[FIXED] How to count days belonging to a given month in the range of two Python datetimes?

Issue

I have two Python datetime and I want to count the days between those dates, counting ONLY the days belonging to the month I choose. The range might overlap multiple months/years.

Example: If I have 2017-10-29 & 2017-11-04 and I chose to count the days in October, I get 3 (29, 30 & 31 Oct.).

I can't find a straightforward way to do this so I think I'm going to iterate over the days using datetime.timedelta(days=1), and increment a count each time the day belongs to the month I chose.

Do you know a more performant method?

I'm using Python 2.7.10 with the Django framework.

Solution

Iterating over the days would be the most straightforward way to do it. Otherwise, you would need to know how many days are in a given month and you would need different code for different scenarios:

1. The given month is the month of the first date
2. The given month is the month of the second date
3. The given month is between the first and the second date (if dates span more than two months)

If you want to support dates spanning more than one year then you would need the input to include month and year.

Your example fits scenario #1, which I guess you could do like this:

>>> from datetime import datetime, timedelta
>>>
>>> first_date = datetime(2017, 10, 29)
>>>
>>> first_day_of_next_month = first_date.replace(month=first_date.month + 1, day=1)
>>> last_day_of_this_month = first_day_of_next_month - timedelta(1)
>>> number_of_days_in_this_month = last_day_of_this_month.day
>>> number_of_days_in_this_month - first_date.day + 1
3

This is why I would suggest implementing it the way you originally intended and only turning to this if there's a performance concern.

Answered By - Jared