def getfo(self, remotepath, fl, callback=None):
        """
        Copy a remote file (``remotepath``) from the SFTP server and write to
        an open file or file-like object, ``fl``.  Any exception raised by
        operations will be passed through.  This method is primarily provided
        as a convenience.

        :param object remotepath: opened file or file-like object to copy to
        :param str fl:
            the destination path on the local host or open file object
        :param callable callback:
            optional callback function (form: ``func(int, int)``) that accepts
            the bytes transferred so far and the total bytes to be transferred
        :return: the `number <int>` of bytes written to the opened file object

        .. versionadded:: 1.10
        """
        with self.open(remotepath, 'rb') as fr:
            file_size = self.stat(remotepath).st_size
            fr.prefetch()
            size = 0
            while True:
                data = fr.read(32768)
                fl.write(data)
                size += len(data)
                if callback is not None:
                    callback(size, file_size)
                if len(data) == 0:
                    break
        return size