Building a list of dates between two dates

Question

My solution to this feels 'icky' and I've got calendar math falling out of my ears after working on similar problems for a week so I can't think straight about this.

Is there a better way to code this?

import datetime
from dateutil.relativedelta import relativedelta
def date_count(start, end, day_of_month=1):
 """
 Return a list of datetime.date objects that lie in-between start and end.
 The first element of the returned list will always be start and the last
 element in the returned list will always be:
 datetime.date(end.year, end.month, day_of_month)
 If start.day is equal to day_of_month the second element will be:
 start + 1 month
 If start.day is after day_of_month then the second element will be:
 the day_of_month in the next month
 If start.day is before day_of_month then the second element will be:
 datetime.date(start.year, start.month, day_of_month)
 >>> start = datetime.date(2012, 1, 15)
 >>> end = datetime.date(2012, 4, 1)
 >>> date_count(start, end, day_of_month=1) #doctest: +NORMALIZE_WHITESPACE
 [datetime.date(2012, 1, 15), datetime.date(2012, 2, 1),
 datetime.date(2012, 3, 1), datetime.date(2012, 4, 1)]
 Notice that it's not a full month between the first two elements in the
 list.
 If you have a start day before day_of_month:
 >>> start = datetime.date(2012, 1, 10)
 >>> end = datetime.date(2012, 4, 1)
 >>> date_count(start, end, day_of_month=15) #doctest: +NORMALIZE_WHITESPACE
 [datetime.date(2012, 1, 10), datetime.date(2012, 1, 15),
 datetime.date(2012, 2, 15), datetime.date(2012, 3, 15),
 datetime.date(2012, 4, 15)]
 Notice that it's not a full month between the first two elements in the
 list and that the last day is rounded to
 datetime.date(end.year, end.month, day_of_month)
 """
 last_element = datetime.date(end.year, end.month, day_of_month)
 if start.day == day_of_month:
 second_element = start + relativedelta(start, months=+1)
 elif start.day > day_of_month:
 _ = datetime.date(start.year, start.month, day_of_month)
 second_element = _ + relativedelta(_, months=+1)
 else:
 second_element = datetime.date(start.year, start.month, day_of_month)
 dates = [start, second_element]
 if last_element <= second_element:
 return dates
 while dates[-1] < last_element:
 next_date = dates[-1] + relativedelta(dates[-1], months=+1)
 next_date = datetime.date(next_date.year, next_date.month, day_of_month)
 dates.append(next_date)
 dates.pop()
 dates.append(last_element)
 return dates

Answer 1

How about...

def date_count(start, end, day_of_month=1):
 dates = [start]
 next_date = start.replace(day=day_of_month)
 if day_of_month > start.day:
 dates.append(next_date)
 while next_date < end.replace(day=day_of_month):
 next_date += relativedelta(next_date, months=+1)
 dates.append(next_date)
 return dates

And by the way it seems like a nice opportunity to use yield, if you wanted to.

def date_count2(start, end, day_of_month=1):
 yield start
 next_date = start.replace(day=day_of_month)
 if day_of_month > start.day:
 yield next_date
 while next_date < end.replace(day=day_of_month):
 next_date += relativedelta(next_date, months=+1)
 yield next_date

Another possibility - discard the first value if it is earlier than the start date:

def date_count(start, end, day_of_month=1):
 dates = [start.replace(day=day_of_month)]
 while dates[-1] < end.replace(day=day_of_month):
 dates.append(dates[-1] + relativedelta(dates[-1], months=+1))
 if dates[0] > start:
 return [start] + dates
 else:
 return [start] + dates[1:]

Or use a list comprehension to iterate over the number of months between start and end.

def date_count(start, end, day_of_month=1):
 round_start = start.replace(day=day_of_month)
 gap = end.year * 12 + end.month - start.year * 12 - start.month + 1
 return [start] + [round_start + relativedelta(round_start, months=i) 
 for i in range(day_of_month <= start.day, gap)]

Finally, I don't know dateutil but it seems you can use rrule:

from dateutil import rrule
def date_count(start, end, day_of_month=1):
 yield start
 for date in rrule(MONTHLY, 
 dtstart=start.replace(day=day_of_month),
 until=end.replace(day=day_of_month)):
 if date > start:
 yield date

• Dustin Wyatt: Excellent answer. Not sure which solution I will use, but thanks!