[Numpy-discussion] PyArray_DescrConverter - alignment / trailingunused bytes

Martin Wiechert martin.wiechert at gmx.de
Fri Sep 15 13:22:44 CDT 2006


On Friday 15 September 2006 20:27, Travis Oliphant wrote:
> Martin Wiechert wrote:
> > On Friday 15 September 2006 19:14, Travis Oliphant wrote:
> >> Martin Wiechert wrote:
> >>> Thanks Albert! Do you also know the corresponding C-API function? It
> >>> cannot be PyArray_DescrConverter (PyObject *, PyArray_Descr **), whose
> >>> signature has no "align", right?
> >>
> >> The DescrConverter function is meant for "O&"-style conversions.  It
> >> can't accept an align function.  We could possibly add something to the
> >> converter to allow specification of alignment through the object to be
> >> converted.
> >
> > I begin to see the light....
> >
> > For dictionaries one could maybe just add an optional key "align".
> > Also an optional key "elsize" or "itemsize" to force the total size of
> > the record may sometimes be useful. E.g. one may want to faithfully map a
> > given C struct. (That's why I was asking for trailing unused bytes.)
> >
> > But I understand that other things have higher priority.
> >
> >> Or, you can just call the __new__ method of the PyArrayDescr_Type object
> >>
> >> res = PyObject_CallMethod((PyObject *)&PyArrayDescr_Type, "__new__",
> >> "Oi", dict_object, 1))
> >>
> >> or call the tp->new method directly:
> >>
> >> args = Py_BuildValue("Oi", dict_object, 1);
> >> PyArrayDescr_Type->tp_new(&PyArrayDescr_Type, args, NULL);
> >> Py_DECREF(args);
> >
> > Thank you! I'll try this.
> >
> >> (I think passing in NULL for the keywords is O.K., but I haven't checked
> >> it).
> >>
> >> -Travis
> >
> > One final question. To me the repr of a dtype with gaps is a little bit
> >
> > puzzling:
> >>>> dtype ({'names': ['a', 'b', 'c'], 'formats': ['<a4', '<f8', '<f4'],
> >
> > 'offsets': [0, 16, 24]})
> > dtype([('a', '|S4'), ('', '|V12'), ('b', '<f8'), ('', '|V12'), ('c',
> > '<f4')])
> >
> > There should be no gap between "b" and "c" but still the repr has ('',
> > '|V12') between them. Am I missing something?
>
> There was a bug I just fixed in the representation of these structures
> with gaps.  It should be fixed in SVN, now.
>
>
> -Travis
>
>

Thanks again.

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