[Numpy-discussion] Regression in choose()

[Olivier Delalleau]

If performance is not critical you could just write your own function for a
quick fix, doing something like
numpy.array([choices[j][i] for i, j in enumerate([2, 3, 1, 0])])

This 32-array limitation definitely looks weird to me though, doesn't seem
to make sense.

-=- Olivier

2011/6/16 Ed Schofield <ed at pythoncharmers.com>

> Hi all,
>
> I have been investigation the limitation of the choose() method (and
> function) to 32 elements. This is a regression in recent versions of NumPy.
> I have tested choose() in the following NumPy versions:
>
> 1.0.4: fine
> 1.1.1: bug
> 1.2.1: fine
> 1.3.0: bug
> 1.4.x: bug
> 1.5.x: bug
> 1.6.x: bug
> Numeric 24.3: fine
>
> (To run the tests on versions of NumPy prior to 1.4.x I used Python 2.4.3.
> For the other tests I used Python 2.7.)
>
> Here 'bug' means the choose() function has the 32-element limitation. I
> have been helping an organization to port a large old Numeric-using codebase
> to NumPy, and the choose() limitation in recent NumPy versions is throwing a
> spanner in the works. The codebase is currently using both NumPy and Numeric
> side-by-side, with Numeric only being used for its choose() function, with a
> few dozen lines like this:
>
> a = numpy.array(Numeric.choose(b, c))
>
> Here is a simple example that triggers the bug. It is a simple extension of
> the example from the choose() docstring:
>
> ----------------
>
> import numpy as np
>
> choices = [[0, 1, 2, 3], [10, 11, 12, 13],
>           [20, 21, 22, 23], [30, 31, 32, 33]]
>
> np.choose([2, 3, 1, 0], choices * 8)
>
> ----------------
>
> A side note: the exception message (defined in
> core/src/multiarray/iterators.c) is also slightly inconsistent with the
> actual behaviour:
>
> Traceback (most recent call last):
>   File "chooser.py", line 6, in <module>
>     np.choose([2, 3, 1, 0], choices * 8)
>   File "/usr/lib64/python2.7/site-packages/numpy/core/fromnumeric.py", line
> 277, in choose
>     return _wrapit(a, 'choose', choices, out=out, mode=mode)
>   File "/usr/lib64/python2.7/site-packages/numpy/core/fromnumeric.py", line
> 37, in _wrapit
>     result = getattr(asarray(obj),method)(*args, **kwds)
> ValueError: Need between 2 and (32) array objects (inclusive).
>
> The actual behaviour is that choose() passes with 31 objects but fails with
> 32 objects, so this should read "exclusive" rather than "inclusive". (And
> why the parentheses around 32?)
>
> Does anyone know what changed between 1.2.1 and 1.3.0 that introduced the
> 32-element limitation to choose(), and whether we might be able to lift this
> limitation again for future NumPy versions? I have a couple of days to work
> on a patch ... if someone can advise me how to approach this.
>
> Best wishes,
>     Ed
>
>
> --
> Dr. Edward Schofield
> Python Charmers
> +61 (0)405 676 229
> http://pythoncharmers.com
>
>
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>
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