Why does Python crash when my PyQt window becomes inactive/is resized?

Question

While playing around with different ways to show a webcam feed (obtained using imageio/ffmpeg) in a PyQt4 window, I stumbled upon this answer. After implementing this in Python 2.7 as an ImageDisplayWidget class (as summarized below), everything seems to work just fine: A window opens, showing my webcam feed without a glitch. If I close the window, everything is stopped and closed neatly.

But... Whenever I click anywhere outside this PyQt window (while it is showing the webcam feed), causing it to lose focus, Python.exe crashes with an unhandled win32 exception. The same happens when I try to resize the window.

I am probably making some kind of exceedingly silly beginner's mistake, but I just don't see it. Can anyone point me in the right direction? Am I breaking some basic rule of (Py)Qt or even Python?

Here's a minimal example:

import sys
import numpy
from PIL import Image, ImageQt  # pillow
from PyQt4 import QtGui, QtCore


class DummyVideoGrabber(QtCore.QTimer):
    signal_image_available = QtCore.pyqtSignal(QtGui.QImage)

    def __init__(self):
        super(DummyVideoGrabber, self).__init__()
        self.timeout.connect(self.update_image)
        self.counter = 0

    def update_image(self):
        # Dummy rgb image (in reality we get a numpy array from imageio's Reader)
        self.counter += 1
        numpy_image = numpy.zeros(shape=(480, 640, 3), dtype=numpy.int8)
        numpy_image[:, :, self.counter%3] = 255
        qt_image = ImageQt.ImageQt(Image.fromarray(numpy_image, mode='RGB'))

        # Emit image
        self.signal_image_available.emit(qt_image)


class ImageDisplayWidget(QtGui.QWidget):
    """
    Custom widget that displays an image using QPainter.

    Mostly copied from: https://stackoverflow.com/a/22355028/4720018

    """
    def __init__(self, size_wxh=None, parent=None):
        super(ImageDisplayWidget, self).__init__(parent)
        self.image = QtGui.QImage()

    def set_image(self, qimage, resize_window=False):
        self.image = qimage
        self.repaint()

    def paintEvent(self, QPaintEvent):
        if not self.image:
            return
        painter = QtGui.QPainter(self)
        painter.drawImage(self.rect(), self.image, self.image.rect())

app = QtGui.QApplication(sys.argv)

# instantiate a display object
display = ImageDisplayWidget()
display.resize(640, 480)
display.show()

# instantiate a grabber object
grabber = DummyVideoGrabber()
grabber.signal_image_available.connect(display.set_image)
grabber.start(100)  # timer interval in ms

# start the event loop
app.exec_()

I found that the crash can be prevented by adding a wasActiveWindow flag (initialized to True in the constructor) and encapsulating the drawImage() call in some logic like so:

        if self.isActiveWindow():
            if self.wasActiveWindow:
                painter.drawImage(self.rect(), self.image, self.image.rect())
            self.wasActiveWindow = True
        else:
            self.wasActiveWindow = False

However, resizing the window still crashes python.

Answer 1

Problem solved by keeping a reference to the qt_image as self.qt_image:

...
# Emit image
self.qt_image = ImageQt.ImageQt(Image.fromarray(numpy_image, mode='RGB'))
self.signal_image_available.emit(self.qt_image)
...    

This way it works as it should. Don't need the self.wasActiveWindow workaround anymore.

Still not sure why not keeping a reference would lead to a low-level python crash though...