Python NamedTemporaryFile-读取时出现ValueError

Question

我在用Python写入NamedTemporaryFile并将其读回时遇到问题。 该函数通过tftpy将文件下载到临时文件，进行读取，对内容进行哈希处理，然后将哈希摘要与原始文件进行比较。 有问题的函数如下：

def verify_upload(self, image, destination):
    # create a tftp client
    client = TftpClient(ip, 69, localip=self.binding_ip)
    # generate a temp file to hold the download info
    if not os.path.exists("temp"):
        os.makedirs("temp")
    with NamedTemporaryFile(dir="temp") as tempfile, open(image, 'r') as original:
        try:
            # attempt to download the target image
            client.download(destination, tempfile, timeout=self.download_timeout)
        except TftpTimeout:
            raise RuntimeError("Could not download {0} from {1} for verification".format(destination, self.target_ip))
        # hash the original file and the downloaded version
        original_digest = hashlib.sha256(original.read()).hexdigest()
        uploaded_digest = hashlib.sha256(tempfile.read()).hexdigest()
        if self.verbose:
            print "Original SHA-256: {0}\nUploaded SHA-256: {1}".format(original_digest, uploaded_digest)
        # return the hash comparison
        return original_digest == uploaded_digest

问题是每次我尝试执行该行uploaded_digest = hashlib.sha256(tempfile.read()).hexdigest() ，应用程序都会ValueError - I/O Operation on a closed file出错。 由于with块未完成，因此我在努力理解为什么将关闭临时文件。 我能想到的唯一可能性是tftpy在完成下载后正在关闭文件，但是我无法在tftpy源代码中找到发生这种情况的任何地方。 注意，我也尝试插入tempfile.seek(0) ，以将文件放回适当的状态以进行读取，但是，这也给了我ValueError 。

tftpy是否可能关闭文件？ 我读到NamedTemporaryFile中可能存在一个错误导致此问题？ 为什么在with块定义的引用超出范围之前关闭文件？

Answer 1

TFTPy正在关闭文件。 在查看源代码时，您错过了以下代码路径：

class TftpClient(TftpSession):
    ...
    def download(self, filename, output, packethook=None, timeout=SOCK_TIMEOUT):
        ...
        self.context = TftpContextClientDownload(self.host,
                                                 self.iport,
                                                 filename,
                                                 output,
                                                 self.options,
                                                 packethook,
                                                 timeout,
                                                 localip = self.localip)
        self.context.start()
        # Download happens here
        self.context.end()  # <--

TftpClient.download调用TftpContextClientDownload.end ：

class TftpContextClientDownload(TftpContext):
    ...
    def end(self):
        """Finish up the context."""
        TftpContext.end(self)  # <--
        self.metrics.end_time = time.time()
        log.debug("Set metrics.end_time to %s", self.metrics.end_time)
        self.metrics.compute()

TftpContextClientDownload.end调用TftpContext.end ：

class TftpContext(object):
    ...
    def end(self):
        """Perform session cleanup, since the end method should always be
        called explicitely by the calling code, this works better than the
        destructor."""
        log.debug("in TftpContext.end")
        self.sock.close()
        if self.fileobj is not None and not self.fileobj.closed:
            log.debug("self.fileobj is open - closing")
            self.fileobj.close()  # <--

然后TftpContext.end关闭文件。