Busy indication with PyQt progress bar

First, it's a bad idea to directly edit the code created with QtDesigner. You may have seen the line # WARNING! All changes made in this file will be lost! at the top of the document. For such a simple widget, you're better off with manual coding.

Secondly, take a closer look at what the action slot actually does.

def action(self):
    self.pb.setRange(0, 0) # Un
    sleep(3) # <-- Your slot blocks HERE
    self.pb.setRange(0, 100)
    self.pb.setValue(100)
    QtGui.qApp.processEvents()

There is no reason for your progressBar to update its value while your slot is blocked in sleep. When action is called, the slot thread sleeps for 3 sec then sets the progress bar to a full 100.

You can't expect the progressBar to magically update itself while your task is running. If you have no idea how long it will take and you can't subdivise it in steps, you should consider using a pulsed ProgressBar instead (see example 1 below). If you can easily get the progression of your task (say copying n files), you should update the value of your progressBar accordingly.

Either way, you should use QThread to get a non-blocking behaviour, and signals to communicate between your thread(s) and your main application.

• The main application starts the QThread implementing the long-running task.
• The QThread notifies the task progression (if available) or completion to the main application

---

Example 1 - Pulse ProgressBar:

If minimum and maximum are both set to 0, the progress bar will show a busy indicator instead of a percentage of steps.

class MyCustomWidget(QtGui.QWidget):

    def __init__(self, parent=None):
        super(MyCustomWidget, self).__init__(parent)
        layout = QtGui.QVBoxLayout(self)

        # Create a progress bar and a button and add them to the main layout
        self.progressBar = QtGui.QProgressBar(self)
        self.progressBar.setRange(0,1)
        layout.addWidget(self.progressBar)
        button = QtGui.QPushButton("Start", self)
        layout.addWidget(button)      

        button.clicked.connect(self.onStart)

        self.myLongTask = TaskThread()
        self.myLongTask.taskFinished.connect(self.onFinished)

    def onStart(self): 
        self.progressBar.setRange(0,0)
        self.myLongTask.start()

    def onFinished(self):
        # Stop the pulsation
        self.progressBar.setRange(0,1)


class TaskThread(QtCore.QThread):
    taskFinished = QtCore.pyqtSignal()
    def run(self):
        time.sleep(3)
        self.taskFinished.emit()  

Example 2 - Classic ProgressBar:

class MyCustomWidget(QtGui.QWidget):

    def __init__(self, parent=None):
        super(MyCustomWidget, self).__init__(parent)
        layout = QtGui.QVBoxLayout(self)       

        self.progressBar = QtGui.QProgressBar(self)
        self.progressBar.setRange(0,100)
        button = QtGui.QPushButton("Start", self)
        layout.addWidget(self.progressBar)
        layout.addWidget(button)

        button.clicked.connect(self.onStart)

        self.myLongTask = TaskThread()
        self.myLongTask.notifyProgress.connect(self.onProgress)


    def onStart(self):
        self.myLongTask.start()

    def onProgress(self, i):
        self.progressBar.setValue(i)


class TaskThread(QtCore.QThread):
    notifyProgress = QtCore.pyqtSignal(int)
    def run(self):
        for i in range(101):
            self.notifyProgress.emit(i)
            time.sleep(0.1)

A bit late, but I've written detailed documentation on this very issue since a lot of people seem to face this problem.

Introduction to Progress Bars