Datetime utility functions

[Paul Moore]

I was just writing some code which did date/time manipulations, and I
found that the Python 2.3 datetime module does not supply a number of
fairly basic functions. I understand the reasoning (the datetime
module provides a set of datatypes, and doesn't attempt to get into
the murky waters of date algorithms) but as these things can be quite
tricky to get right, I was wondering if anyone has already implemented
any date algorithms, or alternatively if I'd missed some easy way of
doing what I'm after.

My specific requirements were:

1. Get the last day of the month contatining a given date(time). I
really was surprised to find this one missing, as it seems to me that
the datetime module must know what the last day for each month is, so
exposing it wouldn't have been hard.

2. Add a number of months to a date. This is messy, as there are
options (what is one month after 31st Jan). The trouble is that the
calculation, while simple, is tricky to get right (month is 1-based,
so off-by-1 errors are easy to make, and divmod isn't as useful as
you'd first think).

Other "nice to have" functions which I didn't need for this program,
but which I have found useful in the past, are

- Round a date(time) to a {year,month,day ,quarter,week,. ..} Or
truncate. Or (occasionally) chop to the next higher boundary
(ceiling).
- Calculate the number of {years,months,d ays,...} between two dates.
(Which is more or less the equivalent of rounding a timedelta).

These latter two aren't very well defined right now, because I have no
immediate use case.

In the absence of anything else, I'll probably write a "date
utilities" module for myself at some point. But I'd hate to be
reinventing the wheel.

Paul.

[Paul Moore]

Carel Fellinger <ca************ *@chello.nl> writes:

On Tue, Sep 16, 2003 at 05:19:50PM +0300, Christos TZOTZIOY Georgiou wrote:
...

I sent my own version without having seen your own --and mine might seem
obfuscated, compared to yours.

you both use the same indirect way of getting to the end of the month.
why not do what datetime.c does, like:

Mainly because that seems even more like reinventing the wheel.
Writing code that already exists is something I dislike doing at the
best of times. Writing *tricky* code that already exists feels even
more unpleasant. (Yes, I know the leap year calculation isn't that
hard - but lots of people have got it wrong in the past...)

I guess that's my real issue. I know the datetime module already has
this information. But persuading it to tell me requires me to jump
through hoops, whereas reimplementing it feels like admitting defeat.

None of it's hard, though. I've spent more time on emails about the
issue than I'd ever need to spend in implementing anything :-)

Paul.
--
This signature intentionally left blank

[Christos TZOTZIOY Georgiou]

On Tue, 16 Sep 2003 20:26:01 +0100, rumours say that Paul Moore
<pf******@yahoo .co.uk> might have written:

[I suggesting dt.__class__() instead of dt.replace()]

dt.replace doesn't have side effects - it creates a new object. Also,
by using replace(), my version works with date, datetime, or
subclasses. Yours does too, but it sets the time component of a
datetime to zero. It's a matter of preference which behaviour is
"better", I guess.

You are right about dt.replace not having side-effects, just like some
anonymous<wink> major python contributor commented. Also .replace feels
"better" since it doesn't lose the time information.

PS ...but I still like my use of divmod() ;-)
--
TZOTZIOY, I speak England very best,
Microsoft Security Alert: the Matrix began as open source.

[Gustavo Niemeyer]

> > you both use the same indirect way of getting to the end of the month.

why not do what datetime.c does, like:

Mainly because that seems even more like reinventing the wheel.
Writing code that already exists is something I dislike doing at the

[...]

Why not using calendar.monthr ange()?

--
Gustavo Niemeyer
http://niemeyer.net

[Dan Bishop]

pa********@atos origin.com (Paul Moore) wrote in message news:<18******* *************** ****@posting.go ogle.com>...

I was just writing some code which did date/time manipulations, and I
found that the Python 2.3 datetime module does not supply a number of
fairly basic functions. I understand the reasoning (the datetime
module provides a set of datatypes, and doesn't attempt to get into
the murky waters of date algorithms) but as these things can be quite
tricky to get right, I was wondering if anyone has already implemented
any date algorithms, or alternatively if I'd missed some easy way of
doing what I'm after.

[description of needed functions]

I wrote a module just like that at my last job. Given a date object,
you could find the first or last day of the month, quarter, or year.
There were also functions to answer questions like "What was the date
5 months ago?" or "What date is the first Monday after October 8?"

Unfortunately, I don't have a copy of the source code here.

[Invalid User]

Try 'mxDateTime' available at

http://www.egenix.com/files/python/e...ownload-mxBASE

It most likely has what you are looking for.

Paul Moore wrote:

I was just writing some code which did date/time manipulations, and I
found that the Python 2.3 datetime module does not supply a number of
fairly basic functions. I understand the reasoning (the datetime
module provides a set of datatypes, and doesn't attempt to get into
the murky waters of date algorithms) but as these things can be quite
tricky to get right, I was wondering if anyone has already implemented
any date algorithms, or alternatively if I'd missed some easy way of
doing what I'm after.

My specific requirements were:

1. Get the last day of the month contatining a given date(time). I
really was surprised to find this one missing, as it seems to me that
the datetime module must know what the last day for each month is, so
exposing it wouldn't have been hard.

2. Add a number of months to a date. This is messy, as there are
options (what is one month after 31st Jan). The trouble is that the
calculation, while simple, is tricky to get right (month is 1-based,
so off-by-1 errors are easy to make, and divmod isn't as useful as
you'd first think).

Other "nice to have" functions which I didn't need for this program,
but which I have found useful in the past, are

- Round a date(time) to a {year,month,day ,quarter,week,. ..} Or
truncate. Or (occasionally) chop to the next higher boundary
(ceiling).
- Calculate the number of {years,months,d ays,...} between two dates.
(Which is more or less the equivalent of rounding a timedelta).

These latter two aren't very well defined right now, because I have no
immediate use case.

In the absence of anything else, I'll probably write a "date
utilities" module for myself at some point. But I'd hate to be
reinventing the wheel.

Paul.