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

CHAPTER 17 ■ FTP
these last few commands that FTP can be used to back file-browser applications that let users drag and
drop files and directories seamlessly between their local system and the remote host.
Doing FTP Securely
Though we noted at the beginning of this chapter that there are far better protocols to adopt than FTP
for pretty much anything you could use FTP to accomplish—in particular the robust and secure SFTP
extension to SSH (see Chapter 16)—we should be fair and note that some few FTP servers support TLS
encryption (see Chapter 6) and that Python’s ftplib does provide this protection if you want to take
advantage of it.
To use TLS, create your FTP connection with the FTP_TLS class instead of the plain FTP class; simply
by doing this, your username and password and, in fact, the entire FTP command channel will be
protected from prying eyes. If you then additionally run the class’s prot_p() method (it takes no
arguments), then the FTP data connection will be protected as well. Should you for some reason want to
return to using an un-encrypted data connection during the session, there is a prot_c() method that
returns the data stream to normal. Again, your commands will continue to be protected as long as you
are using the FTP_TLS class.
Check the Python Standard Library documentation for more details (they include a small code
sample) if you wind up needing this extension to FTP:
http://docs.python.org/library/ftplib.html#ftplib.FTP_TLS
Summary
FTP lets you transfer files between a client running on your machine and a remote FTP server. Though
the protocol is insecure and outdated when compared to better choices like SFTP, you might still find
services and machines that require you to use it. In Python, the ftplib library is used to talk to FTP
servers.
FTP supports binary and ASCII transfers. ASCII transfers are usually used for text files, and permit
line endings to be adjusted as the file is transferred. Binary transfers are used for everything else. The
retrlines() function is used to download a file in ASCII mode, while retrbinary() downloads a file in
binary mode.
You can also upload files to a remote server. The storlines() function uploads a file in ASCII mode,
and storbinary() uploads a file in binary mode.
The ntransfercmd() function can be used for binary uploads and downloads. It gives you more
control over the transfer process and is often used to support a progress bar for the user.
The ftplib module raises exceptions on errors. The special tuple ftplib.all_errors can be used to
catch any error that it might raise.
You can use cwd() to change to a particular directory on the remote end. The nlst() command
returns a simple list of all entries (files or directories) in a given directory. The dir() command returns a
more detailed list but in server-specific format. Even with only nlst(), you can usually detect whether an
entry is a file or directory by attempting to use cwd() to change to it and noting whether you get an error.
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