Recipe13.3.Filtering a List of FTP Sites

Recipe 13.3. Filtering a List of FTP Sites

Credit: Mark Nenadov

Problem

Several of the FTP sites on your list of sites could be down at any time. You want to filter that list and obtain the list of those sites that are currently up.

Solution

Clearly, we first need a function to check whether one particular site is up:

import socket, ftplib def isFTPSiteUp(site):     try:         ftplib.FTP(site).quit( )     except socket.error:         return False     else:         return True

Now, a simple list comprehension can perform the recipe's task, but we may as well wrap that list comprehension inside another function:

def filterFTPsites(sites):     return [site for site in sites if isFTPSiteUp(site)]

Alternatively, filter(isFTPSiteUp, sites) returns exactly the same resulting list as the list comprehension.

Discussion

Lists of FTP sites are sometimes difficult to maintain, since sites may be closed or temporarily down for all sorts of reasons. The code in this recipe is simple and suitable, for example, for use inside a small interactive program that must let the user choose among FTP siteswe may as well not even present for choice those sites we know are down! If you run this code regularly a few times a day and append the results to a file, the results may also be a basis for long-term maintenance of a list of FTP sites. Any site that has been down for more than a certain number of days should probably be moved away from the main list and into a list of sites that may well have croaked.

Very similar ideas could be used to filter lists of sites that serve protocols other than FTP, by using, instead of standard Python library module ftplib, other such modules, such as nntplib for the NNTP protocol, httplib for the Hypertext Transport Protocol (HTTP), and so on.

When you're checking many FTP sites within one program run, it could be much faster to use multiple threads to check on multiple sites at once (so that the delays while waiting for the various sites to respond can overlap), or else use an asynchronous approach. The simple approach presented in this recipe is easiest to program and to understand, but for most real-life networking programs, you do want to enhance performance by using either multithreading or asynchronous approaches, as other recipes in this chapter demonstrate.

Recipe13.3.Filtering a List of FTP Sites