[Python-Dev] Issue5434: datetime.monthdelta

Paul Moore p.f.moore at gmail.com
Thu Apr 16 16:54:08 CEST 2009

2009/4/16 Jess Austin <jess.austin at gmail.com>:
> I'm new to python core development, and I've been advised to write to
> python-dev concerning a feature/patch I've placed at
> http://bugs.python.org/issue5434, with Rietveld at
> http://codereview.appspot.com/25079.
> This patch adds a "monthdelta" class and a "monthmod" function to the
> datetime module.  The monthdelta class is much like the existing
> timedelta class, except that it represents months offset from a date,
> rather than an exact period offset from a date.  This allows us to
> easily say, e.g. "3 months from now" without worrying about the number
> of days in the intervening months.
>    >>> date(2008, 1, 30) + monthdelta(1)
>    datetime.date(2008, 2, 29)
>    >>> date(2008, 1, 30) + monthdelta(2)
>    datetime.date(2008, 3, 30)
> The monthmod function, named in (imperfect) analogy to divmod, allows
> us to round-trip by returning the interim between two dates
> represented as a (monthdelta, timedelta) tuple:
>    >>> monthmod(date(2008, 1, 14), date(2009, 4, 2))
>    (datetime.monthdelta(14), datetime.timedelta(19))
> Invariant: dt + monthmod(dt, dt+td)[0] + monthmod(dt, dt+td)[1] == dt + td

I like the idea in principle. In practice, of course, month
calculations are inherently ill-defined, so you need to be very
specific in documenting all of the edge cases, and you should have
strong use cases to ensure that the behaviour implemented matches user
requirements. (I haven't yet had time to read the patch - you may well
already have these points covered, certainly your comments above
indicate that you appreciate the subtleties involved).

> These also work with datetimes!  There are more details in the
> documentation included in the patch.  In addition to the C module
> file, I've updated the datetime CAPI, the documentation, and tests.
> I feel this would be a good addition to core python.  In my work, I've
> often ended up writing annoying one-off "add-a-month" or similar
> functions.  I think since months work differently than most other time
> periods, a new object is justified rather than trying to shoe-horn
> something like this into timedelta.  I also think that the round-trip
> functionality provided by monthmod is important to ensure that
> monthdeltas are "first-class" objects.

I agree that ultimately it would be useful in the core. However, I'd
suggest that you release the functionality as an independent module in
the first instance, to establish it outside of the core. Once it has
matured somewhat as a 3rd party module, it would then be ready for
integration in the core. This also has the benefit that it makes the
functionality available to users of Python 2.6 (and possibly earlier)
rather than just in 2.7/3.1 onwards.

> Please let me know what you think of the idea and/or its execution.

I hope the above comments help. Ultimately, I'd like to see this added
to the core. It's tricky enough that having a "standard"
implementation is a definite benefit in itself. But equally, I'd give
it time to iron out the corner cases on a faster development cycle
than the core offers before "freezing" it as part of the stdlib.


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