rfc822() vs. rfc1123_date()

To set the Expires header in my web application I used to use the Zope DateTime function rfc822() which doesn't return the date in GMT format. Here's how I used it:

>>> from DateTime import DateTime
>>> hours = 5
>>> then = DateTime() + hours/24.0
>>> then.rfc822()
'Thu, 16 Aug 2007 20:43:59 +0100'

Then I found out (from using YSlow) that it's better to use the GMT format (RFC 1123), and here's how to do that in Zope:

>>> from App.Common import rfc1123_date
>>> from time import time
>>> rfc1123_date(time() + 3600*hours)
'Thu, 16 Aug 2007 19:45:12 GMT'

(notice that even though my locale is here in London, because of the summer time an hour is added)

Well, I thought I'd do a quick benchmark to compare the two approaches because I suspected that rfc1123_date() was faster because you don't have to create a DateTime() object. Here's what the two methods looked like to benchmark it:

   def foo1(self, hours):
       t0 = time()
       now = DateTime()
       then = now+float(hours/24.0)
       x=then.rfc822()
       return time()-t0

   def foo2(self, hours):
       t0 = time()
       x = rfc1123_date(time() + 3600*hours)
       return time()-t0

The result was as I expected, rfc1123_date() was much faster. Here are the results for 10,000 iterations:

Times1: 1.25969386101
Times2: 0.16867017746

round(1.25969386101/0.16867017746)=7.0

But a benchmark on something like this is a bit non-sense. Why? Because even if there's a 7 times difference, you'll only ever need one of these iterations per page. Not 10,000. The first function foo1() takes 0.00013 seconds.

Conclusion, worry more about getting the right output rather than speed at this kind of level.