[Numpy-discussion] Views of a different dtype

[Sebastian Berg]

On Fr, 2015-01-30 at 19:52 -0800, Jaime Fernández del Río wrote:
> On Thu, Jan 29, 2015 at 8:57 AM, Nathaniel Smith <njs at pobox.com>
> wrote:
>         On Thu, Jan 29, 2015 at 12:56 AM, Jaime Fernández del Río
>         <jaime.frio at gmail.com> wrote:
>         [...]

<snip>

>         
>         Could we make it more like: check to see if the last dimension
>         works.
>         If not, raise an error (and let the user transpose some other
>         dimension there if that's what they wanted)? Or require the
>         user to
>         specify which dimension will absorb the shape change? (If we
>         were
>         doing this from scratch, then it would be tempting to just say
>         that we
>         always add a new dimension at the end with newtype.itemsize /
>         oldtype.itemsize entries, or absorb such a dimension if
>         shrinking. As
>         a bonus, this would always work, regardless of contiguity!
>         Except that
>         when shrinking the last dimension would have to be contiguous,
>         of
>         course.)
> 
> 
> When we roll @ in and people start working with stacks of matrices, we
> will probably find ourselves having to create an alias, similar to .T,
> for .swapaxes(-1, -2). Searching for the smallest stride allows to
> take views of such arrays, which does not work right now because the
> array is no longer contiguous globally.
>  

That is true, but I agree with Nathaniel at least as far as that I would
prefer a user to be able to safely use `view` even he has not even an
inkling about what his memory layout is. One option would be an
`axis=-1` default (maybe FutureWarn this from `axis=None` which would
look at order, see below -- or maybe have axis='A', 'C' and 'F' and
default to 'A' for starters).

This even now could start creating bugs when enabling relaxed
strides :(, because your good old fortran order complex array being
viewed as a float one could expand along the wrong axis, and even
without such arrays swap order pretty fast when operating on them, which
can create impossibly to find bugs, because even a poweruser is likely
to forget about such things.

Of course you could argue that view is a poweruser feature and a user
using it should keep these things in mind.... Though if you argue that,
you can almost just use `np.ndarray` directly ;) -- ok, not really
considering how cumbersome it is, but still.

- Sebastian

>         
>         I guess the main consideration for this is that we may be
>         stuck with
>         stuff b/c of backwards compatibility. Can you maybe say a
>         little bit
>         about what is allowed now, and what constraints that puts on
>         things?
>         E.g. are we already grovelling around in strides and picking
>         random
>         dimensions in some cases?
> 
> 
> Just to restate it: right now we only allow new views if the array is
> globally contiguous, so either along the first or last dimension.
> 
> 
> Jaime
>  
>         
>         -n
>         
>         --
>         Nathaniel J. Smith
>         Postdoctoral researcher - Informatics - University of
>         Edinburgh
>         http://vorpus.org
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>         
> 
> 
> 
> 
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