Label in PyQt4 GUI not updating with every loop of FOR loop

Question

I'm having a problem, where I wish to run several command line functions from a python program using a GUI. I don't know if my problem is specific to PyQt4 or if it has to do with my bad use of python code.

What I wish to do is have a label on my GUI change its text value to inform the user which command is being executed. My problem however, arises when I run several commands using a for loop. I would like the label to update itself with every loop, however, the program is not updating the GUI label with every loop, instead, it only updates itself once the entire for loop is completed, and displays only the last command that was executed.

I am using PyQt4 for my GUI environment. And I have established that the text variable for the label is indeed being updated with every loop, but, it is not actually showing up visually in the GUI.

Is there a way for me to force the label to update itself? I have tried the update() and repaint() methods within the loop, but they don't make any difference.

I would really appreciate any help. Thank you.

Ronny.

Here is the code I am using:

# -*- coding: utf-8 -*-
import sys, os
from PyQt4 import QtGui, QtCore
Gui = QtGui
Core = QtCore

# ================================================== CREATE WINDOW OBJECT CLASS
class Win(Gui.QWidget):
    def __init__(self, parent = None):
        Gui.QWidget.__init__(self, parent)

        # --------------------------------------------------- SETUP PLAY BUTTON
        self.but1 = Gui.QPushButton("Run Commands",self)
        self.but1.setGeometry(10,10, 200, 100)

        # -------------------------------------------------------- SETUP LABELS
        self.label1 = Gui.QLabel("No Commands running", self)
        self.label1.move(10, 120)

        # ------------------------------------------------------- SETUP ACTIONS
        self.connect(self.but1, Core.SIGNAL("clicked()"), runCommands)


# =======================================================  RUN COMMAND FUNCTION
def runCommands():
    for i in commands:
        win.label1.setText(i)       # Make label display the command being run
        print win.label1.text()     # This shows that the value is actually
                                    # changing with every loop, but its just not
                                    # being reflected in the GUI label
        os.system(i)

# ======================================================================== MAIN

# ------------------------------------------------------  THE TERMINAL COMMANDS
com1 = "espeak 'senntence 1'"
com2 = "espeak 'senntence 2'"
com3 = "espeak 'senntence 3'"
com4 = "espeak 'senntence 4'"
com5 = "espeak 'senntence 5'"
commands = (com1, com2, com3, com4, com5)

# --------------------------------------------------- SETUP THE GUI ENVIRONMENT
app = Gui.QApplication(sys.argv)
win = Win()
win.show()

sys.exit(app.exec_())

Answer 1

The label gets updated all right, but the GUI isn't redrawn before the end of your loop.

Here's what you can do about it:

• Move your long-running loop to a secondary thread, drawing the GUI is happening in the main thread.

• Call app.processEvents() in your loop. This gives Qt the chance to process events and redraw the GUI.

• Break up your loop and let it run using a QTimer with a timeout of 0.

Using a thread is the best option, but involves quite a bit more work than just calling processEvents. Doing it with a timer is the old fashioned way and is not recommanded anymore. (see the documentation)

Answer 2

You have a basic misunderstanding of how such a GUI works. A Qt GUI has to run in an event loop of its own. Your loop runs instead, and the GUI can't do its work between the executions of your loop. That is, while your for loop is running the GUI code doesn't get CPU time and won't update.

You can set up a timer with an event, and execute your code in handlers of this event a set amount of time - this will solve your problem.