Add Qt GUI example to the logging cookbook.

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 <https://www.qt.io/>`_ framework is a popular
+cross-platform UI framework with Python bindings using `PySide2
+<https://pypi.org/project/PySide2/>`_ or `PyQt5
+<https://pypi.org/project/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()