Post Recurring monthly events in Python

I was working on something in one of my little Django sites and wondered how you make a recurring monthly event in Python? What I mean by recurring event is "every fourth Saturday" or "every first and second Wednesday" and so on.

I did not want to make a dependency on some huge calender server module like Calcore or Twisted's caldav. All I wanted was a function that accepts "every fourth Saturday" and returns me an actual date that I can use for scheduling things.

A quick google didn't come up with anything, so I decided to do it myself. Here are my first and second attempts. The first attempt just works it out mathematically, the second attempt uses a module from the Python standard library.

"""Helper for recurring date."""

DAYS = [
'Monday',
'Tuesday',
'Wednesday',
'Thursday',
'Friday',
'Saturday',
'Sunday',
]

from datetime import date

def eventdate(year, month, target_day, target_ordinal):
    """Convert a human event date to a real date.
    For example, 'the third Thursday of the month'
    the target_ordinal is 3 and the target_day is 'Thursday'.
    """

    day = DAYS.index(target_day.title())

    match = 0
    for i in range(1, 32):
        try:
            if date(year, month, i).weekday() == day:
                match += 1
                if match == target_ordinal:
                    return date(year, month, i)

        except ValueError:
            return None

def main():
    """Example when called directly."""
    today = date.today()
    if today.month == 12:
        year = today.year + 1
        month = 1
    else:
        year = today.year
        month = today.month + 1
    print "Next Month's Linux group is", eventdate(year, month, 'Thursday', 3)
    print "Next Month's Python group is", eventdate(year, month, 'Saturday', 4)

# start the ball rolling
if __name__ == "__main__":
    main()

That worked quite fine, but like all good Python programmers, I want to be as efficient (/lazy) as possible, surely the standard library can do this for me? Well I found that the calendar module will return a matrix of dates organised by week and day. This works as follows:

"""Event helpers."""

def eventdate(year, month, target_day, target_ordinal):
    """Convert a human event date to a real date.
    For example, 'the third Thursday of the month'
    the target_ordinal is 3 and the target_day is 'Thursday'.
    """

    import calendar
    day = getattr(calendar, target_day.upper())
    cal = calendar.Calendar()
    return cal.monthdatescalendar(year, month)[target_ordinal - 1][day]

def main():
    """Example when called directly."""
    from datetime import date
    today = date.today()
    if today.month == 12:
        year = today.year + 1
        month = 1
    else:
        year = today.year
        month = today.month + 1
    print "Next Month's Linux group is", eventdate(year, month, 'Thursday', 3)
    print "Next Month's Python group is", eventdate(year, month, 'Saturday', 4)

# start the ball rolling

if __name__ == "__main__":
    main()

This seems to work identically as the above but in less lines of code. I still get the feeling I am trying too hard and I am missing something obvious, but maybe I am just being too much of perfectionist (as always).

If anyone knows or can work out a more efficient method, please do let me know.

4 thoughts on “Recurring monthly events in Python”

You should look at <a class="reference external" href="http://labix.org/python-dateutil">http://labix.org/python-dateutil</a>, it does this and much more.

<div class="highlight"><pre>DAYS = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday',
'Saturday']

def eventdate(year, month, target_day, target_ordinal):
import sqlite3
db = sqlite3.connect(':memory:')
query = ("select date('%d-01-01', '%d months', '%d days', 'weekday %d');"
% (year, month-1, 7*(target_ordinal-1), DAYS.index(target_day)))
return db.execute(query).fetchone()[0]
</pre></div>

I agree with all in this post! Thank you

If you put the following code before returning the value you can handle things like "the last Saturday" by using a negative target_ordinal. Also, it checks to see if the date that is being returned is a zero indicating that that day doesn't come until the next (or came in the previous) week:
<blockquote>
<dl class="docutils">
<dt>if target_ordinal > 0 and weeks[target_ordinal - 1][day] == 0:</dt>
<dd>target_ordinal+=1</dd>
<dt>elif target_ordinal < 0 and weeks[target_ordinal][day] == 0:</dt>
<dd>target_ordinal-=1</dd>
<dt>if target_ordinal < 0:</dt>
<dd>target_ordinal += len(weeks) + 1</dd>
</dl>
</blockquote>

Stavros on August 9, 2008 at 07:14 p.m. said:

You should look at <a class="reference external" href="http://labix.org/python-dateutil">http://labix.org/python-dateutil</a>, it does this and much more.
Tom Lynn on September 24, 2008 at 03:01 p.m. said:

<div class="highlight"><pre>DAYS = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday',
'Saturday']

def eventdate(year, month, target_day, target_ordinal):
import sqlite3
db = sqlite3.connect(':memory:')
query = ("select date('%d-01-01', '%d months', '%d days', 'weekday %d');"
% (year, month-1, 7*(target_ordinal-1), DAYS.index(target_day)))
return db.execute(query).fetchone()[0]
</pre></div>
Blg on February 19, 2009 at 02:30 p.m. said:

I agree with all in this post! Thank you
Chris on May 4, 2009 at 12:44 p.m. said:

If you put the following code before returning the value you can handle things like "the last Saturday" by using a negative target_ordinal. Also, it checks to see if the date that is being returned is a zero indicating that that day doesn't come until the next (or came in the previous) week:
<blockquote>
<dl class="docutils">
<dt>if target_ordinal > 0 and weeks[target_ordinal - 1][day] == 0:</dt>
<dd>target_ordinal+=1</dd>
<dt>elif target_ordinal < 0 and weeks[target_ordinal][day] == 0:</dt>
<dd>target_ordinal-=1</dd>
<dt>if target_ordinal < 0:</dt>
<dd>target_ordinal += len(weeks) + 1</dd>
</dl>
</blockquote>

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Recurring monthly events in Python

4 thoughts on “Recurring monthly events in Python”