[Python-Dev] Python-Dev Digest, Vol 69, Issue 143 (original) (raw)

Jess Austin jess.austin at gmail.com
Thu Apr 16 23:30:13 CEST 2009

Jared Grubb <jared.grubb at gmail.com> wrote:

On 16 Apr 2009, at 11:42, Paul Moore wrote:

The key thing missing (I believe) from dateutil is any equivalent of monthmod.

I agree with that. It's well-defined and it makes a lot of sense. +1 But, I dont think monthdelta can be made to work... what should the following be?

print(date(2008,1,30) + monthdelta(1)) 2008-02-29 print(date(2008,1,30) + monthdelta(2)) 2008-03-30 print(date(2008,1,30) + monthdelta(1) + monthdelta(1)) 2008-03-29

This is a perceptive observation: in the absence of parentheses to dictate a different order of operations, the third quantity will differ from the second. Furthermore, this won't always be true, just for dates near the end of the month, which is nonintuitive. (Incidentally, this is another reason why this functionality should not just be lumped into timedelta; guarantees that have long existed for operations with timedelta would no longer hold if it tried to deal with months.)

I find that date calculations involving months involve a certain amount of inherent confusion. I've tried to reduce this by introducing well-specified functionality that will allow accurate reasoning, as part of the core's included batteries. I think that one who uses these objects will develop an intuition and write accurate code quickly. It is nonintuitive that order of operation matters for addition of months, just as it matters for subtraction and division of all objects, but with the right tools we can deal with this. An interesting consequence is that if I want to determine if date b is more than a month after date a, sometimes I should use:

b - monthdelta(1) > a

rather than

a + monthdelta(1) < b

[Consider a list of run dates for a process that should run the last day of every month: "a" might be date(2008, 2, 29) while "b" is date(2008, 3, 31). In this case the two expressions would have different values.]

cheers, Jess

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