Hi, On Thu, Mar 8, 2012 at 3:14 PM, Matthew Brett <matthew.brett@gmail.com> wrote:

Hi,

On Wed, Mar 7, 2012 at 4:08 PM, Matthew Brett <matthew.brett@gmail.com> wrote:

...

Hi,

I noticed a casting change running the test suite on our image reader, nibabel: https://github.com/nipy/nibabel/blob/master/nibabel/tests/test_casting.py

For this script:

<pre> import numpy as np

Adata = np.zeros((2,), dtype=np.uint8) Bdata = np.zeros((2,), dtype=np.int16) Bzero = np.int16(0) Bbig = np.int16(256)

print np.__version__ print 'Array add', (Adata + Bdata).dtype print 'Scalar 0 add', (Adata + Bzero).dtype print 'Scalar 256 add', (Adata + Bbig).dtype </pre>

1.4.1 Array add int16 Scalar 0 add uint8 Scalar 256 add uint8

1.5.1 Array add int16 Scalar 0 add uint8 Scalar 256 add uint8

1.6.1 Array add int16 Scalar 0 add uint8 Scalar 256 add int16

1.7.0.dev-aae5b0a Array add int16 Scalar 0 add uint8 Scalar 256 add uint16

I can understand the uint8 outputs from numpy < 1.6 - the rule being not to upcast for scalars.

I can understand the int16 output from 1.6.1 on the basis that the value is outside uint8 range and therefore we might prefer a type that can handle values from both uint8 and int16.

Was the current change intended?  It has the following odd effect:

In [5]: Adata + np.int16(257) Out[5]: array([257, 257], dtype=uint16)

In [7]: Adata + np.int16(-257) Out[7]: array([-257, -257], dtype=int16)

In [8]: Adata - np.int16(257) Out[8]: array([65279, 65279], dtype=uint16)

but I guess you can argue that there are odd effects for other choices too,

In case it wasn't clear, this, in numpy 1.6.1:

In [2]: (np.zeros((2,), dtype=np.uint8) + np.int16(257)).dtype Out[2]: dtype('int16')

changed to this in current trunk:

In [2]: (np.zeros((2,), dtype=np.uint8) + np.int16(257)).dtype Out[2]: dtype('uint16')

which is different still in previous versions of numpy (e.g. 1.4.1):

In [2]: (np.zeros((2,), dtype=np.uint8) + np.int16(257)).dtype Out[2]: dtype('uint8')

My impression had been that the plan was to avoid changes in the casting rules if possible.

Was this change in trunk intentional?  If not, I am happy to bisect,

OK - I will assume it was unintentional and make a bug report, Best, Matthew