Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 17 ■ FTP
In the listing, the cwd() function selects a new working directory on the remote system. Then the
retrlines() function begins the transfer. Its first parameter specifies a command to run on the remote
system, usually RETR, followed by a file name. Its second parameter is a function that is called, over and
over again, as each line of the text file is retrieved; if omitted, the data is simply printed to standard
output. The lines are passed with the end-of-line character stripped, so the homemade writeline()
function simply appends your system’s standard line ending to each line as it is written out.
Try running this program; there should be a file in your current directory named README after the
program is done.
Basic binary file transfers work in much the same way as text-file transfers; Listing 17–3 shows an
example.
Listing 17–3. Downloading a Binary File
#!/usr/bin/env python
# Binary upload - Chapter 17 - binarydl.py
import os
from ftplib import FTP
if os.path.exists('patch8.gz'):
» raise IOError('refusing to overwrite your patch8.gz file')
f = FTP('ftp.kernel.org')
f.login()
f.cwd('/pub/linux/kernel/v1.0')
fd = open('patch8.gz', 'wb')
f.retrbinary('RETR patch8.gz', fd.write)
fd.close()
f.quit()
When run, it deposits a file named patch8.gz in your current working directory. The retrbinary()
function simply passes blocks of data to the specified function. This is convenient, since a file object’s
write() function expects just such data—so in this case, no custom function is necessary.
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. Listing 17–4 shows a sample
program that uses ntransfercmd().
Listing 17–4. Binary Download with Status Updates
#!/usr/bin/env python
# Advanced binary download - Chapter 17 - advbinarydl.py
import os, sys
from ftplib import FTP
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