[Moved] How to write a cache class from QAbstractNetworkCache with python

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I tried and get Segmentation fault, I write this with c++, and it works fine. Anybody know what happened there? Thanks for any help.
I am using Debian sid, qt4 4.7.3, libqtwebkit 2.1.0~2011week13-1 , python-qt4 4.8.3-2+b1 , python 2.6.6-14
Here is my python code:
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import sys, os
try:
from hashlib import md5, sha1
except:
import md5, sha1
from PyQt4.QtGui import QApplication
from PyQt4.QtCore import QUrl, QIODevice, QFile
from PyQt4.QtNetwork import QAbstractNetworkCache, QNetworkCacheMetaData
from PyQt4.QtWebKit import QWebPage, QWebView

def l(msg):
print >> sys.stderr, msg

class MyCache1(QAbstractNetworkCache):
def init(self, basedir, uid=None):
QAbstractNetworkCache.init(self)
self.basedir = basedir
self.uid = uid

def get_filename(self, url):
    m = md5(url.toString()).hexdigest()
    f = os.path.join(self.basedir, m)
    return f + ".meta", f + ".data"

def cacheSize(self):
    l("cacheSize")
    return 100 * 1024 * 1024    # fake

def clear(self):
    l("clear")
    return 0

def data(self, url):
    l("get data" + url.toString())
    mfn, dfn = self.get_filename(url)
    try:
        df = QFile(dfn)
        df.open(QIODevice.ReadOnly)
        return df
    except:
        l("get_data rturn 0")
        return 0

def metaData(self, url):
    l("metaData " + url.toString())
    return QNetworkCacheMetaData()

def insert(self, device):
    l("insert")
    device.close()

def prepare(self, metaData):
    l("get url: " + metaData.url().toString())
    mfn, dfn = self.get_filename(metaData.url())
    l("open file " + dfn)
    df = QFile(dfn)
    df.open(QFile.ReadWrite)
    l("file opened " + dfn)
    #df.write("I am here")
    return df

def remove(self, url):
    l("remove")
    return True

def updateMetaData(self, metaData):
    l("updateMetaData")
    return

app = QApplication(sys.argv)
v = QWebView()
c = MyCache1("/home/networkcache")

v.page().networkAccessManager().setCache(c)
v.load(QUrl("http://google.com/"))
v.show()

app.exec_()
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After doing some debugging, I notice the problem is after network/access/qnetworkreplyimpl.cpp:521, after this point, the cacheSaveDevice is a QIODevice * (this is return from python code "prepare"), but can not call openMode() method, whenever you call it, it will get Segment Fault, but you can access cacheSaveDevice->openMode, I still don't know how to fix it.

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OK, answer by myself:
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def prepare(self, metaData):
l("get url: " + metaData.url().toString())
mfn, dfn = self.get_filename(metaData.url())
l("open file " + dfn)
df = QFile(dfn)
df.open(QFile.ReadWrite)
l("file opened " + dfn)
self.save_df = df
return df
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