Python: Do two dates span a particular day of the week or month? (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Wed, 19 Sep 2012

Python: Do two dates span a particular day of the week or month?

When I'm using my RSS reader FeedMe, I normally check every feed every day. But that can be wasteful: some feeds, like World Wide Words, only update once a week. A few feeds update even less often, like serialized books that come out once a month or whenever the author has time to add something new.

So I decided it would be nice to add some "when" logic to FeedMe, so I could add when = Sat in the config section for World Wide Words and have it only update once a week.

That sounded trivial -- a little python parsing logic to tell days from numbers, a few calls to time.localtime() and I was done.

Except of course I wasn't. Because sometimes, like when I'm on vacation, I don't always update every day. If I missed a Saturday, then I'd never see that week's edition of World Wide Words. And that would be terrible!

So what I really needed was a way to ask, "Has a Saturday occurred (including today) since the last time I ran feedme?"

The last time I ran feedme is easy to determine: it's in the last modified date of the cache file. Or, in more Pythonic terms, it's statbuf = os.stat(cachefile).st_mtime. And of course I can get the current time with time.localtime(). But how do I figure out whether a given week or month day falls between those two dates?

I'm sure this particular wheel has been invented many times. There's probably even a nifty Python library somewhere to do it. But how do you google for that? I tried to think of keywords and found nothing. So I went for a nice walk in the redwoods and thought about it for a bit, and came up with a solution.

Turns out for the week day case, you can just use modular arithmetic: if (weekday_2 - target_weekday) % 7 < (weekday_2 - weekday_1) then the day does indeed fall between the two dates.

Things are a little more complicated for the day of the month, though, because you don't know whether you need mod 30 or 31 or 29 or 28, so you either have to make your own table, or import the calendar module just so you can call calendar.monthrange().

I decided it was easier to use logic: if the difference between the two dates is greater than 31, then it definitely includes any month day. Otherwise, check whether they're in the same month or not, and do greater than/less than comparisons on the three dates.

Throw in a bunch of conversion to make it easy to call, and a bunch of unit tests to make sure everything works and my later tweaks don't break anything, and I had a nice function I could call from Feedme. on github

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[ 22:07 Sep 19, 2012    More programming | permalink to this entry | ]

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