Question

I’m currently need GUI library for a project. I’m familiar with python and found PyQt might be a good choice.

I’m reading a tutorial about PyQt, and quite confused about the following example program

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

"""
ZetCode PyQt4 tutorial 

In this example, we draw text in Russian azbuka.

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

import sys
from PyQt4 import QtGui, QtCore

class Example(QtGui.QWidget):

    def __init__(self):
        super(Example, self).__init__()
    
        self.initUI()
    
    def initUI(self):      

        self.text = u'\u041b\u0435\u0432 \u041d\u0438\u043a\u043e\u043b\u0430\
\u0435\u0432\u0438\u0447 \u0422\u043e\u043b\u0441\u0442\u043e\u0439: \n\
\u0410\u043d\u043d\u0430 \u041a\u0430\u0440\u0435\u043d\u0438\u043d\u0430'

        self.setGeometry(300, 300, 280, 170)
        self.setWindowTitle('Draw text')
        self.show()

    def paintEvent(self, event):

        qp = QtGui.QPainter()
        qp.begin(self)
        self.drawText(event, qp)
        qp.end()
    
    def drawText(self, event, qp):
  
        qp.setPen(QtGui.QColor(168, 34, 3))
        qp.setFont(QtGui.QFont('Decorative', 10))
        qp.drawText(event.rect(), QtCore.Qt.AlignCenter, self.text)        
            
    
def main():

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


if __name__ == '__main__':
    main()

Here, in main function, an Example object is created, thus __init__() function, initUI() is called.
My question is where does paintEvent() function is called?? since if we run the program, self.text(some Russian letters) will exactly appear on the widget.

In other words, what does sys.exit(app.exec_()) actually do? why it will call paintEvent() function?

Thanks!

Answer 1

From PyQt docs:

int QApplication.exec_ ()

Enters the main event loop and waits until exit() is called, then
returns the value that was set to exit() (which is 0 if exit() is
called via quit()).

It is necessary to call this function to start event handling. The
main event loop receives events from the window system and dispatches
these to the application widgets.

From another source:

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.

About painting:

4.2.1. When Painting Occurs

The paintEvent() method is called automatically when

• Your widget is shown for the first time.

• After a window has been moved to reveal some part (or all) of the
  widget.

• The window in which the widget lies is restored after being minimized.

• The window in which the widget lies is resized.

• The user switches from another desktop to the desktop on which the
  widget’s window lies.

You can generate paint events manually by calling QWidget::update().
QWidget::update() erases the widget before generating the paint event.
You can pass arguments to update(), which can restrict painting only
to areas (rectangles, in particular) that need it. The two equivalent
forms of the method are QWidget::update (int x, int y, int width, int
height) and QWidget::update (QRect rectangle), where x and y give the
upper-left corner of the rectangle, and width and height are obvious.
Because update() places a paint event into the event queue, no
painting occurs until the current method exits and control returns to
the event handler. This is a good thing because other events may be
waiting there to be processed, and events need to be processed in a
timely manner for the GUI to operate smoothly.

You can also invoke painting of the widget by calling QWidget::repaint (int x, int y, int width, int height, bool erase) (or one of several
convenience-method forms), where all the arguments mean the same as in
the case of the update() method, and erase tells repaint whether to
erase the rectangle before painting it. repaint() calls paintEvent()
directly. It does not place a paint event into the event queue, so use
this method with care. If you try to call repaint() repeatedly from a
simple loop to create an animation, for example, the animation will be
drawn, but the rest of your user interface will be unresponsive
because the events corresponding to mouse button clicks, keyboard
presses, and so on will be waiting in the queue. Even if you are not
performing a task as potentially time-consuming as animation, it is
generally better to use update() to help keep your GUI alive.

If you paint something on your widget outside the paintEvent(), you
still need to include the logic and commands necessary to paint that
same thing in paintEvent(). Otherwise, the painting you did would
disappear the next time the widget is updated.