[PyQt] deleteLater vs setParent(None)

Elvis Stansvik elvstone at gmail.com
Tue May 10 16:21:07 BST 2016

Hi all,

I have this slot of mine which fires up a process:

def moveToSlot(self, slot):
    assert slot in range(1, 5)

    def succeeded():
        if self._slot != slot:
            self._slot = slot

    def failed():
        log.error('moveToSlot: process failed')

    process = Process(timeout=10000, parent=self)
    process.start(_control('--carousel {}'.format(slot)))

(The Process class here is just a subclass of QProcess with a built-in
timeout and a couple of convenient signals (failed/succeeded).)

My question is about:


Can I count on that the Process object being destroyed eventually
here, or should I rather use

    def cleanup():


That is, setting its parent to None.

The reason I'm asking is this old post


where Phil explains:

"When a QObject has a parent it is owned by that parent. When the C++ object is
owned an extra reference is kept to the Python object under the covers. This
is needed in case the object is either a Python sub-class or you have stashed
extra attributes in the instance dictionary.

Therefore you need to either call the C++ dtor (using sip.delete()), arrange
for it to be called (using deleteLater()), or change the ownership back to
Python (using setParent()). Changing the ownership also removes that extra
reference so that the object is available to be garbage collected when it
goes out of scope."

It's that extra reference talked about here that I'm thinking of, is
that a good reason to use setParent(None) + letting the variable go
out of scope in Python, than calling deleteLater?

This slot of mine will be called many times over a long period of
time, so I don't want to leak memory.

Thanks in advance!

More information about the PyQt mailing list