[Numpy-discussion] The date/time dtype and the casting issue

Francesc Alted faltet@pytables....
Wed Jul 30 12:16:25 CDT 2008

A Wednesday 30 July 2008, Pierre GM escrigué:
> > > Now, what format do you consider for this reference ?
> >
> > Whatever that can be converted into a datetime64 scalar.  Some
> > examples:
> >
> > ref = '2001-04-01'
> > ref = datetime.datetime(2001, 4, 1)
> Er, should I see ref as having a 'day' unit or 'business day' unit in
> that case? I know that 'business days' spoil the game, but Matt
> really needs them, so...

OK.  I was wrong.  Of course you need to specify the resolution, so the 
reference *should* be a NumPy scalar:

ref = numpy.datetime64('2001-04-01', unit="B")  # 'B'usiness days

> > > Moreover, could you give some more examples of interaction
> > > between datetime and timedelta ?
> >
> > In the second proposal there are some examples of this interaction
> > and I'm populating the third proposal with more examples yet.  Just
> > wait a bit (maybe a couple of hours) to see the new proposal.
> OK, with pleasure. It's just that I have trouble understanding the
> meaning of something like
>  t2 = numpy.ones(5, dtype="datetime64[s]")
> That's five times one second after the epoch, right ? But in what
> circumstances would you need t2 ?

I'm not sure I follow you.  This is just an example so as to produce an 
array of time objects quickly.  In general, you should also be able to 
produce the same result by doing:

t2 = numpy.array(['1970-01-01T00:00:05', '1970-01-01T00:00:05', 
                  '1970-01-01T00:00:05', '1970-01-01T00:00:05', 
                  '1970-01-01T00:00:05', dtype="datetime64[s]")

which is more visual, but has the drawback that it's just too long for 
documenting purposes.  When you don't need the values for some 
examples, conciseness is a virtue.

Francesc Alted

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