[Numpy-discussion] Fixing inconsistent behaviour of reduceat()

Dive Nex divenex at gmail.com
Thu Apr 13 07:02:33 EDT 2017

Hi all,

I would like to try to reach a consensus about a long standing inconsistent
behavior of reduceat() reported and discussed here


In summary, it seems an elegant and logical design choice, that all users
will expect, for

    out = ufunc.reduceat(a, indices)

to produce, for all indices j (except for the last one) the following

    out[j] = ufunc.reduce(a[indices[j]:indices[j+1]])

However the current documented and actual behavior is for the case

    indices[i] >= indices[i+1]

to return simply

    out[j] = a[indices[i]]

I cannot see any application where this behavior is useful or where this
choice makes sense. This seems just a bug that should be fixed.

What do people think?

PS: A quick fix for the current implementation is

    out = ufunc.reduceat(a, indices)
    out *= np.diff(indices) > 0
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