[Numpy-discussion] Change in behavior of np.concatenate for upcoming release

[Matthew Brett]

Hi,

On Thu, Sep 13, 2012 at 7:00 PM, Matthew Brett <matthew.brett at gmail.com> wrote:
> Hi,
>
> On Thu, Sep 13, 2012 at 11:31 AM, Matthew Brett <matthew.brett at gmail.com> wrote:
>> On Wed, Sep 12, 2012 at 4:19 PM, Nathaniel Smith <njs at pobox.com> wrote:
>>> On Wed, Sep 12, 2012 at 2:46 PM, Matthew Brett <matthew.brett at gmail.com> wrote:
>>>> Hi,
>>>>
>>>> I just noticed that this works for numpy 1.6.1:
>>>>
>>>> In [36]: np.concatenate(([2, 3], [1]), 1)
>>>> Out[36]: array([2, 3, 1])
>>>>
>>>> but the beta release branch:
>>>>
>>>> In [3]: np.concatenate(([2, 3], [1]), 1)
>>>> ---------------------------------------------------------------------------
>>>> IndexError                                Traceback (most recent call last)
>>>> /Users/mb312/<ipython-input-3-0fa244c8aaa8> in <module>()
>>>> ----> 1 np.concatenate(([2, 3], [1]), 1)
>>>>
>>>> IndexError: axis 1 out of bounds [0, 1)
>>>>
>>>> In the interests of backward compatibility maybe it would be better to
>>>> raise a warning for this release, rather than an error?
>>>
>>> Yep, that'd be a good idea. Want to write a patch? :-)
>>
>> https://github.com/numpy/numpy/pull/440
>
> Thinking about the other thread, and the 'number of elements' check, I
> noticed this:
>
> In [51]: np.__version__
> Out[51]: '1.6.1'
>
> In [52]: r4 = range(4)
>
> In [53]: r3 = range(3)
>
> In [54]: np.concatenate((r4, r3), None)
> Out[54]: array([0, 1, 2, 3, 0, 1, 2])
>
> but:
>
> In [46]: np.__version__
> Out[46]: '1.7.0rc1.dev-ea23de8'
>
> In [47]: np.concatenate((r4, r3), None)
> ---------------------------------------------------------------------------
> ValueError                                Traceback (most recent call last)
> /Users/mb312/tmp/<ipython-input-47-e354b8880702> in <module>()
> ----> 1 np.concatenate((r4, r3), None)
>
> ValueError: all the input arrays must have same number of elements
>
> The change requiring the same number of elements appears to have been
> added explicitly by Mark in commit 9194b3af  .  Mark - what was the
> reason for that check?

Appealing for anyone who might understand that part of the code :
there's a check in multiarraymodule.c at around line 477:

    /*
     * Figure out the final concatenated shape starting from the first
     * array's shape.
     */
    for (iarrays = 1; iarrays < narrays; ++iarrays) {
        if (PyArray_SIZE(arrays[iarrays]) != shape[1]) {
            PyErr_SetString(PyExc_ValueError,
                            "all the input arrays must have same "
                            "number of elements");
            return NULL;
        }
    }

I don't understand the following code so I don't know what this check is for...

Cheers,

Matthew