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Editorial Team
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Asked: 2026-05-28T20:34:36+00:00

For a simple chat program i use a c lib that is wrapped via

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For a simple chat program i use a c lib that is wrapped via boost::python.

A simple GUI is written using PyQT. Receiving messages is done via a blocking call to
said lib. For the GUI to refresh independently the communication part is in a QThread.

While i would assume the GUI and the communication to be independent, the GUI is extremely unresponsive and seems to only update when messages are coming in.

#!/usr/bin/env python

import sys

from PyQt4.QtCore import *
from PyQt4.QtGui import *

import pynetcom2
import time


class NetCom(QThread):

  def __init__(self):
    QThread.__init__(self)
    self.client = pynetcom2.Client()
    self.client.init('127.0.0.1', 4028)
    self.client.provide('myChat', 1)
    self.client.subscribe('myChat', 100)

  def run(self):
    while (1):
      print "Waiting for message..."
      text = self.client.recvStr('myChat', True)
    return



class Netchat(QMainWindow):

    def __init__(self, argv):

        if (len(argv) != 2):
            print "Usage: %s <nickname>" %(argv[0])
            sys.exit(1)
        self.nickname = argv[1]
        print "Logging in with nickname '%s'" %(self.nickname)

        super(Netchat, self).__init__()
        self.setupUI()

        rect = QApplication.desktop().availableGeometry()
        self.resize(int(rect.width() * 0.3), int(rect.height() * 0.6))
        self.show()

        self.com = NetCom()
        self.com.start()

    def setupUI(self):
        centralWidget = QWidget()
        self.setCentralWidget(centralWidget)

        self.testList = QListWidget()

        mainLayout = QHBoxLayout()
        mainLayout.addWidget(self.testList)
        centralWidget.setLayout(mainLayout)

if __name__ == "__main__":
  app = QApplication(sys.argv)
  netchat = Netchat(sys.argv)
  app.exec_()
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1 Answer

  1. Editorial Team

    This might be caused by the infamous Global Interpreter Lock (GIL). Python does not allow two threads to execute Python code at the same time. In your C function, you have to explicitly release and re-acquire the GIL if you want your GUI code to run in parallel.

    This is explained in the Python C API documentation: Thread State and the Global Interpreter Lock.

    It boils down to using the following macros in your C extension:

    Py_BEGIN_ALLOW_THREADS

// Your expensive computation goes here.

Py_END_ALLOW_THREADS
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