Erle Robotics Python Networking Gitbook Free

Advanced Binary Downloading

The ftplib module provides a second function that can be used for binary downloading: ntransfercmd(). This command provides a lower-level interface, but can be useful if you want to know a little bit more about what’s going on during the download. In particular, this more advanced command lets you keep track of the number of bytes transferred, and you can use that information to display status updates for the user. shows a sample program that uses ntransfercmd().

import os, sys
from ftplib import FTP

if os.path.exists('linux-1.0.tar.gz'):
    raise IOError('refusing to overwrite your linux-1.0.tar.gz file')

f = FTP('')

f.voidcmd("TYPE I")

datasock, size = f.ntransfercmd("RETR linux-1.0.tar.gz")
bytes_so_far = 0
fd = open('linux-1.0.tar.gz', 'wb')

while 1:
    buf = datasock.recv(2048)
    if not buf:
    bytes_so_far += len(buf)
    print "\rReceived", bytes_so_far,
    if size:
        print "of %d total bytes (%.1f%%)" % (
            size, 100 * bytes_so_far / float(size)),
        print "bytes",


There are a few new things to note here. First comes the call to voidcmd(). This passes an FTP command directly to the server, checks for an error, but returns nothing. In this case, the raw command is TYPE I. That sets the transfer mode to “image,” which is how FTP refers internally to binary files. In the previous example, retrbinary() automatically ran this command behind the scenes, but the lower-level ntransfercmd() does not. Next, note that ntransfercmd() returns a tuple consisting of a data socket and an estimated size. Always bear in mind that the size is merely an estimate, and should not be considered authoritative; the file may end sooner, or it might go on much longer, than this value. Also, if a size estimate from the FTP server is simply not available, then the estimated size returned will be None.

After receiving the data, it is important to close the data socket and call voidresp(), which reads the command response code from the server, raising an exception if there was any error during transmission. Even if you do not care about detecting errors, failing to call voidresp() will make future commands likely to fail because the server’s output socket will be blocked waiting for you to read the results. Here is an example of running this program:

root@erlerobot:~/Python_files# python
Received 1259161 of 1259161 bytes (100.0%)