[Numpy-discussion] can_cast with structured array output - bug?

Mark Wiebe mwwiebe@gmail....
Mon Feb 13 21:02:38 CST 2012


I took a look into the code to see what is causing this, and the reason is
that nothing has ever been implemented to deal with the fields. This means
it falls back to treating all struct dtypes as if they were a plain "void"
dtype, which allows anything to be cast to it.

While I was redoing the casting subsystem for 1.6, I did think on this
issue, and decided that it wasn't worth tackling it at the time because the
'safe'/'same_kind'/'unsafe' don't seem sufficient to handle what might be
desired. I tried to leave this alone as much as possible.

Some random thoughts about this are:

* Casting a scalar to a struct dtype: should it be safe if the scalar can
be safely cast to each member of the struct dtype? This is the NumPy
broadcasting rule applied to dtypes as if the struct dtype is another
dimension.
* Casting one struct dtype to another: If the fields of the source are a
subset of the target, and the types can safely convert, should that be a
safe cast? If the fields of the source are not a subset of the target,
should that still be a same_kind cast? Should a second enum which
complements the safe/same_kind/unsafe one, but is specific for how
adding/removing struct fields be added?

This is closely related to adding ufunc support for struct dtypes, and the
choices here should probably be decided at the same time as designing how
the ufuncs should work.

-Mark

On Mon, Feb 13, 2012 at 5:20 PM, Matthew Brett <matthew.brett@gmail.com>wrote:

> Hi,
>
> I've also just noticed this oddity:
>
> In [17]: np.can_cast('c', 'u1')
> Out[17]: False
>
> OK so far, but...
>
> In [18]: np.can_cast('c', [('f1', 'u1')])
> Out[18]: True
>
> In [19]: np.can_cast('c', [('f1', 'u1')], 'safe')
> Out[19]: True
>
> In [20]: np.can_cast(np.ones(10, dtype='c'), [('f1', 'u1')])
> Out[20]: True
>
> I think this must be a bug.
>
> In the other direction, it makes more sense to me:
>
> In [24]: np.can_cast([('f1', 'u1')], 'c')
> Out[24]: False
>
> In [25]: np.can_cast([('f1', 'u1')], [('f1', 'u1')])
> Out[25]: True
>
> Best,
>
> Matthew
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