C API: array of floats/ints from python to C and back

[Daniel Fetchinson]

>>> As others already said, using a Numpy array or an array.array object
>>> would
>>> be more efficient (and even easier - the C code gets a pointer to an
>>> array
>>> of integers, as usual).
>>
>> I looked for this in the C API docs but couldn't find anything on how
>> to make an array.array python object appear as a pointer to integers
>> (or floats, etc) in C code. On
>>
>> http://docs.python.org/c-api/concrete.html#sequence-objects
>>
>> There is only list and tuple or maybe you mean byte array? That has
>> only been introduced in python 2.6 and I'm working on 2.5.
>
> array is a library module, and isn't really part of the API. You're
> looking for the buffer protocol:
> PyObject_AsReadBuffer/PyObject_AsWriteBuffer; see
> http://docs.python.org/c-api/objbuffer.html
>
> Given an array.array('l') (containing C long integers):
>
> int do_something(PyObject* obj)
> {
>      long *vec;
>      Py_ssize_t nbytes, nitems, i;
>
>      if (PyObject_AsReadBuffer(obj, (const void **)&vec, &nbytes) != 0)
>          return NULL;
>      nitems = nbytes/sizeof(long);
>      for (i=0; i<nitems; i++) {
>        /* do something with vec[i] */
>      }
>      return ret;
> }
>
>  From Python you can get "vec" and "nitems" using the buffer_info() method
> of array objects.

Thanks very much, this was very helpful!

Cheers,
Daniel

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