Using threads in QGIS python plugins

I really wanted to write this post since a long time but things got in the way, and now an email finally triggered me.
As part of a consultancy I got to work with threads in a python plugin for QGIS. Since it was a pretty tedious process, I decided to write  the whole thing fairly generic so that it could be used easily by others.
Before using this, please note that there are ongoing efforts to get something like this directly in QGIS 3.
The code below (or maybe a more recent version – the one posted here is 1da300f from May 6, 2015) can be found on github
The usage is pretty self explanatory but it goes like this. You create your worker class that inherits from AbstractWorker and implement the work method:

class ExampleWorker(AbstractWorker):
    """worker, implement the work method here and raise exceptions if needed"""
    def __init__(self, steps):
        AbstractWorker.__init__(self)
        self.steps = steps
        # if a worker cannot define the length of the work it can set an
        # undefined progress by using
        # self.toggle_show_progress.emit(False)
    def work(self):
        print('Doing some long running job')

in your code you import all the needed modules and functions and call your ExampleWorker like this:

def run_example_worker():
    # create a new worker instance that does 7 steps
    worker = ExampleWorker(7)
    start_worker(worker, self.iface, 'testing the worker')

here, for reference, the whole code

# -*- coding: utf-8 -*-
"""
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <https://www.gnu.org/licenses/>.
    this was inspired by:
    https://snorf.net/blog/2013/12/07/multithreading-in-qgis-python-plugins/
    https://eli.thegreenplace.net/2011/04/25/passing-extra-arguments-to-pyqt-slot
    https://gis.stackexchange.com/questions/64831/how-do-i-prevent-qgis-from-being-detected-as-not-responding-when-running-a-hea/64928#64928
"""
__author__ = 'marco@opengis.ch'
import time
import traceback
from random import randint
from PyQt4 import QtCore
from PyQt4.QtCore import QThread, Qt
from PyQt4.QtGui import QProgressBar, QPushButton
from qgis.core import QgsMessageLog
from qgis.gui import QgsMessageBar
###########################################################################
# This is what you need to call when you want to start a work in a thread #
###########################################################################
def run_example_worker():
    # create a new worker instance that does 7 steps
    worker = ExampleWorker(7)
    start_worker(worker, self.iface, 'testing the worker')
###########################################################################
# This could be in a separate file example_worker.py                      #
###########################################################################
class ExampleWorker(AbstractWorker):
    """worker, implement the work method here and raise exceptions if needed"""
    def __init__(self, steps):
        AbstractWorker.__init__(self)
        self.steps = steps
        # if a worker cannot define the length of the work it can set an
        # undefined progress by using
        # self.toggle_show_progress.emit(False)
    def work(self):
        if randint(0, 100) > 70:
            raise RuntimeError('This is a random mistake during the '
                               'calculation')
        self.toggle_show_progress.emit(False)
        self.toggle_show_cancel.emit(False)
        self.set_message.emit(
            'NOT showing the progress because we dont know the length')
        sleep(randint(0, 10))
        self.toggle_show_cancel.emit(True)
        self.toggle_show_progress.emit(True)
        self.set_message.emit(
            'Doing long running job while showing the progress')
        for i in range(1, self.steps+1):
            if self.killed:
                self.cleanup()
                raise UserAbortedNotification('USER Killed')
            # wait one second
            time.sleep(1)
            self.progress.emit(i * 100/self.steps)
        return True
    def cleanup(self):
        print "cleanup here"
###########################################################################
# This could be in a separate file abstract_worker.py                     #
###########################################################################
class AbstractWorker(QtCore.QObject):
    """Abstract worker, ihnerit from this and implement the work method"""
    # available signals to be used in the concrete worker
    finished = QtCore.pyqtSignal(object)
    error = QtCore.pyqtSignal(Exception, basestring)
    progress = QtCore.pyqtSignal(float)
    toggle_show_progress = QtCore.pyqtSignal(bool)
    set_message = QtCore.pyqtSignal(str)
    toggle_show_cancel = QtCore.pyqtSignal(bool)
    # private signal, don't use in concrete workers this is automatically
    # emitted if the result is not None
    successfully_finished = QtCore.pyqtSignal(object)
    def __init__(self):
        QtCore.QObject.__init__(self)
        self.killed = False
    def run(self):
        try:
            result = self.work()
            self.finished.emit(result)
        except UserAbortedNotification:
            self.finished.emit(None)
        except Exception, e:
            # forward the exception upstream
            self.error.emit(e, traceback.format_exc())
            self.finished.emit(None)
    def work(self):
        """ Reimplement this putting your calculation here
            available are:
                self.progress.emit(0-100)
                self.killed
            :returns a python object - use None if killed is true
        """
        raise NotImplementedError
    def kill(self):
        self.is_killed = True
        self.set_message.emit('Aborting...')
        self.toggle_show_progress.emit(False)
class UserAbortedNotification(Exception):
    pass
def start_worker(worker, iface, message, with_progress=True):
    # configure the QgsMessageBar
    message_bar_item = iface.messageBar().createMessage(message)
    progress_bar = QProgressBar()
    progress_bar.setAlignment(Qt.AlignLeft | Qt.AlignVCenter)
    if not with_progress:
        progress_bar.setMinimum(0)
        progress_bar.setMaximum(0)
    cancel_button = QPushButton()
    cancel_button.setText('Cancel')
    cancel_button.clicked.connect(worker.kill)
    message_bar_item.layout().addWidget(progress_bar)
    message_bar_item.layout().addWidget(cancel_button)
    iface.messageBar().pushWidget(message_bar_item, iface.messageBar().INFO)
    # start the worker in a new thread
    # let Qt take ownership of the QThread
    thread = QThread(iface.mainWindow())
    worker.moveToThread(thread)
    worker.set_message.connect(lambda message: set_worker_message(
        message, message_bar_item))
    worker.toggle_show_progress.connect(lambda show: toggle_worker_progress(
        show, progress_bar))
    worker.toggle_show_cancel.connect(lambda show: toggle_worker_cancel(
        show, cancel_button))
    worker.finished.connect(lambda result: worker_finished(
        result, thread, worker, iface, message_bar_item))
    worker.error.connect(lambda e, exception_str: worker_error(
        e, exception_str, iface))
    worker.progress.connect(progress_bar.setValue)
    thread.started.connect(worker.run)
    thread.start()
    return thread, message_bar_item
def worker_finished(result, thread, worker, iface, message_bar_item):
        # remove widget from message bar
        iface.messageBar().popWidget(message_bar_item)
        if result is not None:
            # report the result
            iface.messageBar().pushMessage('The result is: %s.' % result)
            worker.successfully_finished.emit(result)
        # clean up the worker and thread
        worker.deleteLater()
        thread.quit()
        thread.wait()
        thread.deleteLater()
def worker_error(e, exception_string, iface):
    # notify the user that something went wrong
    iface.messageBar().pushMessage(
        'Something went wrong! See the message log for more information.',
        level=QgsMessageBar.CRITICAL,
        duration=3)
    QgsMessageLog.logMessage(
        'Worker thread raised an exception: %s' % exception_string,
        'SVIR worker',
        level=QgsMessageLog.CRITICAL)
def set_worker_message(message, message_bar_item):
    message_bar_item.setText(message)
def toggle_worker_progress(show_progress, progress_bar):
    progress_bar.setMinimum(0)
    if show_progress:
        progress_bar.setMaximum(100)
    else:
        # show an undefined progress
        progress_bar.setMaximum(0)
def toggle_worker_cancel(show_cancel, cancel_button):
    cancel_button.setVisible(show_cancel)

Hope this could help you, if you need professional help don’t hesitate contacting us