[Numpy-discussion] Regression in choose()

[Ed Schofield]

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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