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What is the 2nd last Friday in May 2005 ? What is the first Monday in August 2006 ?

cutoff_date = dow_date_finder(SECONDLAST,FRI,OCT,2005)

Python, 50 lines
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import datetime   # a thing of beauty and a joy forever

FIRST = 0
SECOND = 1
THIRD = 2
FOURTH = FORTH = 3  # for people who have finger trouble
FIFTH = 4
LAST = -1
SECONDLAST = -2
THIRDLAST = -3

MONDAY = MON = 0
TUESDAY = TUE = TUES = 1
WEDNESDAY = WED = 2
THURSDAY = THU = THUR = 3
FRIDAY = FRI = 4
SATURDAY = SAT = 5
SUNDAY = SUN = 6

JANUARY = JAN = 1
FEBRUARY = FEB = 2
MARCH = MAR = 3
APRIL = APR = 4
MAY = 5
JUNE = JUN = 6
JULY = JUL = 7
AUGUST = AUG = 8
SEPTEMBER = SEP = 9
OCTOBER = OCT = 10
NOVEMBER = NOV = 11
DECEMBER = DEC = 12

def dow_date_finder(which_weekday_in_month=FIRST,day=MONDAY,month=JANUARY,year=2000):
    dt = datetime.date(year,month,1)
    dow_lst = []
    while dt.weekday() != day:
        dt = dt + datetime.timedelta(days=1)
    while dt.month == month:
        dow_lst.append(dt)
        dt = dt + datetime.timedelta(days=7)
    return dow_lst[which_weekday_in_month]  # may raise an exception if slicing is wrong


if __name__ == "__main__":
    print "2nd tuesday of may 2005"
    print dow_date_finder(SECOND,TUESDAY,MAY,2005)
    print "last wednesday of april 2005"
    print dow_date_finder(LAST,WEDNESDAY,APRIL,2005)
    print "secondlast friday of october 2005 - short form"
    print dow_date_finder(SECONDLAST,FRI,OCT,2005)

Commercial programs often need to know these days : usually they are special because they're holidays or financial periods or whatever.

I personally don't like the Monday=0 that we find in Python - it goes against "cron" standard, so I always battle a but with it. So "hash-defining" it as above makes it slightly more palatable.

The function should really be hardened a lot more : checking for invalid days of week, days of month and the "which value". But it runs in a consistent environment so I haven't felt the need for that.

3 comments

Raymond Hettinger 16 years, 5 months ago  # | flag

Alternate version using the calendar module.

from calendar import monthrange

def dow_date_finder(which_weekday_in_month=FIRST,day=MONDAY,month=JANUARY,year=2000):
    bom, days = monthrange(year, month)
    firstmatch = (day - bom) % 7 + 1
    return xrange(firstmatch, days+1, 7)[which_weekday_in_month]
Chris Smith 15 years, 8 months ago  # | flag

You might want to embed that last line in a try/except to catch the case when the requested day does not exist, e.g. there's not a 5th Thursday of Nov 1977 but there is in 1978:

try:
    return xrange(firstmatch, days+1, 7)[which_weekday_in_month]
except:
    return None
greg p 14 years, 4 months ago  # | flag

I put this into an online utility here:

http://www.utilitymill.com/utility/Nth_Weekday_in_Month

Hopefully it will be useful to people. Feel free to improve it, some of the parameters still aren't working.

Created by Mark Pettit on Mon, 13 Jun 2005 (PSF)
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