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

[Warren Weckesser]

On Thu, Sep 13, 2012 at 9:01 AM, 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.



Heh, I guess "expected" is subjective:

In [23]: np.__version__
Out[23]: '1.6.1'

In [24]: a = zeros((2,2))

In [25]: b = ones((2,3))

In [26]: concatenate((a, b), axis=0)  # Expected error.
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/Users/warren/gitwork/class-material/demo/pytables/<ipython-input-26-7cefb735e507>
in <module>()
----> 1 concatenate((a, b), axis=0)  # Expected error.

ValueError: array dimensions must agree except for d_0

In [27]: concatenate((a, b), axis=1)   # Normal behavior.
Out[27]:
array([[ 0.,  0.,  1.,  1.,  1.],
       [ 0.,  0.,  1.,  1.,  1.]])

In [28]: concatenate((a, b), axis=2)   # Cryptic error message.
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/Users/warren/gitwork/class-material/demo/pytables/<ipython-input-28-0bce84c34ef1>
in <module>()
----> 1 concatenate((a, b), axis=2)   # Cryptic error message.

ValueError: bad axis1 argument to swapaxes

In [29]: concatenate((a, b), axis=32)   # What the... ?
Out[29]: array([ 0.,  0.,  0.,  0.,  1.,  1.,  1.,  1.,  1.,  1.])


I would expect an error, consistent with the behavior when 1 < axis < 32.


Warren




> 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.
>
> -Travis
>
>
>
>
>
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