[Numpy-discussion] suggestion for generalizing numpy functions

[Darren Dale]

On Fri, Jul 17, 2009 at 10:03 AM, Darren Dale <dsdale24@gmail.com> wrote:

> On Mon, Jul 13, 2009 at 7:12 PM, Darren Dale <dsdale24@gmail.com> wrote:
>
>> 2009/7/13 Stéfan van der Walt <stefan@sun.ac.za>
>>
>>> Hi Darren
>>>
>>> 2009/7/13 Darren Dale <dsdale24@gmail.com>:
>>> > I've put together a first cut at implementing __array_prepare__, which
>>> > appears to work, and I would like to request feedback. Here is an
>>> overview
>>> > of the approach:
>>>
>>> This is pretty neat!  Do you have a quick snippet at hand illustrating
>>> its use?
>>
>>
>> That would be helpful, wouldn't it? The attached script is a modified
>> version of RealisticInfoArray from
>> http://docs.scipy.org/doc/numpy/user/basics.subclassing.html . It should
>> yield the following output:
>>
>>
>> starting with [0 1 2 3 4]
>> which is of type <class '__main__.MyArray'>
>> and has info attribute = "information"
>> subtracting 3 from [0 1 2 3 4]
>> subtract calling __array_prepare__ on [0 1 2 3 4] input
>> output array is now of type <class '__main__.MyArray'>
>> output array values are still uninitialized:
>>         [139911601789568        39578752 139911614885536        39254560
>>               48]
>> __array_prepare__ is updating info attribute on output
>> __array_prepare__ finished, subtract ufunc is taking over
>> subtract calling __array_wrap__ on [0 1 2 3 4] input
>> output array has initial value: [-3 -2 -1  0  1]
>> __array_wrap__ is setting output endpoints to 0
>> yielding [ 0 -2 -1  0  0]
>> which is of type <class '__main__.MyArray'>
>> and has info attribute = "new_information"
>>
>
> This is a gentle ping, hoping to get some feedback so this feature has a
> chance of being included in the next release.
>

I have a question about the C-api. If I want to make the default
implementation of __array_prepare__ (or __array_wrap__, is anyone out
there?) simply pass through the output array:

 static PyObject *
array_preparearray(PyArrayObject *self, PyObject *args)
{
    PyObject *arr;

    if (PyTuple_Size(args) < 1) {
        PyErr_SetString(PyExc_TypeError,
                        "only accepts 1 argument");
        return NULL;
    }
    arr = PyTuple_GET_ITEM(args, 0);
    if (!PyArray_Check(arr)) {
        PyErr_SetString(PyExc_TypeError,
                        "can only be called with ndarray object");
        return NULL;
    }
    return arr;
}

Is this sufficient, or do I need to worry about calling Py_INCREF?

Thanks,
Darren
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