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This PySide tutorial shows you how to use native Python threads (non-QThread, i.e. threading.Thread) to carry out work in the background (e.g. downloading files). In the special case of downloading, you might want to use QNetworkAccessManager and friends, but in this case, we assume that you can't use it for whatever reason (e.g. because you want to use Twisted or because you already have your special download code that you want to reuse).


Importing required modules

We will be using standard Python modules for threading (threading) and for downloading (urllib). From PySide, we need the standard module QtCore, QtGui and QtDeclarative:

import os
import sys
import threading
import urllib
from PySide import QtCore, QtGui, QtDeclarative

The Downloader object

We now subclass QObject (so we can have Signals, Slots and Properties in our downloader) and implement all the properties that we need for downloading the file and also for displaying the current status in the UI:

class Downloader(QtCore.QObject):
 def ''init''(self, url, filename=None):
 self._url = url
 if filename is None:
 filename = os.path.basename(self._url)
 self._filename = filename
 self._progress = 0.
 self._running = False
 self._size = -1
 def _download(self):
 def reporthook(pos, block, total):
 if self.size != total:
 self._size = total
 self.progress = float(pos*block)/float(total)
 urllib.urlretrieve(self._url, self._filename, reporthook)
 self.running = False
 def start_download(self):
 if not self.running:
 self.running = True
 thread = threading.Thread(target=self._download)
 def _get_progress(self):
 return self._progress
 def _set_progress(self, progress):
 self._progress = progress
 def _get_running(self):
 return self._running
 def _set_running(self, running):
 self._running = running
 def _get_filename(self):
 return self._filename
 def _get_size(self):
 return self._size
 on_progress = QtCore.Signal()
 on_running = QtCore.Signal()
 on_filename = QtCore.Signal()
 on_size = QtCore.Signal()
 progress = QtCore.Property(float, _get_progress, _set_progress,
 running = QtCore.Property(bool, _get_running, _set_running,
 filename = QtCore.Property(str, _get_filename, notify=on_filename)
 size = QtCore.Property(int, ''get_size, notify=on_size)

Creating a new Downloader instance

As an example, we create a new Downloader here that downloads a kernel image for the N900 from the MeeGo repository:

downloader = Downloader

QApplication, QDeclarativeView and context properties

As usual, we simply instantiate a new QApplication and a QDeclarativeView. Our Downloader is exposed to the QML context by setting it as context property downloader on the rootContext of our view. We then simply load the QML file via setSource, show the view and execute the application:

app = QtGui.QApplication(sys.argv)
view = QtDeclarative.QDeclarativeView()
view.rootContext().setContextProperty('downloader', downloader)
view.setSource(file.replace('.py', '.qml'))


This is the QML UI for our downloader example. The interesting parts here are:

  • When the button is clicked, downloader.start_download() (a PySide Slot) is called, which starts the thread
  • The UI elements use properties of the downloader to determine visiblity and content- they are automatically updated when the properties are notified to be updated
import Qt 4.7
Rectangle {
 width: 200; height: 160
function formatProgress(size, progress) {
 return "" + parseInt(progress*size/1024) +
 " KiB ("''parseInt(progress*100.)'' "%)";
Text {
 x: progressBar.x; y: 20
 width: progressBar.width
 font.pixelSize: 8
 text: downloader.filename
 elide: Text.ElideRight
Rectangle {
 id: progressBar
 color: "#aaa"
x: 20; y: 60
 width: parent.width-40
 height: 20
Rectangle {
 color: downloader.progress<1?"#ee8":"#8e8"
 clip: true
anchors {
 top: parent.top
 bottom: parent.bottom
 left: parent.left
width: parent.width*downloader.progress
Text {
 anchors {
 fill: parent
 rightMargin: 5
 color: "black"
 text: formatProgress(downloader.size, downloader.progress)
 verticalAlignment: Text.AlignVCenter
 horizontalAlignment: Text.AlignRight
Rectangle {
 anchors.left: progressBar.left
 anchors.right: progressBar.right
color: "#aad"
 y: progressBar.y + progressBar.height + 20
 height: 40
Text {
 anchors.fill: parent
 color: "#003"
 text: downloader.running?"Please wait…":"Start download"
verticalAlignment: Text.AlignVCenter
 horizontalAlignment: Text.AlignHCenter
MouseArea {
 anchors.fill: parent
 onClicked: { downloader.start_download() }

How the example app looks

Save the files WorkingOnIt.py and WorkingOnIt.qml in the same folder and start the app using python WorkingOnIt.py: Screenshotupdate.jpg