[Numpy-discussion] Implicit conversion of python datetime to numpy datetime64?

[Mark Wiebe]

On Tue, Feb 14, 2012 at 9:37 PM, Benjamin Root <ben.root at ou.edu> wrote:

> On Tuesday, February 14, 2012, Mark Wiebe <mwwiebe at gmail.com> wrote:
> > On Tue, Feb 14, 2012 at 8:17 PM, Benjamin Root <ben.root at ou.edu> wrote:
> >>
> >> Just a thought I had.  Right now, I can pass a list of python ints or
> floats into np.array() and get a numpy array with a sensible dtype.  Is
> there any reason why we can't do the same for python's datetime?  Right
> now, it is very easy for me to make a list comprehension of datetime
> objects using strptime(), but it is very awkward to make a numpy array out
> of it.
> >
> > I would consider this a bug, it's not behaving sensibly at present.
> Here's what it does for me:
> >
> > In [20]: np.array([datetime.datetime.strptime(date, "%m/%d/%y") for date
> in ["02/03/12",
> >
> >     ...: "07/22/98", "12/12/12"]], dtype="M8")
>
> Well, I guess it would be nice if I didn't even have to provide the dtype
> (I.e., inferred from the datetime type, since we aren't talking about
> strings).  But I hadn't noticed the above, I was just making object arrays.
>
>
> >
> >
> ---------------------------------------------------------------------------
> >
> > TypeError Traceback (most recent call last)
> >
> > C:\Python27\Scripts\<ipython-input-20-d3b7b5392190> in <module>()
> >
> > 1 np.array([datetime.datetime.strptime(date, "%m/%d/%y") for date in
> ["02/03/12",
> >
> > ----> 2 "07/22/98", "12/12/12"]], dtype="M8")
> >
> > TypeError: Cannot cast datetime.datetime object from metadata [us] to
> [D] according to the rule 'same_kind'
> >
> > In [21]: np.array([datetime.datetime.strptime(date, "%m/%d/%y") for date
> in ["02/03/12",
> >
> >     ...: "07/22/98", "12/12/12"]], dtype="M8[us]")
> >
> > Out[21]:
> >
> > array(['2012-02-02T16:00:00.000000-0800',
> >
> > '1998-07-21T17:00:00.000000-0700', '2012-12-11T16:00:00.000000-0800'],
> dtype='datetime64[us]')
> >
> > In [22]: np.array([datetime.datetime.strptime(date, "%m/%d/%y") for date
> in ["02/03/12",
> >
> >     ...: "07/22/98", "12/12/12"]], dtype="M8[us]").astype("M8[D]")
> >
> > Out[22]: array(['2012-02-03', '1998-07-22', '2012-12-12'],
> dtype='datetime64[D]')
> >>
> >> The only barrier I can think of are those who have already built code
> around a object dtype array of datetime objects.
> >>
> >> Thoughts?
> >> Ben Root
> >>
> >> P.S. - what ever happened to arange() and linspace() for datetime64?
> >
> > arange definitely works:
> > In[28] np.arange('2011-03-02', '2011-04-01', dtype='M8')
> > Out[28]:
> > array(['2011-03-02', '2011-03-03', '2011-03-04', '2011-03-05',
> >        '2011-03-06', '2011-03-07', '2011-03-08', '2011-03-09',
> >        '2011-03-10', '2011-03-11', '2011-03-12', '2011-03-13',
> >        '2011-03-14', '2011-03-15', '2011-03-16', '2011-03-17',
> >        '2011-03-18', '2011-03-19', '2011-03-20', '2011-03-21',
> >        '2011-03-22', '2011-03-23', '2011-03-24', '2011-03-25',
> >        '2011-03-26', '2011-03-27', '2011-03-28', '2011-03-29',
> >        '2011-03-30', '2011-03-31'], dtype='datetime64[D]')
> > I didn't get to implementing linspace. I did look at it, but the current
> code didn't make it a trivial thing to put in.
> > -Mark
>
> Sorry, I wasn't clear about arange, I meant that it would be nice if it
> could take python datetimes as arguments (and timedelat for the step?)
> because that is much more intuitive than remembering the exact dtype code
> and string format.
>
> I see it as the numpy datetime64 type could take three types for it's
> constructor: another datetime64, python datetime, and The standard
> unambiguous datetime string.  I should be able to use these interchangeably
> in numpy.  The same would be true for timedelta64.
>
> Easy interchange between python datetime and datetime64 would allow numpy
> to piggy-back on established functionality in the python system libraries,
> allowing for focus to be given to extended features.
>

Ben Walsh actually implemented this and the code is in a pull request here:

https://github.com/numpy/numpy/pull/111

This didn't go in, because the datetime properties don't exist on the
arrays after you convert them to datetime64, so there could be some
unintuitive consequences from that. When Martin implemented the quaternion
dtype, we discussed the possibility that dtypes could expose properties
that show up on the array object, and if this were implemented I think the
conversion and compatibility between python datetime and datetime64 could
be made quite natural.

-Mark


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