Pyside GUI Application Development- Second Edition: Develop more dynamic and robust GUI applications using PySide, an open source cross-platform UI framework, Second Edition

Many of the modern programming languages are backed up by a set of libraries (commonly referred to as toolkits) to create GUI applications, such as Qt, Tcl/Tk, and so on. PySide is a Python binding of the cross-platform GUI toolkit Qt, and it runs on all platforms that are supported by Qt, including Windows, Mac OS X, and Linux. It is one of the alternatives to toolkits such as Tkinter for GUI programming in Python.

PySide combines the advantages of Qt and Python. A PySide programmer has all the power of Qt, but it is able to exploit it with the simplicity of Python. PySide is licensed under the LGPL version 2.1 license, allowing both Free/Open Source software and proprietary software development. PySide is evolving continuously, like any other open source product, and you are free to contribute to its development. Some of the applications, such as matplotlib, PhotoGrabber, QBitTorrent, Lucas Chess, Fminer and so on, certify the wide spread usage of PySide in the software industry.

PySide has also become an enabler of mobile development. Qt Mobility is a project that is creating a new suite of Qt APIs for mobile device functionality. The project Pyside Mobility is a set of bindings that allows Python to access the Qt Mobility API. The Qt Mobility API enables the developer to access the bread and butter of services provided by the underlying operating system that are essential for any mobile application. Learning PySide, you learn this for free. Without further ado, let's get hacking!

In computing terms, GUI (pronounced as gooey, or Graphical User Interface) is used to denote a set of interfaces with computing systems that involves user-friendly images rather than boring text commands. GUI comes to the rescue of the numerous command-line interfaces that have always been coupled with a steep learning curve because learning and mastering commands requires a lot of effort due to their nonintuitive nature. Moreover, GUI layers make it easy for the end users to fulfill their needs without knowing much about the underlying implementation, which is unnecessary for them.

Every other application in the modern world is designed with interactive graphics to attract the end users. Simplicity and usability are the two main ingredients for a successful GUI system. The demanding feature of a GUI is to allow the user to concentrate on the task at hand. To achieve this, it must serve the interaction between the human and the computer, and make it no less than seamless and flowing. Therefore, learning to create GUIs will not only make you a successful developer, but it will also help in getting some revenue for yourself.

At a very basic level, a GUI is seen as a window (visibly noticeable or not) consisting of the following parts: controls, menu, layout, and interaction. A GUI is represented as a window on the screen and contains a number of different controls, as follows:

Development of a GUI application revolves around defining and controlling these components, and designing the area of interaction is the most challenging part of all. The correct exploitation of events, listeners, and handlers will help in developing better GUI applications. Many frameworks have been developed to support GUI development, such as the Model-View-Controller framework that is used in many web-based applications. Using some of these frameworks can make the GUI programming easier and will come in handy for future implementations. A good user-interface design relates to the user, not to the system architecture.

This is your first step in this series of learning. PySide is compatible with Python 2.6 or later and Qt 4.6 or better. So, before getting to install PySide, we must make sure that minimum version compatibility is achieved. This section will teach you two ways of installing PySide. One, being the most common and easiest way, is using simple point and click installers and package managers. This will install the most stable version of PySide on your system, which you can comfortably use without worrying too much about the stability. However, if you are an advanced programmer, you may prefer to build PySide from scratch from the latest builds that are available when you are reading this book. Both these methods are explained here for Windows, Mac OS X, and Linux systems, and you are free to choose your own setup style.

Installation of PySide on Windows is pretty much easy with the help of an installer. Perform the following steps for setup:

The binaries for MAC OS X installers of PySide are available at:

http://qt-project.org/wiki/PySide_Binaries_MacOSX

Download the latest version that is compatible with your system and perform a similar installation as explained in the previous section.

You can also choose to install PySide from the command line with the help of Homebrew or using MacPorts. The commands, respectively, are as follows:

brew install pyside
port-install pyXX-pyside

Replace XX with your Python version.

Installing PySide on a Debian-based system is much easier with the synaptic package manager. Issuing the following command will fetch and install the latest stable version available in the aptitude distribution:

sudo apt-get install python-pyside

On an RPM-based system, you can use the RPM-based distribution, yum, as follows:

yum install python-pyside pyside-tools

If you want to make sure that PySide is installed properly on your system, issue the following commands in the Python shell environment, as shown in Figure 1. The import pyside command should not return any errors.

PySide.__version__ should output something similar to 1.1.2:

Figure 1

Let's move on to see how we can build PySide from scratch.

Before starting to build PySide on Windows, ensure that the following prerequisites are installed:

Make sure that the Git and cmake executables are set in your system path. Now, perform the following steps to start building PySide:

On completion of these steps, the PySide should have been successfully built on your system.

The following are the prerequisites to build PySide in Linux:

PySide is a collection of four interdependent packages, namely API Extractor, Generator Runner, Shiboken Generator, and Pyside Qt bindings. In order to build PySide, you have to download and install these packages in that order:

Always, make sure that you have downloaded and built these packages in this order because each of these packages is interdependent. The build steps for each of these are:

Once you complete these steps in this order for each of these packages, PySide should be successfully built on your system.

export DYLD_LIBRARY_PATH=~/my_dir/install/lib

Congratulations on setting up Pyside successfully on your system. Now, it's time to do some real work using PySide. We have set up PySide and now we want to use it in our application. To do this, you have to import the PySide modules in your program to access the PySide data and functions. Here, let's learn some basics of importing modules in your Python program.

There are basically two ways that are widely followed when importing modules in Python. The first is to use a direct import <module> statement. This statement will import the module and creates a reference to the module in the current namespace. If you have to refer to entities (functions and data) that are defined in module, you can use module.function. The second is to use from module import*. This statement will import all of the entities that the module provides and set up references in the current namespace to all the public objects defined by that module. In this case, referencing an object within the module will boil down to simply stating its literal name in code.

Therefore, in order to use PySide functions and data in your program, you have to import it by saying either import PySide or from PySide import*. In the former case, if you have to refer to some function from PySide you have to prefix it with PySide, such as PySide.<function_name>. In the latter, you can simply call the function by <function_name>. Also, please note that in the latter statement, * can be replaced by specific functions or objects. The use of * denotes that we are trying to import all the available functions from that module. Throughout this book, I would prefer to use the latter format as I do not have to prefix the module name every time when I have to refer to something inside that module.

It's time to roll up our sleeves and get our hands dirty with some real coding now. We are going to learn how to create our first and the traditional Hello World application. Have a look at the code first, and we will dissect the program line by line for a complete explanation of what it does. The code may look a little strange to you at first but you will gain understanding as we move through:

# Import the necessary modules required
import sys
from PySide.QtCore import *
from PySide.QtGui import *

# Main Function
if __name__ == '__main__':

    # Create the main application
    myApp = QApplication(sys.argv)

    # Create a Label and set its properties
    appLabel = QLabel()
    appLabel.setText("Hello, World!!!\n Look at my first app using PySide")
    appLabel.setAlignment(Qt.AlignCenter)
    appLabel.setWindowTitle("My First Application")
    appLabel.setGeometry(300, 300, 250, 175)

    # Show the Label
    appLabel.show()

    # Execute the Application and Exit
    myApp.exec_()
    sys.exit()

On interpretation, you will get an output window, as shown in the figure:

Now, let's get into the working of the code. We start with importing the necessary objects into the program.

Lines 1, 2 and 3 imports the necessary modules that are required for the program. Python is supported with a library of standard modules that are built into the interpreter and provide access to operations that are not a part of the core language. One such standard module is sys, which provides access to some variables and functions that are used closely by the interpreter. In the preceding program, we need the sys module to pass command-line arguments sys.argv as a parameter to the QApplication class. It contains the list of command-line arguments that are passed to a Python script. Any basic GUI application that uses PySide should have two classes imported for basic functionality. They are QtCore and QtGui. The QtCore module contains functions that handle signals and slots and overall control of the application, whereas QtGui contains methods to create and modify various GUI window components and widgets.

In the main program, we are creating an instance of the QApplication class. QApplication creates the main event loop, where all events from the window system and other sources are processed and dispatched. This class is responsible for an application's initialization, finalization, and session management. It also handles the events and sets the application's look and feel. It parses the command-line arguments (sys.argv) and sets its internal state, accordingly. There should be only one QApplication object in the whole application even though the application creates one or many windows at any point in time.

Once the main application instance is created, we move on by creating a QLabel instance that will display the required message on the screen. This class is used to display a text or an image. The appearance of the text or image can be controlled in many ways by the functions provided by this class. The next two lines that follow the instantiation of this class set the text to be displayed and align it in a way that is centered on the application window.

As Python is an object-oriented programming language, we take the advantage of many object-oriented features, such as polymorphism, inheritance, object initialization, and so on. The complete Qt modules are designed in an object-oriented paradigm that supports these features. QLabel is a base class that is inherited from the QFrame super class whose parent class is QWidget (the details will be covered in forthcoming chapters). So, the functions that are available in QWidget and QFrame are inherited to QLabel. The two functions, setWindowTitle and setGeometry, are functions of QWidget, which are inherited by the QLabel class. These are used to set the title of the window and position it on the screen.

Now that all the instantiation and setup is done, we are calling the show function of the QLabel object to present the label on the screen. At this point only, the label becomes visible to the user and they are able to view it on the screen. Finally, we call the exec_() function of the QApplication object, which will enter the Qt main loop and start executing the Qt code. In reality, this is where the label will be shown to the user but the details can be safely ignored as of now. Finally, we exit the program by calling sys.exit().

It is not always possible to foresee all the errors in your programs and deal with them. Python comes with an excellent feature called exception handling to deal with all runtime errors. The aim of the book is not to explain this feature in detail but to give you some basic ideas so that you can implement it in the code that you write.

In general, the exceptions that are captured while executing a program are handled by saving the current state of the execution in a predefined place and switching the execution to a specific subroutine known as exception handler. Once they are handled successfully, the program takes the normal execution flow using the saved information. Sometimes, the normal flow may be hindered due to some exceptions that could not be resolved transparently. In any case, exception handling provides a mechanism for smooth flow of the program altogether.

In Python, the exception handling is carried out in a set of try and except statements. The try statements consist of a set of suspicious code that we think may cause an exception. On hitting an exception, the statement control is transferred to the except block where we can have a set of statements that handles the exception and resolves it for a normal execution of a program. The syntax for the same is as follows:

  try : suite
  except exception <, target> : suite
  except : suite

Here, suite is an indented block of statements. We can also have a set of try, except block in a try suite. The former except statement provides a specific exception class that can be matched with the exception that is raised. The latter except statement is a general clause that is used to handle a catch-all version. It is always advisable to write our code in the exception encapsulation.

In the previous example, consider that we have missed instantiating the appLabel object. This might cause an exception confronting to a class of exception called NameError. If we did not encapsulate our code within the try block, this raises a runtime error. However, if we had put our code in a try block, an exception can be raised and handled separately, which will not cause any hindrance to the normal execution of the program. The following set of code explains this with the possible output:

# Import the necessary modules required
import sys
from PySide.QtCore import *
from PySide.QtGui import *

# Main Function
if __name__ == '__main__':

    # Create the main application
    myApp = QApplication(sys.argv)

    # Create a Label and set its properties
    try:
        #appLabel = QLabel()
        appLabel.setText("Hello, World!!!\n Look at my first app using PySide")
        appLabel.setAlignment(Qt.AlignCenter)
        appLabel.setWindowTitle("My First Application")
        appLabel.setGeometry(300, 300, 250, 175)

        # Show the Label
        appLabel.show()

        # Execute the Application and Exit
        myApp.exec_()
        sys.exit()
    except NameError:
        print("Name Error:", sys.exc_info()[1])
        pass

In the preceding program, if we did not handle the exceptions, the output would be as shown in the figure:

Conversely, if we execute the preceding code, we will not run into any of the errors shown in the preceding figure. Instead, we will have captured the exception and given some information about it to the user, as follows:

Hence, it is always advised to implement exception handling as a good practice in your code.

The combination of Qt with Python provides the flexibility of Qt developers, develops GUI programs in a more robust language, and presents a rapid application development platform available on all major operating systems. We introduced to you the basics of PySide and its installation procedure on Windows, Linux, and Mac systems. We went on to create our first application, which introduced the main components of creating a GUI application and the event loop. We have concluded this chapter with an awareness on how to introduce exception handling as a best practice. Moving on, we are set to create some real-time applications in PySide.