How to get non-blocking/real-time behavior from Python logging module? (output to PyQt QTextBrowser)

Question

Description: I have written a custom log handler for capturing log events and writing them to a QTextBrowser object (working sample code shown below).

Answer 1

Here is another method. In this example, I add a StreamHandler to the logger that writes to a buffer by inheriting from both QObject and StringIO: When the handler encounters a non-empty string, the bufferMessage signal is emitted and captured in the on_bufferMessage slot.

#!/usr/bin/env python
#-*- coding:utf-8 -*-

import logging, StringIO, time

from PyQt4 import QtCore, QtGui

class logBuffer(QtCore.QObject, StringIO.StringIO):
    bufferMessage = QtCore.pyqtSignal(str)

    def __init__(self, *args, **kwargs):
        QtCore.QObject.__init__(self)
        StringIO.StringIO.__init__(self, *args, **kwargs)

    def write(self, message):
        if message:
            self.bufferMessage.emit(unicode(message))

        StringIO.StringIO.write(self, message)

class myThread(QtCore.QThread):
    def __init__(self, parent=None):
        super(myThread, self).__init__(parent)
        self.iteration = None

    def start(self):
        self.iteration = 3

        return super(myThread, self).start()

    def run(self):        
        while self.iteration:
            logging.info("Hello from thread {0}! {1}".format(0, self.iteration))
            self.iteration -= 1

            time.sleep(3)

class myThread1(QtCore.QThread):
    def __init__(self, parent=None):
        super(myThread1, self).__init__(parent)
        self.iteration = None
        self.logger = logging.getLogger(__name__)

    def start(self):
        self.iteration = 3

        return super(myThread1, self).start()

    def run(self):        
        time.sleep(1)
        while self.iteration:
            self.logger.info("Hello from thread {0}! {1}".format(1, self.iteration))
            self.iteration -= 1

            time.sleep(3)


class myWindow(QtGui.QWidget):
    def __init__(self, parent=None):
        super(myWindow, self).__init__(parent)

        self.pushButton = QtGui.QPushButton(self)
        self.pushButton.setText("Send Log Message")
        self.pushButton.clicked.connect(self.on_pushButton_clicked)

        self.pushButtonThread = QtGui.QPushButton(self)
        self.pushButtonThread.setText("Start Threading")
        self.pushButtonThread.clicked.connect(self.on_pushButtonThread_clicked)

        self.lineEdit = QtGui.QLineEdit(self)
        self.lineEdit.setText("Hello!")

        self.label = QtGui.QLabel(self)

        self.layout = QtGui.QVBoxLayout(self)
        self.layout.addWidget(self.lineEdit)
        self.layout.addWidget(self.pushButton)
        self.layout.addWidget(self.pushButtonThread)
        self.layout.addWidget(self.label)

        self.logBuffer = logBuffer()
        self.logBuffer.bufferMessage.connect(self.on_logBuffer_bufferMessage)

        logFormatter = logging.Formatter('%(levelname)s: %(message)s')

        logHandler = logging.StreamHandler(self.logBuffer)
        logHandler.setFormatter(logFormatter)

        self.logger = logging.getLogger()
        self.logger.setLevel(logging.INFO)
        self.logger.addHandler(logHandler)

        self.thread = myThread(self)
        self.thread1 = myThread1(self)

    @QtCore.pyqtSlot()
    def on_pushButtonThread_clicked(self):
        self.thread.start()
        self.thread1.start()

    @QtCore.pyqtSlot(str)
    def on_logBuffer_bufferMessage(self, message):
        self.label.setText(message)

    @QtCore.pyqtSlot()
    def on_pushButton_clicked(self):
        message = self.lineEdit.text()
        self.logger.info(message if message else "No new messages")

if __name__ == "__main__":
    import sys

    app = QtGui.QApplication(sys.argv)
    app.setApplicationName('myWindow')

    main = myWindow()
    main.show()

    sys.exit(app.exec_())

The best thing about this method is that you can log messages from modules/threads of you main app without having to keep any reference to the logger, for example, by calling logging.log(logging.INFO, logging_message) or logging.info(logging_message)