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First programs in PyQt4 toolkit

In this part of the PyQt4 tutorial we will learn some basic functionality.

Simple example

The code example is very simplistic. It only shows a small window. Yet we can do a lot with this window. We can resize it. Maximize it. Minimize it. This requires a lot of coding. Someone already coded this functionality. Because it repeats in most applications, there is no need to code it over again. So it has been hidden from a programmer. PyQt4 is a high level toolkit. If we would code in a lower level toolkit, the following code example could easily have dozens of lines.

#!/usr/bin/python
# -*- coding: utf-8 -*-

"""
ZetCode PyQt4 tutorial 

In this example, we create a simple
window in PyQt4.

author: Jan Bodnar
website: zetcode.com 
last edited: October 2011
"""

import sys
from PyQt4 import QtGui


def main():
    
    app = QtGui.QApplication(sys.argv)

    w = QtGui.QWidget()
    w.resize(250, 150)
    w.move(300, 300)
    w.setWindowTitle('Simple')
    w.show()
    
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

The above code shows a small window on the screen.

import sys
from PyQt4 import QtGui

Here we provide the necessary imports. The basic GUI widgets are located in QtGui module.

app = QtGui.QApplication(sys.argv)

Every PyQt4 application must create an application object. The application object is located in the QtGui module. The sys.argv parameter is a list of arguments from the command line. Python scripts can be run from the shell. It is a way, how we can control the startup of our scripts.

w = QtGui.QWidget()

The QtGui.QWidget widget is the base class of all user interface objects in PyQt4. We provide the default constructor for QtGui.QWidget. The default constructor has no parent. A widget with no parent is called a window.

w.resize(250, 150)

The resize() method resizes the widget. It is 250px wide and 150px high.

w.move(300, 300)

The move() method moves the widget to a position on the screen at x=300, y=300 coordinates.

w.setWindowTitle('Simple')

Here we set the title for our window. The title is shown in the titlebar.

w.show()

The show() method displays the widget on the screen. A widget is first created in memory and later shown on the screen.

sys.exit(app.exec_())

Finally, we enter the mainloop of the application. The event handling starts from this point. The mainloop receives events from the window system and dispatches them to the application widgets. The mainloop ends, if we call the exit() method or the main widget is destroyed. The sys.exit() method ensures a clean exit. The environment will be informed, how the application ended.

The exec_() method has an underscore. It is because the exec is a Python keyword. And thus, exec_() was used instead.

An application icon

The application icon is a small image, which is usually displayed in the top left corner of the titlebar. In the following example we will show, how we do it in PyQt4. We will also introduce some new methods.

#!/usr/bin/python
# -*- coding: utf-8 -*-

"""
ZetCode PyQt4 tutorial 

This example shows an icon
in the titlebar of the window.

author: Jan Bodnar
website: zetcode.com 
last edited: October 2011
"""

import sys
from PyQt4 import QtGui


class Example(QtGui.QWidget):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):
        
        self.setGeometry(300, 300, 250, 150)
        self.setWindowTitle('Icon')
        self.setWindowIcon(QtGui.QIcon('web.png'))        
    
        self.show()
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()    

The previous example was coded in a procedural style. Python programming language supports both procedural and object oriented programming styles. Programming in PyQt4 means programming in OOP.

class Example(QtGui.QWidget):
    
    def __init__(self):
        super(Example, self).__init__()

The three most important things in object oriented programming are classes, data and methods. Here we create a new class called Example. The Example class inherits from QtGui.QWidget class. This means, that we must call two constructors. The first one for the Example class and the second one for the inherited class. The super() method returns the parent object of the Example class and we call it's constructor. The __init__() method is a constructor method in Python language.

self.initUI() 

The creation of the GUI is delegated to the initUI() method.

self.setGeometry(300, 300, 250, 150)
self.setWindowTitle('Icon')
self.setWindowIcon(QtGui.QIcon('web.png'))  

All three methods have been inherited from the QtGui.QWidget class. The setGeometry() does two things. It locates the window on the screen and sets the size of the window. The first two parameters are the x and y positions of the window. The third is the width and the fourth is the height of the window. In fact, it combines the resize() and move() methods in one method. The last method sets the application icon. To do this, we have created a QtGui.QIcon object. The QtGui.QIcon receives the path to our icon to be displayed.

def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()   

The startup code has been placed in the main() method. This is a Python idiom.

Showing a tooltip

We can provide a balloon help for any of our widgets.

#!/usr/bin/python
# -*- coding: utf-8 -*-

"""
ZetCode PyQt4 tutorial 

This example shows a tooltip on 
a window and a button

author: Jan Bodnar
website: zetcode.com 
last edited: October 2011
"""

import sys
from PyQt4 import QtGui


class Example(QtGui.QWidget):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):
        
        QtGui.QToolTip.setFont(QtGui.QFont('SansSerif', 10))
        
        self.setToolTip('This is a <b>QWidget</b> widget')
        
        btn = QtGui.QPushButton('Button', self)
        btn.setToolTip('This is a <b>QPushButton</b> widget')
        btn.resize(btn.sizeHint())
        btn.move(50, 50)       
        
        self.setGeometry(300, 300, 250, 150)
        self.setWindowTitle('Tooltips')    
        self.show()
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

In this example, we show a tooltip for a two PyQt4 widgets.

QtGui.QToolTip.setFont(QtGui.QFont('SansSerif', 10))

This static method sets a font used to render tooltips. We use a 10px SansSerif font.

self.setToolTip('This is a <b>QWidget</b> widget')

To create a tooltip, we call the setTooltip() method. We can use rich text formatting.

btn = QtGui.QPushButton('Button', self)
btn.setToolTip('This is a <b>QPushButton</b> widget')

We create a button widget and set a tooltip for it.

btn.resize(btn.sizeHint())
btn.move(50, 50)       

The button is being resized and moved on the window. The sizeHint() method gives a recommended size for the button.

Closing a window

The obvious way to how to close a window is to click on the x mark on the titlebar. In the next example, we will show, how we can programatically close our window. We will briefly touch signals and slots.

The following is the constructor of a QtGui.QPushButton, that we will use in our example.

QPushButton(string text, QWidget parent = None)

The text parameter is a text that will be displayed on the button. The parent is a widget, onto which we place our button. In our case it will be a QtGui.QWidget.

#!/usr/bin/python
# -*- coding: utf-8 -*-

"""
ZetCode PyQt4 tutorial 

This program creates a quit
button. When we press the button,
the application terminates. 

author: Jan Bodnar
website: zetcode.com 
last edited: October 2011
"""

import sys
from PyQt4 import QtGui, QtCore


class Example(QtGui.QWidget):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):               
        
        qbtn = QtGui.QPushButton('Quit', self)
        qbtn.clicked.connect(QtCore.QCoreApplication.instance().quit)
        qbtn.resize(qbtn.sizeHint())
        qbtn.move(50, 50)       
        
        self.setGeometry(300, 300, 250, 150)
        self.setWindowTitle('Quit button')    
        self.show()
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

In this example, we create a quit button. Upon clicking on the window, the application terminates.

from PyQt4 import QtGui, QtCore

In this example, we will use an object from the QtCore module.

qbtn = QtGui.QPushButton('Quit', self)

We create a push button. The button is an instance of the QtGui.QPushButton class. The first parameter of the constructor is the label of the button. The second parameter is the parent widget. The parent widget is the Example widget, which is a QtGui.QWidget by inheritance.

qbtn.clicked.connect(QtCore.QCoreApplication.instance().quit)

The event processing system in PyQt4 is built with the signal & slot mechanism. If we click on the button, the signal clicked is emitted. The slot can be a Qt slot or any Python callable. The QtCore.QCoreApplication contains the main event loop. It processes and dispatches all events. The instance() method gives us its current instance. Note that QtCore.QCoreApplication is created with the QtGui.QApplication. The clicked signal is connected to the quit() method, which terminates the application. The communication is done between two objects. The sender and the receiver. The sender is the push button, the receiver is the application object.

Message Box

By default, if we click on the x button on the titlebar, the QtGui.QWidget is closed. Sometimes we want to modify this default behaviour. For example, if we have a file opened in an editor to which we did some changes. We show a message box to confirm the action.

#!/usr/bin/python
# -*- coding: utf-8 -*-

"""
ZetCode PyQt4 tutorial 

This program shows a confirmation 
message box when we click on the close
button of the application window. 

author: Jan Bodnar
website: zetcode.com 
last edited: October 2011
"""

import sys
from PyQt4 import QtGui


class Example(QtGui.QWidget):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):               
        
        self.setGeometry(300, 300, 250, 150)        
        self.setWindowTitle('Message box')    
        self.show()
        
    def closeEvent(self, event):
        
        reply = QtGui.QMessageBox.question(self, 'Message',
            "Are you sure to quit?", QtGui.QMessageBox.Yes | 
            QtGui.QMessageBox.No, QtGui.QMessageBox.No)

        if reply == QtGui.QMessageBox.Yes:
            event.accept()
        else:
            event.ignore()        
        
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

If we close the QtGui.QWidget, the QtGui.QCloseEvent is generated. To modify the widget behaviour we need to reimplement the closeEvent() event handler.

reply = QtGui.QMessageBox.question(self, 'Message',
    "Are you sure to quit?", QtGui.QMessageBox.Yes | 
    QtGui.QMessageBox.No, QtGui.QMessageBox.No)

We show a message box with two buttons. Yes and No. The first string appears on the titlebar. The second string is the message text displayed by the dialog. The third argument specifies the combination of buttons appearing in the dialog. The last parameter is the default button. It is the button, which has initially the keyboard focus. The return value is stored in the reply variable.

if reply == QtGui.QMessageBox.Yes:
    event.accept()
else:
    event.ignore()  

Here we test the return value. If we clicked Yes button, we accept the event which leads to the closure of the widget and to the termination of the application. Otherwise we ignore the close event.

Centering window on the screen

The following script shows, how we can center a window on the desktop screen.

#!/usr/bin/python
# -*- coding: utf-8 -*-

"""
ZetCode PyQt4 tutorial 

This program centers a window 
on the screen. 

author: Jan Bodnar
website: zetcode.com 
last edited: October 2011
"""

import sys
from PyQt4 import QtGui


class Example(QtGui.QWidget):
    
    def __init__(self):
        super(Example, self).__init__()
        
        self.initUI()
        
    def initUI(self):               
        
        self.resize(250, 150)
        self.center()
        
        self.setWindowTitle('Center')    
        self.show()
        
    def center(self):
        
        qr = self.frameGeometry()
        cp = QtGui.QDesktopWidget().availableGeometry().center()
        qr.moveCenter(cp)
        self.move(qr.topLeft())
        
        
def main():
    
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()     

Centering a window on the screen.

self.center()

The code that will center the window is placed in the custom center() method.

qr = self.frameGeometry()

We get a rectangle specifying the geometry of the main window. This includes any window frame.

cp = QtGui.QDesktopWidget().availableGeometry().center()

We figure out the screen resolution of our monitor. And from this resolution, we get the center point.

qr.moveCenter(cp)

Our rectangle has already its width and height. Now we set the center of the rectangle to the center of the screen. The rectangle's size is unchanged.

self.move(qr.topLeft())

We move the top-left point of the application window to the top-left point of the qr rectangle, thus centering the window on our screen.

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In this part of the PyQt4 tutorial, we covered some basics.