PyQt Quickie: Don't Get Garbage Collected

There is one area where Qt and Python (and in consequence PyQt) have major disagreements. That area is memory management.

While Qt has its own mechanisms to handle object allocation and disposal (the hierarchical QObject trees, smart pointers, etc.), PyQt runs on Python, so it has garbage collection.

Let's consider a simple example:

from PyQt4 import QtCore

def finished():
    print "The process is done!"
    # Quit the app
    QtCore.QCoreApplication.instance().quit()

def launch_process():
    # Do something asynchronously
    proc = QtCore.QProcess()
    proc.start("/bin/sleep 3")
    # After it finishes, call finished
    proc.finished.connect(finished)

def main():
    app = QtCore.QCoreApplication([])
    # Launch the process
    launch_process()
    app.exec_()

main()

If you run this, this is what will happen:

QProcess: Destroyed while process is still running.
The process is done!

Plus, the script never ends. Fun! The problem is that proc is being deleted at the end of launch_process because there are no more references to it.

Here is a better way to do it:

from PyQt4 import QtCore

processes = set([])

def finished():
    print "The process is done!"
    # Quit the app
    QtCore.QCoreApplication.instance().quit()

def launch_process():
    # Do something asynchronously
    proc = QtCore.QProcess()
    processes.add(proc)
    proc.start("/bin/sleep 3")
    # After it finishes, call finished
    proc.finished.connect(finished)

def main():
    app = QtCore.QCoreApplication([])
    # Launch the process
    launch_process()
    app.exec_()

main()

Here, we add a global processes set and add proc there so we always keep a reference to it. Now, the program works as intended. However, it still has an issue: we are leaking QProcess objects.

While in this case the leak is very short-lived, since we are ending the program right after the process ends, in a real program this is not a good idea.

So, we would need to add a way to remove proc from processes in finished. This is not as easy as it may seem. Here is an idea that will not work as you expect:

def launch_process():
    # Do something asynchronously
    proc = QtCore.QProcess()
    processes.add(proc)
    proc.start("/bin/sleep 3")
    # Remove the process from the global set when done
    proc.finished.connect(lambda: processes.remove(proc))
    # After it finishes, call finished
    proc.finished.connect(finished)

In this version, we will still leak proc, even though processes is empty! Why? Because we are keeping a reference to proc in the lambda!

I don't really have a good answer for that that doesn't involve turning everything into members of a QObject and using sender to figure out what process is ending, or using QSignalMapper. That version is left as an exercise.

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