multiprocessing problem [pyqt, py2exe]

Question

I am writing a GUI program using PyQt4. There is a button in my main window and by clicking this button. I hope to launch a background process which is an instance of a class derived from processing.Process.

class BackgroundTask(processing.Process):
    def __init__(self, input):
        processing.Process.__init__(self)
        ...

    def run(self):
        ...

(Note that I am using the Python2.5 port of the python-multiprocessing obtained from http://code.google.com/p/python-multiprocessing/ that is why it is processing.Process instead of multiprocessing.Process. I guess this should not make a difference. Am I right?)

The code connected to the button click signal is something simply like

 processing.freezeSupport()
 task = BackgroundTask(input)
 task.start()

The program works as expected under the python intepreter, i.e. if it is started from the command line "python myapp.py".

However, after I package the program using py2exe, everytime when I click that button, instead of starting the background task, a copy of the main window pops up. I am not sure what is the reason of this behavior. I guess it is related to the following note addressed at http://docs.python.org/library/multiprocessing.html#multiprocessing-programming

"Functionality within this package requires that the main method be importable by the children. This is covered in Programming guidelines however it is worth pointing out here. This means that some examples, such as the multiprocessing.Pool examples will not work in the interactive interpreter "

The only place I have if name == "main" is in the main module as in a typical pyqt program

if __name__ == "__main__":
    a = QApplication(sys.argv)
    QObject.connect(a,SIGNAL("lastWindowClosed()"),a,SLOT("quit()"))
    w = MainWindow()
    w.show()
    a.exec_()

Any solutions on how to fix this problem? Thanks!

Answer 1

I think your actual problem has to do with this:

The program works as expected under the python intepreter, i.e. if it is started from the command line "python myapp.py".

However, after I package the program using py2exe, every time when I click that button, > instead of starting the background task, a copy of the main window pops up.

You need to add a special call to the freeze_support() function to make the multiprocessing module work with "frozen" executables (eg, those made with py2exe):

if __name__ == "__main__":
    # add freeze support
    processing.freeze_support()
    a = QApplication(sys.argv)
    QObject.connect(a,SIGNAL("lastWindowClosed()"),a,SLOT("quit()"))
    w = MainWindow()
    w.show()
    a.exec_()

Reference: http://docs.python.org/library/multiprocessing.html#multiprocessing.freeze_support

Answer 2

"Functionality within this package requires that the main method be importable by the children."

I think this means you have to have main() function defined somewhere.

Answer 3

The question is concerning Python 2 and was solved. For Python 3, it would look like:

from multiprocessing import freeze_support

if __name__ == '__main__':
    freeze_support()

    a = QApplication(sys.argv)
    ...