The event loop is already running

Question

I have the following 5 files:

gui.py

# -*- coding: utf-8 -*-


from PyQt4 import QtCore, QtGui

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    def _fromUtf8(s):
        return s

try:
    _encoding = QtGui.QApplication.UnicodeUTF8
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig, _encoding)
except AttributeError:
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig)

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName(_fromUtf8("MainWindow"))
        MainWindow.resize(385, 365)
        sizePolicy = QtGui.QSizePolicy(QtGui.QSizePolicy.Preferred, QtGui.QSizePolicy.Preferred)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(MainWindow.sizePolicy().hasHeightForWidth())
        MainWindow.setSizePolicy(sizePolicy)
        MainWindow.setMinimumSize(QtCore.QSize(385, 365))
        MainWindow.setMaximumSize(QtCore.QSize(385, 365))
        self.centralwidget = QtGui.QWidget(MainWindow)
        self.centralwidget.setEnabled(True)
        self.centralwidget.setMinimumSize(QtCore.QSize(385, 318))
        self.centralwidget.setMaximumSize(QtCore.QSize(385, 318))
        self.centralwidget.setObjectName(_fromUtf8("centralwidget"))
        self.verticalLayout = QtGui.QVBoxLayout(self.centralwidget)
        self.verticalLayout.setSizeConstraint(QtGui.QLayout.SetDefaultConstraint)
        self.verticalLayout.setObjectName(_fromUtf8("verticalLayout"))
        self.textEdit = QtGui.QTextEdit(self.centralwidget)
        self.textEdit.setObjectName(_fromUtf8("textEdit"))
        self.verticalLayout.addWidget(self.textEdit)
        self.pushButton_1 = QtGui.QPushButton(self.centralwidget)
        self.pushButton_1.setObjectName(_fromUtf8("pushButton_1"))
        self.verticalLayout.addWidget(self.pushButton_1)
        self.pushButton_2 = QtGui.QPushButton(self.centralwidget)
        self.pushButton_2.setObjectName(_fromUtf8("pushButton_2"))
        self.verticalLayout.addWidget(self.pushButton_2)
        self.pushButton_3 = QtGui.QPushButton(self.centralwidget)
        self.pushButton_3.setObjectName(_fromUtf8("pushButton_3"))
        self.verticalLayout.addWidget(self.pushButton_3)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtGui.QMenuBar(MainWindow)
        self.menubar.setEnabled(False)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 385, 24))
        self.menubar.setObjectName(_fromUtf8("menubar"))
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtGui.QStatusBar(MainWindow)
        self.statusbar.setEnabled(False)
        self.statusbar.setObjectName(_fromUtf8("statusbar"))
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow", None))
        self.pushButton_1.setText(_translate("MainWindow", "Start a thread", None))
        self.pushButton_2.setText(_translate("MainWindow", "Toggle Timer", None))
        self.pushButton_3.setText(_translate("MainWindow", "Exit", None))

functions.py

import variables, logging

def initialize():

    variables.lock.acquire()
    try:
        pass
    finally:
        variables.lock.release()


def refreshgui():
    import start

    if variables.threadcounter != 0:
        start.myapp.ui.textEdit.setText(variables.globalstring + ' with ' + str(variables.threadcounter) + ' running threads' + '\nCounter: ' + str(variables.counter) + ' seconds')
    else:
        start.myapp.ui.textEdit.setText('String not submitted' + ' with ' + str(variables.threadcounter) + ' running threads' + '\nCounter: ' + str(variables.counter) + ' seconds')        
    variables.counter += 1

threadhandler.py

import variables, functions, threading

    def variablesinitialize():
        t = threading.Thread(name='Variables initialize', target=variables.initialize)
        t.start()
        t.join()

    def functionsinitialize():
        t = threading.Thread(name='Functions initialize', target=functions.initialize)
        t.start()
        t.join()

variables.py

import logging, threading

def initialize():
    global globalstring, counter, threadcounter, lock

    lock = threading.Lock()
    lock.acquire()
    try:
        logging.basicConfig(level=logging.INFO, format='%(asctime)s (%(threadName)-2s) %(message)s', datefmt='%d/%m/%Y %H:%M:%S')
        globalstring = 'No Success'
        counter = 0
        threadcounter = 0
    finally:
        lock.release()

and start.py

# -*- coding: utf-8 -*-

import sys
from PyQt4 import QtCore, QtGui
from gui import Ui_MainWindow

import threadhandler, functions

class StartQT4(QtGui.QMainWindow):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)

        QtCore.QObject.connect(self.ui.pushButton_1,QtCore.SIGNAL("clicked()"), exit)
        QtCore.QObject.connect(self.ui.pushButton_2,QtCore.SIGNAL("clicked()"), exit)
        QtCore.QObject.connect(self.ui.pushButton_3,QtCore.SIGNAL("clicked()"), exit)


app = QtGui.QApplication(sys.argv)
myapp = StartQT4()
myapp.show()

threadhandler.variablesinitialize()
threadhandler.functionsinitialize()

timer = QtCore.QTimer()
timer.timeout.connect(functions.refreshgui)
timer.start(1000)    

sys.exit(app.exec_())

---

After executing start.py i get the message: " QCoreApplication:exec: The event loop is already running "

I figured out, that it has something to do with the timer

timer = QtCore.QTimer()
timer.timeout.connect(functions.refreshgui)
timer.start(1000) 

out of start.py and the function

def refreshgui():
    import start

    if variables.threadcounter != 0:
        start.myapp.ui.textEdit.setText(variables.globalstring + ' with ' + str(variables.threadcounter) + ' running threads' + '\nCounter: ' + str(variables.counter) + ' seconds')
    else:
        start.myapp.ui.textEdit.setText('String not submitted' + ' with ' + str(variables.threadcounter) + ' running threads' + '\nCounter: ' + str(variables.counter) + ' seconds')        
    variables.counter += 1

out of functions.py

Could someone explain to me what the problem is? I don't know what's wrong with the code...

Answer 1

I think the problem is with your start.py file. You have a function refreshgui which re imports start.py

import will run every part of the code in the file. It is customary to wrap the main functionality in an ''if __name__ == '__main__': to prevent code from being run on import.

The error you are getting happens everytime you have more than one QApplication or QCoreApplication.

if __name__ == '__main__': # only executes the below code if it python has run it as the main
    app = QtGui.QApplication(sys.argv) # before this was getting called twice
    myapp = StartQT4()
    myapp.show()

    threadhandler.variablesinitialize()
    threadhandler.functionsinitialize()

    timer = QtCore.QTimer()
    timer.timeout.connect(functions.refreshgui) # I believe this re-imports start.py which calls all of this code again.
    timer.start(1000)    

    sys.exit(app.exec_())

Answer 2

Since I often test my code in Spyder/IPython I found it helpful to include the following check to HashSplat's answer to make sure that only one QApplication is running:

if __name__ == '__main__': 
    try:
        app
    except:
        app = QtGui.QApplication(sys.argv)