when and how to use PySide QEventLoop

Question

I have an application in which I tried using a while loop but the application just hangs, and now I think I need to use QEventLoop to make the application work here is some of the relevant code

def ex_initialator(self):
    """ sets up things for the problem runner """

    print "ayadara"
    print "self.start_button_status this is...::" ,self.start_button_status

    #:Pop up dialogs       
    no_options_selected = QtGui.QMessageBox()

    select_range = QtGui.QMessageBox()

    select_op_type = QtGui.QMessageBox()



    #:Shows message if no options selected       
    if len(self.op_types) < 1 and len(self.num_range) < 1 and self.start_button_status == 0:
        print "Please pick from the options provided"
        no_options_selected.exec_()

    #:Shows message if range is not selected
    if len(self.op_types) >= 1 and len(self.num_range) < 1 and self.start_button_status == 0 :
        print "Please pick at least one of the ranges"
        select_range.exec_()

    #:Shows message if op_type/s not selected
    if len(self.op_types) < 1 and len(self.num_range) >= 1 and self.start_button_status == 0:
        print "Please pick at least one of the operation types"        
        select_op_type.exec_()

    #:Correct inputs initiates problem maker
    if len(self.op_types) >= 1 and len(self.num_range) >= 1 and (self.start_button_status == 0 or self.start_button_status == 1):

       self.start_button_status = 1
       self.prob_signal.runner_sig.connect(self.problem_runner)
       self.prob_signal.runner_sig.emit()


    def problem_runner(self):
    """ Expression Makers """
    while self.start_button_status == 1:

        #:picks range types
        range_type = random.choice(self.num_range)

        D = decimal.Decimal

        self.answer_lineEdit.returnPressed.connect(self.ex_evaluator)
        self.answer_lineEdit.clear()

what I want is to be able to start, pause, and stop buttons, I'm a beginner so I might have gone wrong in using QLineEdit in which case I would appreciate some suggestions as to a better implementation

Answer 1

If you are performing long operation, you need either move it to separate thread or call QApplication::processEvents() periodically. It allows GUI to remain responsible. Here is a simple example:

from PySide import QtGui, QtCore
import time

class MyWidget(QtGui.QWidget):
  def __init__(self):
    QtGui.QWidget.__init__(self)
    layout = QtGui.QVBoxLayout(self)
    self.label = QtGui.QLabel()
    self.button_start = QtGui.QPushButton("Start")
    self.button_stop = QtGui.QPushButton("Stop")
    self.button_stop.setEnabled(False)
    self.button_pause = QtGui.QPushButton("Pause")
    self.button_pause.setEnabled(False)
    layout.addWidget(self.label)
    layout.addWidget(self.button_start)
    layout.addWidget(self.button_stop)
    layout.addWidget(self.button_pause)
    self.button_start.clicked.connect(self.processing)
    self.button_pause.clicked.connect(self.pause)
    self.button_stop.clicked.connect(self.stop)

  def processing(self):
    self.button_start.setEnabled(False)
    self.button_stop.setEnabled(True)
    self.button_pause.setEnabled(True)
    self.stop_requested = False
    self.paused = False
    i = 0
    while not self.stop_requested:
      QtGui.QApplication.processEvents()
      if self.paused:
        time.sleep(0.1)
      else:
        i += 1
        self.label.setText(unicode(i))
    self.button_start.setEnabled(True)
    self.button_stop.setEnabled(False)
    self.button_pause.setEnabled(False)

  def pause(self):
    self.paused = not self.paused

  def stop(self):
    self.stop_requested = True

if __name__ == '__main__':
    import sys
    app = QtGui.QApplication(sys.argv)
    widget = MyWidget()
    widget.show()
    sys.exit(app.exec_())