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

CHAPTER 17 ■ FTP
f.login()
print "Current working directory:", f.pwd()
f.quit()
The welcome message will generally have no information that could be usefully parsed by your
program, but you might want to display it if a user is calling your client interactively. The login()
function can take several parameters, including a username, password, and a third, rarely used
authentication token that FTP calls an “account.” Here we have called it without parameters, which
makes it log in as the user “anonymous” with a generic value for the password.
Recall that an FTP session can visit different directories, just like a shell prompt can move between
locations with cd. Here, the pwd() function returns the current working directory on the remote site of
the connection. Finally, the quit() function logs out and closes the connection.
Here is what the program outputs when run:
$ ./connect.py
Welcome: 220 ProFTPD Server (Bring it on...)
Current working directory: /
ASCII and Binary Files
When making an FTP transfer, you have to decide whether you want the file treated as a monolithic
block of binary data, or whether you want it parsed as a text file so that your local machine can paste its
lines back together using whatever end-of-line character is native to your platform.
A file transferred in so-called “ASCII mode” is delivered one line at a time, so that you can glue the
lines back together on the local machine using its own line-ending convention. Take a look at Listing 17–
2 for a Python program that downloads a well-known text file and saves it in your local directory.
Listing 17–2. Downloading an ASCII File
#!/usr/bin/env python
# ASCII download - Chapter 17 - asciidl.py
# Downloads README from remote and writes it to disk.
import os
from ftplib import FTP
if os.path.exists('README'):
» raise IOError('refusing to overwrite your README file')
def writeline(data):
» fd.write(data)
» fd.write(os.linesep)
f = FTP('ftp.kernel.org')
f.login()
f.cwd('/pub/linux/kernel')
fd = open('README', 'w')
f.retrlines('RETR README', writeline)
fd.close()
f.quit()
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