Python PyQt像素图更新崩溃

我有一个简单的PyQt应用程序。您可以使用pageup和pagedown键在图像文件夹中循环。

我正在使用QLabel的像素图显示图像。在您注释processEvents调用之前，以下逻辑实际上是有效的。

关于崩溃原因的任何想法以及为什么添加processEvents可以防止崩溃？

# Python version: 2.7.9 
# Qt version: 4.8.6
# PyQt version: 4.11.3

import sys
import os 
from PIL import Image
from PIL import ImageOps
from PIL.ImageQt import ImageQt
from PyQt4 import QtGui
from PyQt4 import QtCore
from PyQt4 import Qt

imagefolder = "C:/Users/alan/Desktop/imagefolder"

class QExampleLabel (QtGui.QLabel):
    def __init__(self, parentQWidget = None):
        super(QExampleLabel, self).__init__(parentQWidget)
        self.initUI()

    def initUI (self):

        self.imagelist = []

        for path, subdirs, files in os.walk(imagefolder):
            path = path.replace("\\", "/") 
            for file in files:
                base, ext = os.path.splitext(file)
                if ext.lower() == ".jpg":
                    jpg = path + "/" + file
                    self.imagelist.append(jpg)

        self.i = -1

    def keyPressEvent(self, keyevent):
        event = keyevent.key()
        if event == QtCore.Qt.Key_PageUp:
            self.backward()
        if event == QtCore.Qt.Key_PageDown:
            self.forward()

    def forward(self):
        self.i = self.i + 1
        if self.i >= len(self.imagelist):
            self.i = len(self.imagelist) - 1
        self.displayimage()

    def backward(self):    
        self.i = self.i - 1
        if self.i < 0:
            self.i = 0
        self.displayimage()

    def displayimage(self):   
        photo = self.imagelist[self.i]
        img = Image.open(photo)
        mysize = (260,260)
        method = Image.NEAREST if img.size == mysize else Image.ANTIALIAS
        self.cropimage = ImageOps.fit(img, mysize, method = method, centering = (0.5,0.5)) 
        qim = ImageQt(self.cropimage)
        pix = QtGui.QPixmap.fromImage(qim)
        self.setPixmap(pix)  
        QtGui.QApplication.processEvents()  # comment this line to see setPixmap crash

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    myQExampleLabel = QExampleLabel()
    myQExampleLabel.show()
    myQExampleLabel.resize(260,260)
    sys.exit(app.exec_())