how to convert months into days when using relativedelta?

Question

Here is my method

def get_remaining_days_in_financial_month(self, from_day):
    current_financial_day = date(from_day.year, from_day.month,
                                 self.financial_day_of_month)
    end_financial_month = current_financial_day + relativedelta(months=+1)
    delta = relativedelta(end_financial_month, from_day)
    remaining_days_in_financial_month = delta.days
    return remaining_days_in_financial_month

When I debug, I see

current_financial_day = 2013-06-01
delta = relativedelta(months=+1)
end_financial_month = 2013-07-01
from_day = 2013-06-01
remaining_days_in_financial_month = 0

Although this information is correct, I would like to know the number of days, because number of days changes from 28 in Feb, to 30 in June and 31 in August

How can I achieve this? The dateutil library doesn't offer a way it seems

Thank you

Answer 1

Use datetime.timedelta(); simply subtract the two dates:

delta = end_financial_month - from_day
return delta.days

Answer 2

It seems there's some confusion about what a timedelta is versus a relativedelta.

A timedelta is a context-free duration. Think of it as some number of microseconds. When you take a datetime and add a timedelta to it, you get a datetime that number of microseconds in the future, which is defined agnostic to varying month lengths, leap years, etc.

A relativedelta is a context-sensitive duration. Add "a month" to February 1. Add "a month" to August 1. You'll be adding a different number of microseconds in each case, because "a month" (as a duration) has different meanings depending on the reference point or context. The same thing goes for adding "a year" when leap years are involved.

Adding two timedelta instances makes perfect sense and is well-defined. You're just adding two "number of microseconds."

Adding two relativedelta instances isn't so straightforward and, looking at the documentation, appears to be disallowed.

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All that being said, Martijn's answer is of course correct. I just wanted to clarify the difference in meaning between these two concepts.