[Numpy-discussion] Setting custom dtypes and 1.14

[Allan Haldane]

On 01/29/2018 04:02 PM, josef.pktd at gmail.com wrote:
> 
> 
> On Mon, Jan 29, 2018 at 3:44 PM, Benjamin Root <ben.v.root at gmail.com
> <mailto:ben.v.root at gmail.com>> wrote:
> 
>     I <3 structured arrays. I love the fact that I can access data by
>     row and then by fieldname, or vice versa. There are times when I
>     need to pass just a column into a function, and there are times when
>     I need to process things row by row. Yes, pandas is nice if you want
>     the specialized indexing features, but it becomes a bear to deal
>     with if all you want is normal indexing, or even the ability to
>     easily loop over the dataset.
> 
> 
> I don't think there is a doubt that structured arrays, arrays with
> structured dtypes, are a useful container. The question is whether they
> should be more or the foundation for more.
> 
> For example, computing a mean, or reduce operation, over numeric element
> ("columns"). Before padded views it was possible to index by selecting
> the relevant "columns" and view them as standard array. With padded
> views that breaks and AFAICS, there is no way in numpy 1.14.0 to compute
> a mean of some "columns". (I don't have numpy 1.14 to try or find a
> workaround, like maybe looping over all relevant columns.)
> 
> Josef

Just to clarify, structured types have always had padding bytes, that
isn't new.

What *is* new (which we are pushing to 1.15, I think) is that it may be
somewhat more common to end up with padding than before, and only if you
are specifically using multi-field indexing, which is a fairly
specialized case.

I think recfunctions already account properly for padding bytes. Except
for the bug in #8100, which we will fix, padding-bytes in recarrays are
more or less invisible to a non-expert who only cares about
dataframe-like behavior.

In other words, padding is no obstacle at all to computing a mean over a
column, and single-field indexes in 1.15 behave identically as before.
The only thing that will change in 1.15 is multi-field indexing, and it
has never been possible to compute a mean (or any binary operation) on
multiple fields.

Allan

> 
>     Cheers!
>     Ben Root
> 
>     On Mon, Jan 29, 2018 at 3:24 PM, <josef.pktd at gmail.com
>     <mailto:josef.pktd at gmail.com>> wrote:
> 
> 
> 
>         On Mon, Jan 29, 2018 at 2:55 PM, Stefan van der Walt
>         <stefanv at berkeley.edu <mailto:stefanv at berkeley.edu>> wrote:
> 
>             On Mon, 29 Jan 2018 14:10:56 -0500, josef.pktd at gmail.com
>             <mailto:josef.pktd at gmail.com> wrote:
> 
>                 Given that there is pandas, xarray, dask and more, numpy
>                 could as well drop
>                 any pretense of supporting dataframe_likes. Or, adjust
>                 the recfunctions so
>                 we can still work dataframe_like with structured
>                 dtypes/recarrays/recfunctions.
> 
> 
>             I haven't been following the duckarray discussion carefully,
>             but could
>             this be an opportunity for a dataframe protocol, so that we
>             can have
>             libraries ingest structured arrays, record arrays, pandas
>             dataframes,
>             etc. without too much specialized code?
> 
> 
>         AFAIU while not being in the data handling area, pandas defines
>         the interface and other libraries provide pandas compatible
>         interfaces or implementations.
> 
>         statsmodels currently still has recarray support and usage. In
>         some interfaces we support pandas, recarrays and plain arrays,
>         or anything where asarray works correctly.
> 
>         But recarrays became messy to support, one rewrite of some
>         functions last year converts recarrays to pandas, does the
>         manipulation and then converts back to recarrays.
>         Also we need to adjust our recarray usage with new numpy
>         versions. But there is no real benefit because I doubt that
>         statsmodels still has any recarray/structured dtype users. So,
>         we only have to remove our own uses in the datasets and unit tests.
> 
>         Josef
> 
>          
> 
> 
>             Stéfan
> 
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