[Numpy-discussion] Setting custom dtypes and 1.14

[Eric Wieser]

Why is the list of tuples a useful thing to have in the first place? If the
goal is to convert an array into a structured array, you can do that far
more efficiently with:

def make_tup_dtype(arr):
    """
    Attempt to make a type capable of viewing the last axis of an
array, even if it is non-contiguous.
    Unfortunately `.view` doesn't allow us to use this dtype in that
case, which needs a patch...
    """
    n_fields = arr.shape[-1]
    step = arr.strides[-1]
    descr = dict(names=[], formats=[], offsets=[], itemsize=step * n_fields)
    for i in range(n_fields):
        descr['names'].append('f{}'.format(i))
        descr['offsets'].append(step * i)
        descr['formats'].append(arr.dtype)
    return np.dtype(descr)

Used as:

>>> arr = np.arange(6).reshape(3, 2)>>> arr.view(make_tup_dtype(arr)).squeeze(axis=-1)
array([(0, 1), (2, 3), (4, 5)],
      dtype=[('f0', '<i4'), ('f1', '<i4')])

Perhaps this should be provided by recfunctions (or maybe it already is, in
a less rigid form?)

Eric
​

On Fri, 26 Jan 2018 at 10:48 Allan Haldane <allanhaldane at gmail.com> wrote:

> On 01/25/2018 08:53 PM, Chris Barker - NOAA Federal wrote:
> >> On Jan 25, 2018, at 4:06 PM, Allan Haldane <allanhaldane at gmail.com>
> wrote:
> >
> >>> 1) This is a known change with good reason?
> >
> >> . The
> >> change occurred because the old assignment behavior was dangerous, and
> >> was not doing what you thought.
> >
> > OK, that’s a good reason!
> >
> >>> A) improve the error message.
> >>
> >> Good idea. I'll see if we can do it for 1.14.1.
> >
> > What do folks think about a totuple() method — even before this I’ve
> > wanted that. But in this case, it seems particularly useful.
> >
> > -CHB
>
> Two thoughts:
>
> 1. `totuple` makes most sense for 2d arrays. But what should it do for
> 1d or 3+d arrays? I suppose it could make the last dimension a tuple, so
> 1d arrays would give a list of tuples of size 1.
>
> 2. structured array's .tolist() already returns a list of tuples. If we
> have a 2d structured array, would it add one more layer of tuples? That
> would raise an exception if read back in by `np.array` with the same dtype.
>
> These points make me think that instead of a `.totuple` method, this
> might be more suitable as a new function in np.lib.recfunctions. If the
> goal is to help manipulate structured arrays, that submodule is
> appropriate since it already has other functions do manipulate fields in
> similar ways. What about calling it `pack_last_axis`?
>
> def pack_last_axis(arr, names=None):
>     if arr.names:
>         return arr
>     names = names or ['f{}'.format(i) for i in range(arr.shape[-1])]
>     return arr.view([(n, arr.dtype) for n in names]).squeeze(-1)
>
> Then you could do:
>
>     >>> pack_last_axis(uv).tolist()
>
> to get a list of tuples.
>
> Allan
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