Programming Python, Second Edition

Transferring Files over the Net

As we saw in the previous chapter, sockets see plenty of action on the Net. For instance, the getfile example at the end of that chapter allowed us to transfer entire files between machines. In practice, though, higher-level protocols are behind much of what happens on the Net. Protocols run on top of sockets, but hide much of the complexity of the network scripting examples we’ve just seen.

FTP -- the File Transfer Protocol -- is one of the more commonly used Internet protocols. It defines a higher-level conversation model that is based on exchanging command strings and file contents over sockets. By using FTP, we can accomplish the same task as the prior chapter’s getfile script, but the interface is simpler, and standard -- FTP lets us ask for files from any server machine that supports FTP, without requiring that it run our custom getfile script. FTP also supports more advanced operations such as uploading files to the server, getting remote directory listings, and more.

Really, FTP runs on top of two sockets: one for passing control commands between client and server (port 21), and another for transferring bytes. By using a two-socket model, FTP avoids the possibility of deadlocks (i.e., transfers on the data socket do not block dialogs on the control socket). Ultimately, though, Python’s ftplib support module allows us to upload and download files at a remote server machine by FTP, without dealing in raw socket calls or FTP protocol details. ...