[Numpy-discussion] TypeError when using double , longdouble in numpy.dot

[David Cournapeau]

On Thu, Jul 8, 2010 at 6:13 AM, Christoph Gohlke <cgohlke at uci.edu> wrote:
> Dear NumPy developers,
>
> I am trying to solve some scipy.sparse TypeError failures reported in
> [1] and reduced them to the following example:
>
>
>>>> import numpy
>>>> a = numpy.array([[1]])
>
>>>> numpy.dot(a.astype('single'), a.astype('longdouble'))
> array([[1.0]], dtype=float64)
>
>>>> numpy.dot(a.astype('double'), a.astype('longdouble'))
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: array cannot be safely cast to required type
>
>
> Is this exception expected?

No, I don't think so. The error seems to be platform specific - I have
the expected result on my macbook.

>
> Also I noticed this:
>
>>>> numpy.array([1]).astype('longdouble').dtype.num
> 13
>>>> numpy.array([1.0]).astype('longdouble').dtype.num
> 12

This is unexpected. There maybe some untested/buggy codepaths for the
windows case (where sizeof(double) == sizeof(long double)). I will try
to look into it, but please post an issue on trac so that it does not
get lost,

David