[Numpy-discussion] casting from datetime64

Alex Samuel alex at alexsamuel.net
Mon May 13 09:31:47 EDT 2019


Thanks! I understand your hesitation; certainly if I am using undocumented APIs (or even if they are otherwise unused) I understand that it will be on my shoulders to preserve compatibility if you change things. But I would definitely like to get basic functionality working with current (and older, to be honest) NumPy versions.

Ah, I missed previously that the array objects are passed as the void* args to the PyArray_VectorUnaryFunc. But it's documented here. 
https://docs.scipy.org/doc/numpy-1.13.0/reference/c-api.types-and-structures.html#c.PyArray_ArrFuncs.cast
(Why aren't these two args PyArrayObject* then?)

As far as I can tell, get_datetime_metadata_from_dtype() isn't a public API? It certainly isn't spelled like one. It's declared in numpy/core/src/multiarray/_datetime.h, which doesn't appear to be #included in any other header. I suppose I could duplicate the struct layout in my own code to fish out the field I need. Obviously, compatibility is my own problem then.

Regards,
Alex



On Mon, May 13, 2019, at 01:31, Sebastian Berg wrote:
> Hi Alex,
> 
> On Mon, 2019-05-13 at 00:35 -0400, Alex Samuel wrote:
> > Hi,
> > 
> > When registering a custom cast function from datetime64 to another
> > dtype, how can I get the units?
> > 
> > I am calling PyArray_RegisterCastFunc from NPY_DATETIME. Ideally,
> > I'd like to register a separate cast function for each datetime64
> > units (or none at all... I don't want all units to be castable). 
> > Next best thing would be to obtain the units in the cast function and
> > dispatch accordingly.
> > 
> > Is this possible? I glanced through the code, and it looks like
> > there's a lot of hard-coded logic around datetime64, but I didn't go
> > through it carefully. Thought I'd ask before drilling further down.
> > get_datetime_metadata_from_dtype
> 
> No, I do not think that is possible. But you do get the array pointers
> during the cast. I honestly would prefer not to promise that all of
> this will survive if we change this in numpy, since there are almost no
> users. But I think if we change it I could promise to help with
> cleaning it up in ora.
> 
> I think this is public API (but I do not really like anyone using it
> ;)), so you can use:
> 
> ```
> /*
> * This function returns a pointer to the DateTimeMetaData
> * contained within the provided datetime dtype.
> */
> static PyArray_DatetimeMetaData *
> get_datetime_metadata_from_dtype(PyArray_Descr *dtype)
> {
>  /* original error check for DATETIME unnecessary for you */
>  return &(((PyArray_DatetimeDTypeMetaData *)dtype->c_metadata)->meta);
> }
> ```
> 
> And in the castfunc (fromarr is passed in as a void * as far as):
> ```
> NPY_DATETIMEUNIT base = get_datetime_metadata_from_dtype(
>  PyArray_DESCR(fromarr))->base;
> ```
> 
> Where NPY_DATETIMEUNIT is the enum defined in ndarraytypes.h. The logic
> will have to happen inside the cast func.
> 
> I would hope there was a better way, but I cannot think of any, and I
> am scared that supporting this will add yet another ugly hack when we
> want to improve dtypes...
> 
> Best,
> 
> Sebastian
> 
> 
> 
> > Thanks in advance,
> > Alex
> > 
> > 
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