One of the must-have necessities for any blogging or other web publishing software is the RSS (Really Simple Syndication) feed. RSS, or its younger and sexier sibling Atom, is the file format used by news aggregators, which give you a frequent listing of posts and stories published by your favorite blogs, newspapers, journals, whatever you want, without you having to actually go to their website. Usually the RSS feed is generated by some library of code where you construct your feed as an array of objects or data structures. Here's how you put together an RSS feed with PyRSS2Gen, for instance:

rss = PyRSS2Gen.RSS2(
    title = "Andrew's PyRSS2Gen feed",
    link = "http://www.dalkescientific.com/Python/PyRSS2Gen.html",
    description = "The latest news about PyRSS2Gen, a Python library for generating RSS2 feeds",

    lastBuildDate = datetime.datetime.now(),

    items = [
       PyRSS2Gen.RSSItem(
         title = "PyRSS2Gen-0.0 released",
         link = "http://www.dalkescientific.com/news/030906-PyRSS2Gen.html",
         description = "Dalke Scientific today announced PyRSS2Gen-0.0, a library for generating RSS feeds for Python.  ",
         guid = PyRSS2Gen.Guid("http://www.dalkescientific.com/news/030906-PyRSS2Gen.html"),
         pubDate = datetime.datetime(2003, 9, 6, 21, 31)),
       PyRSS2Gen.RSSItem(
         title = "Thoughts on RSS feeds for bioinformatics",
         link = "http://www.dalkescientific.com/writings/diary/archive/2003/09 …

I’ve just been doing something that probably should never be done in Python (again), and I figured it might be useful to record this little snippet for posterity. Many Python programmers are probably familiar with decorators, which modify functions by wrapping them with other functions. For example, if you want to print ‘entering’ and ‘exiting’ to trace when a function’s execution begins and ends, you could do that with a decorator:

def trace(func):
    def wrapper(*args, **kwargs):
        print 'entering'
        func(*args, **kwargs)
        print 'exiting'
    return wrapper

@trace
def f(a,b):
     print 'in f: ' + str(a) + ',' + str(b)

Note that in practice there are better ways to do this, but it’s just an example.

Anyway, it’s also possible to customize the behavior of decorator itself by providing arguments, for instance if you wanted to print different strings instead of ‘entering’ and ‘exiting’. But in that case, your “decorator” function is actually a wrapper that creates the real decorator and returns it:

def trace(enter_string, exit_string):
    def _trace(func):
        def wrapper(*args, **kwargs):
            print enter_string
            func(*args, **kwargs)
            print exit_string
        return wrapper
    return _trace

@trace('calling f', 'returning')
def f(a,b):
     print 'in f: ' + str …