Given a date range how can we break it up into N contiguous sub-intervals?

Question

I am accessing some data through an API where I need to provide the date range for my request, ex. start='20100101', end='20150415'. I thought I would speed this up by breaking up the date range into non-overlapping intervals and use multiprocessing on each interval.

My problem is that how I am breaking up the date range is not consistently giving me the expected result. Here is what I have done:

from datetime import date

begin = '20100101'
end = '20101231'

Suppose we wanted to break this up into quarters. First I change the string into dates:

def get_yyyy_mm_dd(yyyymmdd):
    # given string 'yyyymmdd' return (yyyy, mm, dd)
    year = yyyymmdd[0:4]
    month = yyyymmdd[4:6]
    day = yyyymmdd[6:]
    return int(year), int(month), int(day)

y1, m1, d1 = get_yyyy_mm_dd(begin)
d1 = date(y1, m1, d1)
y2, m2, d2 = get_yyyy_mm_dd(end)
d2 = date(y2, m2, d2)

Then divide this range into sub-intervals:

def remove_tack(dates_list):
    # given a list of dates in form YYYY-MM-DD return a list of strings in form 'YYYYMMDD'
    tackless = []
    for d in dates_list:
        s = str(d)
        tackless.append(s[0:4]+s[5:7]+s[8:])
    return tackless

def divide_date(date1, date2, intervals):
    dates = [date1]
    for i in range(0, intervals):
        dates.append(dates[i] + (date2 - date1)/intervals)
    return remove_tack(dates)

Using begin and end from above we get:

listdates = divide_date(d1, d2, 4)
print listdates # ['20100101', '20100402', '20100702', '20101001', '20101231'] looks correct

But if instead I use the dates:

begin = '20150101'
end = '20150228'

...

listdates = divide_date(d1, d2, 4)
print listdates # ['20150101', '20150115', '20150129', '20150212', '20150226']

I am missing two days at the end of February. I don't need time or timezone for my application and I don't mind installing another library.

Answer 1

I would actually follow a different approach and rely on timedelta and date addition to determine the non-overlapping ranges

Implementation

def date_range(start, end, intv):
    from datetime import datetime
    start = datetime.strptime(start,"%Y%m%d")
    end = datetime.strptime(end,"%Y%m%d")
    diff = (end  - start ) / intv
    for i in range(intv):
        yield (start + diff * i).strftime("%Y%m%d")
    yield end.strftime("%Y%m%d")

Execution

>>> begin = '20150101'
>>> end = '20150228'
>>> list(date_range(begin, end, 4))
['20150101', '20150115', '20150130', '20150213', '20150228']