[capi-sig] Beginner Question PyObject* to int

[Guilherme Polo]

On Tue, Nov 11, 2008 at 12:55 PM, Philipp Heuser <philipp at achtziger.de> wrote:
> Hi all,
>
> I am just starting to use the C API for Python. What I want to do is to use
> an external funtion availabele in C and therfore I want to 'convert' a list
> of floats from Python to an array of floats in C which I can forward to the
> external function. So first step is tp parse a list to C and let the C code
> print the variables! The obviously too simplistic approach is below... When
> I call this function from python, e.g. cpp([1,2,3]) the output is:
> 25180552
> 25180540
> 25180528
>
> Could you be so kind to help me here to get started with using C-functions
> from Python? Where is the error?
>
> Thank you very much and kind regards,
>
> Philipp
>
>
>
> static PyObject *
> cpp_foo(PyObject *self, PyObject* args)
> {
>     PyObject* obj;
>   PyObject* seq;
>     int i, len;
>     if(!PyArg_ParseTuple(args, "O", &obj))
>       return NULL;
>         seq = PySequence_Fast(obj, "expected a sequence");
>   len = PySequence_Size(obj);
>     PyObject* item;
>   int a;
>
>   for (i=0; i<len; i++)
>   {
>       item=PySequence_Fast_GET_ITEM(seq, i);
>       a=item;  // ?????????????????????????????????????????

Add more '?'s here.
PySequence_FAST_GET_ITEM returns a PyObject, you can't just go and
give it to 'a' to turn it into an integer. Also, this variable 'i'
better be of type Py_ssize_t instead of int.

So, to get your value as a long, first check item is indeed a long
using PyLong_Check(item) and then convert it to a long with
PyLong_AsLong(item):

>       printf("%i\n", a);
>   }
>     Py_DECREF(seq);       return None;
> }
>
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-- 
-- Guilherme H. Polo Goncalves