PyQt4 & Flask: 解决 "QObject: Cannot create children for a parent that is in a di

我想通过Flask服务器上的一个HTTP请求将页面的图片保存下来。当我运行这个程序时，得到了如下错误信息：

QObject: Cannot create children for a parent that is in a different thread.

以下是我的代码：

import sys
import time
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4.QtWebKit import *
import Image
from flask import Flask, Response, jsonify,request
app=Flask(__name__)


class Screenshot(QWebView):
    def __init__(self):
        self.app = QApplication(sys.argv)
        QWebView.__init__(self)
        self._loaded = False
        self.loadFinished.connect(self._loadFinished)

    def capture(self,url,width,output_file):
        self.resize(width,300)
        self.load(QUrl(url))
        self.wait_load()
        # set to webpage size
        frame = self.page().mainFrame()
        self.page().setViewportSize(frame.contentsSize())
        # render image
        image = QImage(self.page().viewportSize(), QImage.Format_ARGB32)
        painter = QPainter(image)
        frame.render(painter)
        painter.end()
        print 'saving', output_file
        image.save(output_file)

    def wait_load(self, delay=0):
        # process app events until page loaded
        while not self._loaded:
            self.app.processEvents()
            time.sleep(delay)
        self._loaded = False

    def _loadFinished(self, result):
        self._loaded = True





if __name__=='__main__':
    s = Screenshot()

    @app.route('/image', methods=['GET','POST'])
    def getPicture():

        #reading args
        url=request.args.get('url')
        screenWidth=int(request.args.get('sw'))
        top=int(request.args.get('top'))
        left=int(request.args.get('left'))
        width=int(request.args.get('width'))
        height=int(request.args.get('height'))

        #cropping image
        s.capture(url,screenWidth,"temp.png")
        img=Image.open("temp.png")
        box=(left, top, left+width, top+height)
        area=img.crop(box)
        area.save("output","png")
        return "output.png"

        @app.route('/')
        def api_root():
            return 'Welcome'

    app.run(host='0.0.0.0',port=3000,debug=True)

whenever I hit this using 
curl http://0.0.0.0:3000/image?url=googlecom&sw=1920&top=100&left=100&width=200&height=200

I get the following error message,
QObject: Cannot create children for a parent that is in a different thread.
(Parent is Screenshot(0x7f9dbf121b90), parent's thread is QThread(0x7f9dbdb92240), current thread is QThread(0x7f9dbf11ed50)
QPixmap: It is not safe to use pixmaps outside the GUI thread
QPixmap: It is not safe to use pixmaps outside the GUI thread

解决方案

为了解决这个问题，我们需要确保在GUI线程中创建和使用QWebView对象。我们可以通过以下方式做到这一点：

1. 在主线程中创建QWebView对象：

if __name__=='__main__':
    app = QApplication(sys.argv)
    s = Screenshot()

2. 将QWebView对象的信号与主线程中的槽函数连接：

    s.loadFinished.connect(s._loadFinished)

3. 在主线程中调用QWebView对象的capture方法：

    @app.route('/image', methods=['GET','POST'])
    def getPicture():

        #reading args
        url=request.args.get('url')
        screenWidth=int(request.args.get('sw'))
        top=int(request.args.get('top'))
        left=int(request.args.get('left'))
        width=int(request.args.get('width'))
        height=int(request.args.get('height'))

        #cropping image
        s.capture(url,screenWidth,"temp.png")
        img=Image.open("temp.png")
        box=(left, top, left+width, top+height)
        area=img.crop(box)
        area.save("output","png")
        return "output.png"

这样，就可以确保QWebView对象及其信号和槽函数都是在GUI线程中创建和调用的，从而避免了错误。

以下是更新后的代码：

import sys
import time
from PyQt4.QtCore import *
from PyQt4.QtGui import *
from PyQt4.QtWebKit import *
import Image
from flask import Flask, Response, jsonify,request
app=Flask(__name__)


class Screenshot(QWebView):
    def __init__(self):
        self.app = QApplication(sys.argv)
        QWebView.__init__(self)
        self._loaded = False
        self.loadFinished.connect(self._loadFinished)

    def capture(self,url,width,output_file):
        self.resize(width,300)
        self.load(QUrl(url))
        self.wait_load()
        # set to webpage size
        frame = self.page().mainFrame()
        self.page().setViewportSize(frame.contentsSize())
        # render image
        image = QImage(self.page().viewportSize(), QImage.Format_ARGB32)
        painter = QPainter(image)
        frame.render(painter)
        painter.end()
        print 'saving', output_file
        image.save(output_file)

    def wait_load(self, delay=0):
        # process app events until page loaded
        while not self._loaded:
            self.app.processEvents()
            time.sleep(delay)
        self._loaded = False

    def _loadFinished(self, result):
        self._loaded = True




if __name__=='__main__':
    app = QApplication(sys.argv)
    s = Screenshot()

    @app.route('/image', methods=['GET','POST'])
    def getPicture():

        #reading args
        url=request.args.get('url')
        screenWidth=int(request.args.get('sw'))
        top=int(request.args.get('top'))
        left=int(request.args.get('left'))
        width=int(request.args.get('width'))
        height=int(request.args.get('height'))

        #cropping image
        s.capture(url,screenWidth,"temp.png")
        img=Image.open("temp.png")
        box=(left, top, left+width, top+height)
        area=img.crop(box)
        area.save("output","png")
        return "output.png"

        @app.route('/')
        def api_root():
            return 'Welcome'

    app.run(host='0.0.0.0',port=3000,debug=True)