[DB-SIG] Re: ANN: DateTime type, version 0.1

[Magnus Lycka]

At 22:40 1997-12-13 +0100, M.-A. Lemburg wrote:
>t1 = maketime(1997,12,12)
>t2 = maketime(1998,2,1)
>d = DateTimeDelta(month=+2)

As I've written before one could also imagine this being the
same class with an attribute indicating if the time is relative
or not. Although that might not be a good idea since a absolute
time can be expressed as a number relative to epoch, while +2
months are sometimes the same as 2592000 seconds, and sometimes
something else... Hm, two classes is probably better, they are
after all more different than one having an epoch to relate to,
and the other just lacking that reference.

>(The two categories mutually exclude each other, e.g. if you use
> month=+2,day=+2 you can't use absdate=+30)

I'm not sure I follow you here. What's the difference of day=+40
and absdate=+40? Why should absdate be different from day? Why not
just add the bigger things first? 1900,1,1 + (year=+20,month=+30,
day=+40) => 1922,8,10

You might have a problem with which order you add things anyways,
even if 'days' is small, so you have to make up your mind here.
>>> t = maketime(1998,2,27)
>>> m = DateTimeDelta(month=+1)
>>> d = DateTimeDelta(day=+2)
>>> t1 = t + m
>>> print t1
1998,3,27
>>> print t1 + d
1998,3,29
>>> t2 = t + d
>>> print t2
1998,3,1
>>> print t2+m
1998,4,1

So, t + m + d != t + d + m.

Not funny, but it's not our fault that the months have different lenghts.
The way I see it absdate=+X (if I have understood what you meant by that
term) is just the same as (year=0,month=0,day=+X). I think the user can 
handle year = month = 0 if that's what she wants.

>Questions:
>- What is the outcome of maketime(1997,12,31) + d ? Since february
>  doesn't have 31 days the new date is not well defined. I guess
>  an exception should be raised to indicate this.

This is obviously something a lot of people have run into lots of
times. Banks, authorities etc. Is there not any standard for handling
this? I imagine this is something you want to handle differently in
different occasions. Sometimes you will probably want it to be Feb 28,
and sometimes Mar 3. This probably depends on whether you are paying or 
receiving money ;-) if it's an economical system for instance... I guess
subclassing with the handling of this in the subclass is acceptable.

Another option would be to have some extra attribute in DateTimeDelta
for longer or shorter spans. But maybe it's better to have a simpler and
cleaner system and subclass to handle such things.

BTW, can anyone say something about how much an extra level of subclassing
affects performance in Python? Since there is late binding, I imagine an
extra level (or several) of subclassing will have a performance cost. Is
this significant? (I understand this is not a clear yes/no question, but
maybe someone can say something enlightening on that issue.)

>- What should t2 - t1 result in ? There are two options:
>  a) it returns a relative result like DateTimeDelta(year=+1,month=-10,
>     day=-11)
>  b) it returns an absolute difference, DateTimeDelta(absdate=+51)
>  I think arithmetic should always use option b). Option a) could
>  be returned by a special reldifftime() function.
>- Is there a need for addition of deltas ? 
>- Would having seperate types for the two flavors (relative and
>  absolute) be a better approach. [actually while writing this, I
>  tend to think so].

Hm... My thought was initially that for instance 
>>> t1 = maketime(1997,12,12,12,45,50)
>>> t2 = maketime(1997,12,12,09,32,55)
>>> diff = t2 - t1 # 11575 seconds
would come out like
>>> print diff
3 hours 12 minutes 55 seconds
rather than 3,13,-5 or 11575. Naturally this fails when we come to 
months... :-( I suppose b) is the right way to do it. How much energy
would it take to move the earth closer to the sun so that the year is
reduced to 360 days? That would make things sooo much simpler, and
it would finally be a proper use of all those A-bombs! ;-)


	Magnus

--
Magnus Lycka, S/W Engineer, M.Sc.E.E; Folktrov. 6C, 907 51 Umea, Sweden
Tel: +46(0)90 198 498, GSM: +46(0)70 582 80 65, Fax: +46(0)70 612 80 65
<mailto:magnus.lycka@tripnet.se>         <http://www1.tripnet.se/~mly/>


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