Harsh J

Memoirs of a QWERTY Keyboard

The PyQt Intro

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This article is an introduction to the PyQt GUI Application Development Framework that uses Python and Nokia’s Qt.

Contents:

  1. Pretext
  2. Prerequisites
  3. Sample Code
  4. Breaking Down the Code
  5. Getting Further

Pretext

Ever wondered how to easily write good programs using Python? Programs that would run on any platform – from Windows to Mac OS X along with the various Linux variants? Programs that look good, look native, and also work great?

Here’s one solution – PyQt! With it you can create complete, working applications that may look like this:

Fresco - An application developed with PyQt4

Nokia’s Qt is a free (LGPL) cross-platform C++/Java based application-framework, with GUI being its most prominent feature.

PyQt is simply a python-binding to the C++ libraries that Qt provides. Using PyQt one can write neat looking GUI applications that do a lot, and do so easily.

Nokia's Qt 4

A series of articles, starting with this one will aim to tutor interested programmers into using this wonderful library, and developing applications with it.

Prerequisites

The most-minimal pre-requisites are:

  • Basic Python programming knowledge.
  • A little know-how about GUI and the event-based-programming concepts.

Two primary resources where we start from:

  1. Nokia’s Qt
  2. Riverbank Computing’s PyQt

Downloading and installing PyQt is fairly easy on Linux thanks to the various package managers. Do check out the download links at each of the above-mentioned links to get them on other platforms as well. Installation help is also provided by Riverbank Computing to help you set-up the PyQt4 library on your OS.

Sample Code

So let’s start off in the same style as almost everything do in a programmer’s perspective: Hello World!

Below is a well documented program for the same:


import sys

# Importing the necessary Qt classes.

from PyQt4.QtGui import QLabel, QApplication

# We use the from foo import * syntax here because
# all of Qt's objects begin with a Q
# and thus we shouldn't run into namespace problems.

if __name__=='__main__':

	App = QApplication(sys.argv)

	# All Qt programs need an
	# QApplication instance.

	# We pass the sys.argv as its arguments
	# because Qt is adept at handling some
	# of the default command-line options
	# like style, size, etc by itself.

	Label = QLabel( "Hello World!" )

	# QLabel is the class providing a
	# simple label

	Label.show()

	# Like in most GUI toolkits, we have
	# to manually set it to show

	App.exec_()

	# Notice the _ after exec, this is to
	# avoid the confusion with Python's
	# exec() built-in-function

	# exec_() starts the main application
	# loop. Something like main() of other
	# toolkits.

This is how it looks when run, a simple application that displays some popular text:

A simple Hello World application written with Qt

Breaking it down

The first thing we have initialized under our ‘__main__’ is the QApplication instance, App. Every GUI application needs this instance to be present. It handles the look and feel of the application and is the main thread of the same. Without it Qt will error out and not run any GUI.

Next we create a label. A label is a simple text-displaying widget used in many forms to describe the input fields and other things. In Qt, its called the QLabel and resides under the QtGui namespace. We create one with a simple Hello World text under the object name Label.

Now we call the show() method of the label so that it appears on the screen. This method is applicable to all QWidget-class children, and QLabel is one among them. Calling show() draws the label widget’s window on the screen.

Finally, we start the application loop, the one responsible for making an application run as an infinite loop waiting for user-interaction. Its done by the exec_() method of QApplication (App is the instance name). Note the _underscore_ in the function name, this is to avoid confusion between Python’s own exec() built-in-function.

Getting Further

Tutorials

FAQs

Footnotes

Some more thoughts on the article and the code.

  • Am running this Qt application in a GNOME environment and thus you see the GTK style being used. Qt can adapt to your environment and can use the native style of any platform it runs on. Also, you can try out the -style option to change it too. More on this as we progress.
  • This program is just a quick start for the impatient, a dose for the hyperactive. I shall take a slow, but smooth, approach to the finer details of PyQt as the tutorial progresses.

Written by Harsh

April 26th, 2009 at 8:01 am

Posted in Software

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