How to use QThreads with libtorrent in PyQt?

Question

I want to write some simple PyQt torrent client, but I've got some snag with that. I would like to run some loop which takes care of downloading files (simple code using libtorrent) in a PyQt code. Although, when the torrent downloading works, the UI doesn't show up and when I change the order of called functions, the UI shows but downloading doesn't work. I read about QThreads but it's a bit hard for me - could anyone explain how does QThread work and how to use it with libtorrent? Here's some example of torrent client code:

import libtorrent as lt
import time
import sys

ses = lt.session()
ses.listen_on(6881, 6891)

info = lt.torrent_info(sys.argv[1])
h = ses.add_torrent({'ti': info, 'save_path': './'})
print 'starting', h.name()

while (not h.is_seed()):
   s = h.status()

   state_str = ['queued', 'checking', 'downloading metadata', \
      'downloading', 'finished', 'seeding', 'allocating', 'checking fastresume']
   print '\r%.2f%% complete (down: %.1f kb/s up: %.1f kB/s peers: %d) %s' % \
      (s.progress * 100, s.download_rate / 1000, s.upload_rate / 1000, \
      s.num_peers, state_str[s.state]),
   sys.stdout.flush()

   time.sleep(1)

print h.name(), 'complete'

Answer 1

I would imagine QT runs a message loop that doesn't return until the application is terminated.

What I believe you want to do is to move the loop body (that polls the libtorrent status) into a message handler that's called regularly by QT. Say, a timer function.

Answer 2

Firstly I would recommend the PySide implementation of Qt.

In that situation (and plain pyQt) you should look into signals and slots:

http://qt-project.org/wiki/Signals_and_Slots_in_PySide

http://zetcode.com/gui/pyqt4/eventsandsignals/

Basically you just need to set up what is usually called a 'Worker' class that is just a QThread.

class Worker(QtCore.QThread):
    updateProgress = QtCore.Signal(int) #or whatever you wanna call it 

This ^^^ is where the code above goes. Then what you want to do is connect the signal in the main GUI thread / class.

self.worker.updateProgress.connect(self.setProgress) #notice no ()


def setProgress(self, progress):
    self.progressBar.setValue(progress) 

Then finally inside the function that will output values you want:

self.updateProgress.emit(value)

This example implies something like a progress bar functionality. I know this post is old -- but this would have helped me so hopefully the googlers will find it useful ;)