[Numpy-discussion] Obscure code in concatenate code path?

[Matthew Brett]

Hi,

On Thu, Sep 13, 2012 at 3:01 PM, Travis Oliphant <travis at continuum.io> wrote:
>
> On Sep 13, 2012, at 8:40 AM, Nathaniel Smith wrote:
>
>> On Thu, Sep 13, 2012 at 11:12 AM, Matthew Brett <matthew.brett at gmail.com> wrote:
>>> Hi,
>>>
>>> While writing some tests for np.concatenate, I ran foul of this code:
>>>
>>>    if (axis >= NPY_MAXDIMS) {
>>>        ret = PyArray_ConcatenateFlattenedArrays(narrays, arrays, NPY_CORDER);
>>>    }
>>>    else {
>>>        ret = PyArray_ConcatenateArrays(narrays, arrays, axis);
>>>    }
>>>
>>> in multiarraymodule.c
>>
>> How deeply weird
>
>
> This is expected behavior.   It's how the concatenate Python function manages to handle axis=None to flatten the arrays before concatenation.    This has been in NumPy since 1.0 and should not be changed without deprecation warnings which I am -0 on.
>
> Now, it is true that the C-API could have been written differently (I think this is what Mark was trying to encourage) so that there are two C-API functions and they are dispatched separately from the array_concatenate method depending on whether or not a None is passed in.   But, the behavior is documented and has been for a long time.
>
> Reference PyArray_AxisConverter (which turns a "None" Python argument into an axis=MAX_DIMS).   This is consistent behavior throughout the C-API.

How about something like:

#define NPY_NONE_AXIS NPY_MAXDIMS

to make it clearer what is intended?

Best,

Matthew