diff --git a/Doc/howto/logging-cookbook.rst b/Doc/howto/logging-cookbook.rst
index e62308192d1..ef9ad447f80 100644
--- a/Doc/howto/logging-cookbook.rst
+++ b/Doc/howto/logging-cookbook.rst
@@ -2744,3 +2744,216 @@ And if we want less:
In this case, the commands don't print anything to the console, since nothing
at ``WARNING`` level or above is logged by them.
+
+.. _qt-gui:
+
+A Qt GUI for logging
+--------------------
+
+A question that comes up from time to time is about how to log to a GUI
+application. The `Qt `_ framework is a popular
+cross-platform UI framework with Python bindings using `PySide2
+`_ or `PyQt5
+`_ libraries.
+
+The following example shows how to log to a Qt GUI. This introduces a simple
+``QtHandler`` class which takes a callable, which should be a slot in the main
+thread that does GUI updates. A worker thread is also created to show how you
+can log to the GUI from both the UI itself (via a button for manual logging)
+as well as a worker thread doing work in the background (here, just random
+short delays).
+
+The worker thread is implemented using Qt's ``QThread`` class rather than the
+:mod:`threading` module, as there are circumstances where one has to use
+``QThread``, which offers better integration with other ``Qt`` components.
+
+The code should work with recent releases of either ``PySide2`` or ``PyQt5``.
+You should be able to adapt the approach to earlier versions of Qt. Please
+refer to the comments in the code for more detailed information.
+
+.. code-block:: python3
+
+ import datetime
+ import logging
+ import random
+ import sys
+ import time
+
+ # Deal with minor differences between PySide2 and PyQt5
+ try:
+ from PySide2 import QtCore, QtGui, QtWidgets
+ Signal = QtCore.Signal
+ Slot = QtCore.Slot
+ except ImportError:
+ from PyQt5 import QtCore, QtGui, QtWidgets
+ Signal = QtCore.pyqtSignal
+ Slot = QtCore.pyqtSlot
+
+ logger = logging.getLogger(__name__)
+
+ #
+ # Signals need to be contained in a QObject or subclass in order to be correctly
+ # initialized.
+ #
+ class Signaller(QtCore.QObject):
+ signal = Signal(str)
+
+ #
+ # Output to a Qt GUI is only supposed to happen on the main thread. So, this
+ # handler is designed to take a slot function which is set up to run in the main
+ # thread. In this example, the function takes a single argument which is a
+ # formatted log message. You can attach a formatter instance which formats a
+ # LogRecord however you like, or change the slot function to take some other
+ # value derived from the LogRecord.
+ #
+ # You specify the slot function to do whatever GUI updates you want. The handler
+ # doesn't know or care about specific UI elements.
+ #
+ class QtHandler(logging.Handler):
+ def __init__(self, slotfunc, *args, **kwargs):
+ super(QtHandler, self).__init__(*args, **kwargs)
+ self.signaller = Signaller()
+ self.signaller.signal.connect(slotfunc)
+
+ def emit(self, record):
+ s = self.format(record)
+ self.signaller.signal.emit(s)
+
+ #
+ # This example uses QThreads, which means that the threads at the Python level
+ # are named something like "Dummy-1". The function below gets the Qt name of the
+ # current thread.
+ #
+ def ctname():
+ return QtCore.QThread.currentThread().objectName()
+
+ #
+ # This worker class represents work that is done in a thread separate to the
+ # main thread. The way the thread is kicked off to do work is via a button press
+ # that connects to a slot in the worker.
+ #
+ # Because the default threadName value in the LogRecord isn't much use, we add
+ # a qThreadName which contains the QThread name as computed above, and pass that
+ # value in an "extra" dictionary which is used to update the LogRecord with the
+ # QThread name.
+ #
+ # This example worker just outputs messages sequentially, interspersed with
+ # random delays of the order of a few seconds.
+ #
+ class Worker(QtCore.QObject):
+ @Slot()
+ def start(self):
+ extra = {'qThreadName': ctname() }
+ logger.debug('Started work', extra=extra)
+ i = 1
+ # Let the thread run until interrupted. This allows reasonably clean
+ # thread termination.
+ while not QtCore.QThread.currentThread().isInterruptionRequested():
+ delay = 0.5 + random.random() * 2
+ time.sleep(delay)
+ logger.debug('Message after delay of %3.1f: %d', delay, i, extra=extra)
+ i += 1
+
+ #
+ # Implement a simple UI for this cookbook example. This contains:
+ #
+ # * A read-only text edit window which holds formatted log messages
+ # * A button to start work and log stuff in a separate thread
+ # * A button to log something from the main thread
+ # * A button to clear the log window
+ #
+ class Window(QtWidgets.QWidget):
+
+ def __init__(self, app):
+ super(Window, self).__init__()
+ self.app = app
+ self.textedit = te = QtWidgets.QTextEdit(self)
+ # Set whatever the default monospace font is for the platform
+ f = QtGui.QFont('nosuchfont')
+ f.setStyleHint(f.Monospace)
+ te.setFont(f)
+ te.setReadOnly(True)
+ PB = QtWidgets.QPushButton
+ self.work_button = PB('Start background work', self)
+ self.log_button = PB('Log a message at a random level', self)
+ self.clear_button = PB('Clear log window', self)
+ self.handler = h = QtHandler(self.update_status)
+ # Remember to use qThreadName rather than threadName in the format string.
+ fs = '%(asctime)s %(qThreadName)-12s %(levelname)-8s %(message)s'
+ formatter = logging.Formatter(f)
+ h.setFormatter(formatter)
+ logger.addHandler(h)
+ # Set up to terminate the QThread when we exit
+ app.aboutToQuit.connect(self.force_quit)
+
+ # Lay out all the widgets
+ layout = QtWidgets.QVBoxLayout(self)
+ layout.addWidget(te)
+ layout.addWidget(self.work_button)
+ layout.addWidget(self.log_button)
+ layout.addWidget(self.clear_button)
+ self.setFixedSize(900, 400)
+
+ # Connect the non-worker slots and signals
+ self.log_button.clicked.connect(self.manual_update)
+ self.clear_button.clicked.connect(self.clear_display)
+
+ # Start a new worker thread and connect the slots for the worker
+ self.start_thread()
+ self.work_button.clicked.connect(self.worker.start)
+ # Once started, the button should be disabled
+ self.work_button.clicked.connect(lambda : self.work_button.setEnabled(False))
+
+ def start_thread(self):
+ self.worker = Worker()
+ self.worker_thread = QtCore.QThread()
+ self.worker.setObjectName('Worker')
+ self.worker_thread.setObjectName('WorkerThread') # for qThreadName
+ self.worker.moveToThread(self.worker_thread)
+ # This will start an event loop in the worker thread
+ self.worker_thread.start()
+
+ def kill_thread(self):
+ # Just tell the worker to stop, then tell it to quit and wait for that
+ # to happen
+ self.worker_thread.requestInterruption()
+ if self.worker_thread.isRunning():
+ self.worker_thread.quit()
+ self.worker_thread.wait()
+ else:
+ print('worker has already exited.')
+
+ def force_quit(self):
+ # For use when the window is closed
+ if self.worker_thread.isRunning():
+ self.kill_thread()
+
+ # The functions below update the UI and run in the main thread because
+ # that's where the slots are set up
+
+ @Slot(str)
+ def update_status(self, status):
+ self.textedit.append(status)
+
+ @Slot()
+ def manual_update(self):
+ levels = (logging.DEBUG, logging.INFO, logging.WARNING, logging.ERROR,
+ logging.CRITICAL)
+ level = random.choice(levels)
+ extra = {'qThreadName': ctname() }
+ logger.log(level, 'Manually logged!', extra=extra)
+
+ @Slot()
+ def clear_display(self):
+ self.textedit.clear()
+
+ def main():
+ QtCore.QThread.currentThread().setObjectName('MainThread')
+ logging.getLogger().setLevel(logging.DEBUG)
+ app = QtWidgets.QApplication(sys.argv)
+ example = Window(app)
+ example.show()
+ sys.exit(app.exec_())
+
+ if __name__=='__main__':
+ main()