[Numpy-discussion] Do we want scalar casting to behave as it does at the moment?

Nathaniel Smith njs@pobox....
Tue Jan 8 13:59:16 CST 2013


On 8 Jan 2013 17:24, "Andrew Collette" <andrew.collette@gmail.com> wrote:
>
> Hi,
>
> > I think you are voting strongly for the current casting rules, because
> > they make it less obvious to the user that scalars are different from
> > arrays.
>
> Maybe this is the source of my confusion... why should scalars be
> different from arrays?  They should follow the same rules, as closely
> as possible.  If a scalar value would fit in an int16, why not add it
> using the rules for an int16 array?

The problem is that rule for arrays - and for every other party of
numpy in general - are that we *don't* pick types based on values.
Numpy always uses input types to determine output types, not input
values.

# This value fits in an int8
In [5]: a = np.array([1])

# And yet...
In [6]: a.dtype
Out[6]: dtype('int64')

In [7]: small = np.array([1], dtype=np.int8)

# Computing 1 + 1 doesn't need a large integer... but we use one
In [8]: (small + a).dtype
Out[8]: dtype('int64')

Python scalars have an unambiguous types: a Python 'int' is a C
'long', and a Python 'float' is a C 'double'. And these are the types
that np.array() converts them to. So it's pretty unambiguous that
"using the same rules for arrays and scalars" would mean, ignore the
value of the scalar, and in expressions like
  np.array([1], dtype=np.int8) + 1
we should always upcast to int32/int64. The problem is that this makes
working with narrow types very awkward for no real benefit, so
everyone pretty much seems to want *some* kind of special case. These
are both absolutely special cases:

numarray through 1.5: in a binary operation, if one operand has
ndim==0 and the other has ndim>0, ignore the width of the ndim==0
operand.

1.6, your proposal: in a binary operation, if one operand has ndim==0
and the other has ndim>0, downcast the ndim==0 item to the smallest
width that is consistent with its value and the other operand's type.

-n


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