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. >
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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