[Numpy-discussion] Setting custom dtypes and 1.14

josef.pktd at gmail.com josef.pktd at gmail.com
Mon Jan 29 17:59:29 EST 2018


On Mon, Jan 29, 2018 at 5:50 PM, <josef.pktd at gmail.com> wrote:

>
>
> On Mon, Jan 29, 2018 at 4:11 PM, Allan Haldane <allanhaldane at gmail.com>
> wrote:
>
>> 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.
>>
>
> from the example in the other thread
> a[['b', 'c']].view(('f8', 2)).mean(0)
>
>
> (from the statsmodels usecase:
> read csv with genfromtext to get recarray or structured array
> select/index the numeric columns
> view them as standard array
> do whatever we can do with standard numpy  arrays
> )
>

Or, to phrase it as a question:

How do we get a standard array with homogeneous dtype from the
corresponding elements of a structured dtype in numpy 1.14.0?

Josef




>
> Josef
>
>
>>
>> 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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