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

Nathaniel Smith njs@pobox....
Wed Jan 2 05:24:10 CST 2013


This discussion seems to have petered out without reaching consensus
one way or another. This seems like an important issue, so I've opened
a bug:
  https://github.com/numpy/numpy/issues/2878
Hopefully this way we'll at least not forget about it; also I tried to
summarize the main issues there and would welcome comments.

-n

On Mon, Nov 12, 2012 at 7:54 PM, Matthew Brett <matthew.brett@gmail.com> wrote:
> Hi,
>
> I wanted to check that everyone knows about and is happy with the
> scalar casting changes from 1.6.0.
>
> Specifically, the rules for (array, scalar) casting have changed such
> that the resulting dtype depends on the _value_ of the scalar.
>
> Mark W has documented these changes here:
>
> http://docs.scipy.org/doc/numpy/reference/ufuncs.html#casting-rules
> http://docs.scipy.org/doc/numpy/reference/generated/numpy.result_type.html
> http://docs.scipy.org/doc/numpy/reference/generated/numpy.promote_types.html
>
> Specifically, as of 1.6.0:
>
> In [19]: arr = np.array([1.], dtype=np.float32)
>
> In [20]: (arr + (2**16-1)).dtype
> Out[20]: dtype('float32')
>
> In [21]: (arr + (2**16)).dtype
> Out[21]: dtype('float64')
>
> In [25]: arr = np.array([1.], dtype=np.int8)
>
> In [26]: (arr + 127).dtype
> Out[26]: dtype('int8')
>
> In [27]: (arr + 128).dtype
> Out[27]: dtype('int16')
>
> There's discussion about the changes here:
>
> http://mail.scipy.org/pipermail/numpy-discussion/2011-September/058563.html
> http://mail.scipy.org/pipermail/numpy-discussion/2011-March/055156.html
> http://mail.scipy.org/pipermail/numpy-discussion/2012-February/060381.html
>
> It seems to me that this change is hard to explain, and does what you
> want only some of the time, making it a false friend.
>
> Is it the right behavior for numpy 2.0?
>
> Cheers,
>
> Matthew
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