问为什么QtNetwork.QFtp.get下载在多个文件的for循环中失败？

EN

class FtpWindow(QtGui.QDialog):

def __init__(self, parent=None):
    self.fileList = QtGui.QTreeWidget()
    self.ftp = QtNetwork.QFtp(self)
    self.progressDialog = QtGui.QProgressDialog(self)
    self.progressDialog.canceled.connect(self.ftp.abort)
    self.downloadAllButton.clicked.connect(self.downloadAllFile)
    self.ref_holder = {}
    self.ftp.commandFinished.connect(self.ftpCommandFinished)

def download_file(self, fileName):
    """Non blocking start downloading a file."""
    self.outFile = QtCore.QFile(fileName)

    """ Need this to have files saved locally """
    if not self.outFile.open(QtCore.QIODevice.WriteOnly):  
        QtGui.QMessageBox.information(self, "FTP",
                "Unable to save the file %s." % fileName)
        self.outFile = None
        return
    cmd_id = self.ftp.get(filename, self.outFile) # Non blocking just start downloading

    # This keeps the object alive and doesn't overwrite them.
    self.ref_holder[cmd_id] = [filename, self.outFile]

def downloadAllFile(self):
    self.progressDialog.reset()
    self.num_downloads = self.fileList.topLevelItemCount()
    self.counter=1
    self.progressDialog.setLabelText("Downloading %d/%d files ..." % (self.counter, self.num_downloads))
    self.progressDialog.show()
    for jj in range(num_downloads): # how many files in a particular folder
        fileName = self.fileList.topLevelItem(jj).text(0) 
        self.download_file(fileName) # Non blocking, and doesn't overwrite self.outFile with every iteration

def ftpCommandFinished(self, cmd_id, error=None):
    """Increased the number of items finished."""
    self.progressDialog.setValue(self.progressDialog.value()+1)
    item = self.ref_holder.pop(cmd_id) # Remove the reference for the finished item
    if error:
        self.progressDialog.setLabelText("Error downloading %s" % item[0])

    # Check if all downloads are done
    if len(self.ref_holder) == 0:
        self.progressDialog.close() # This closes the extra window
        self.outFile.close()   # You need this to have the last file saved
    else: 
        self.counter+=1
        self.progressDialog.setLabelText("Downloading %d/%d files ..." % (self.counter, self.num_downloads))

def updateDataTransferProgress(self, readBytes, totalBytes):
    self.progressDialog.setMaximum(totalBytes)
    self.progressDialog.setValue(readBytes)