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Add Qt GUI example to the logging cookbook. (GH-14978) (GH-14979) 

(cherry picked from commit 1ed915e8ae)
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Miss Islington (bot) 2019-07-27 06:04:13 -07:00 committed by Vinay Sajip
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Doc/howto/logging-cookbook.rst
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@ -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()