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

Question

我正在尝试使用for循环从ftp站点下载多个文件。在弹出python.exe关闭窗口之前，以下代码似乎只适用于循环中的前2个文件。两个下载的文件是完美的，但第三个下载的文件在关机时是空的。剩下的文件我都搞不懂。你知道问题出在哪里吗？

from PyQt4 import QtCore, QtGui, QtNetwork


class FtpWindow(QtGui.QDialog):

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

    def downloadAllFile(self):
        for jj in range(self.fileList.topLevelItemCount()): # how many files in a particular folder
            fileName = self.fileList.topLevelItem(jj).text(0) 
            self.outFile = QtCore.QFile(fileName)

            self.ftp.get(fileName, self.outFile) #download one file at a time
            self.progressDialog.setLabelText("Downloading %s..." % fileName)    
            self.progressDialog.exec_()

    def ftpCommandFinished(self, _, error):
        self.setCursor(QtCore.Qt.ArrowCursor)
        if self.ftp.currentCommand() == QtNetwork.QFtp.Get:
            if error:
                self.statusLabel.setText("Canceled download of %s." % self.outFile.fileName())
                self.outFile.close()
                self.outFile.remove()
            else:
                self.statusLabel.setText("Downloaded %s to current directory." % self.outFile.fileName())
                self.outFile.close()

            self.outFile = None
            self.enableDownloadButton()
            self.progressDialog.hide()

Answer 1

self.progressDialog.exec_()应该是阻塞模式对话框。对非阻塞调用使用self.progressDialog.show()。

看起来ftp get是非阻塞的，所以您必须等待，直到使用commandFinished()信号完成下载。

我的猜测是，循环中的每个迭代都会覆盖self.outFile，因此没有任何python对对象的引用。这使得对象在python执行垃圾回收时就会死掉。我的猜测是，前两个文件较小且快速，第三个文件较大，因此其他文件可以在垃圾回收之前下载。对于最后一个文件，要么是垃圾收集，要么是更快。

http://pyside.github.io/docs/pyside/PySide/QtNetwork/QFtp.html#PySide.QtNetwork.PySide.QtNetwork.QFtp.get

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."""
        outFile = QtCore.QFile(filename)
        cmd_id = self.ftp.get(filename, outFile) # Non blocking just start downloading

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

    def downloadAllFile(self):
        self.progressDialog.reset()
        num_downloads = self.fileList.topLevelItemCount()
        self.progressDialog.setMaximum(num_downloads)
        self.progressDialog.setValue(0)
        self.progressDialog.setLabelText("Downloading %d files ..." % 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.setValue(self.progressDialog.maximium())
            self.progressDialog.close() # This shouldn't be needed

我上面的示例将保留文件名和outFile对象引用，直到命令结束。当命令结束时，引用被删除，允许python清理对象。

Answer 2

感谢HashSplat的输入。我有一些更新，使其功能齐全：

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)