Python: Script doesn't run without print - python-2.7

Question

I have written a small executable script. This program only works when I use a print-statement (at the end of the on_start()-method of the QDialog()class). Please, take a look in the on_start()-method of the QDialog-class.
As you can see I create an a task_thread- and work_object-instance. But, when I execute this script without print-statement nothing happens - no Traceback or other error messages.
Where is the bug? I guess the problem is that I create the instances at the local level - I am not sure. How can I fix it?
import sys
from time import sleep
from PyQt4.QtCore import QThread, pyqtSignal, Qt, QStringList, QObject, QTimer
from PyQt4.QtGui import QVBoxLayout, QPushButton, QDialog, QProgressBar, QApplication, \
QMessageBox, QTreeWidget, QTreeWidgetItem, QLabel
def create_items(total):
for elem in range(total):
yield elem
class WorkObject(QObject):
notify_progress = pyqtSignal(object)
fire_label = pyqtSignal(object)
finished = pyqtSignal()
def __init__(self, parent=None):
QObject.__init__(self, parent)
def add_items(self):
total = 190000
x = (total/100*1)
a = x
counter = 0
for element in create_items(10000):
counter += 1
self.notify_progress.emit((element))
self.fire_label.emit((counter))
if counter == x:
x += a
sleep(1)
if not self.keep_running:
self.keep_running = True
break
def run(self):
self.keep_running = True
self.add_items()
def stop(self):
self.keep_running = False
class MyCustomDialog(QDialog):
finish = pyqtSignal()
def __init__(self, parent=None):
QDialog.__init__(self, parent)
layout = QVBoxLayout(self)
self.tree = QTreeWidget(self)
self.label = QLabel(self)
self.pushButton_start = QPushButton("Start", self)
self.pushButton_stopp = QPushButton("Stopp", self)
self.pushButton_close = QPushButton("Close", self)
layout.addWidget(self.label)
layout.addWidget(self.tree)
layout.addWidget(self.pushButton_start)
layout.addWidget(self.pushButton_stopp)
layout.addWidget(self.pushButton_close)
self.pushButton_start.clicked.connect(self.on_start)
self.pushButton_stopp.clicked.connect(self.on_finish)
self.pushButton_close.clicked.connect(self.close)
def fill_tree_widget(self, i):
parent = QTreeWidgetItem(self.tree)
self.tree.addTopLevelItem(parent)
parent.setText(0, unicode(i))
parent.setCheckState(0, Qt.Unchecked)
parent.setFlags(parent.flags() | Qt.ItemIsTristate | Qt.ItemIsUserCheckable)
def on_label(self, i):
self.label.setText("Result: {}".format(i))
def on_start(self):
self.tree.clear()
self.label.clear()
task_thread = QThread(self)
work_object = WorkObject()
work_object.fire_label.connect(self.on_label)
work_object.notify_progress.connect(self.fill_tree_widget)
work_object.finished.connect(task_thread.quit)
self.finish.connect(work_object.stop)
work_object.moveToThread(task_thread)
task_thread.started.connect(work_object.run)
task_thread.finished.connect(task_thread.deleteLater)
timer = QTimer()
# I set the single shot timer on False,
# because I don't want the timer to fires only once,
# it should fires every interval milliseconds
timer.setSingleShot(False)
timer.timeout.connect(work_object.stop)
timer.start(0)
task_thread.start()
print
def on_finish(self):
self.finish.emit()
def main():
app = QApplication(sys.argv)
window = MyCustomDialog()
window.resize(600, 400)
window.show()
sys.exit(app.exec_())
if __name__ == "__main__":
main()

Answer 1

You are right, work_object is created as a local instance.
Here is a simple solution:
work_object = WorkObject()
self.work_object = work_object # Add this line