icons and buttons. The latter two use askXXX instead of showXXX and return a value to indicate which button the user pressed, like so:

res = tMessageBox.askokcancel("Which?", "Ready to stop?") print res

Here are some of the Tkinter message boxes:

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