[Tutor] python lists to C arrays and vice versa

[Danny Yoo]

On Mon, 7 Feb 2005, Viktor Hornak wrote:

> I've been trying to find more resources/documentation about how to
> convert python lists to C arrays (and vice versa) when writing a python
> extension.

Hi Viktor,


There was a post back in 1999 that might be useful for you:

    http://mail.python.org/pipermail/tutor/1999-November/000758.html

I'm not positive if there is a nicer helper function to do this.


> I also found one or two examples of similar operations on this list
> postings (from 6 years ago!)  but they didn't quite explain all I need
> to know.

Ah, so you did see that posting then.  *grin* Ok, good.

What parts of their explanation were incomplete?  Maybe one of us here can
help fill in more details for you.



> Here is the code I was trying to use to convert two lists (one
> is a list of strings with the same length and the other is a list of
> doubles) to C arrays in a C extension:

[Some code cut]

The code there seems sorta ok: you might want to add a string length check
in there somewhere: the code expects that names are at most 4 characters
long, but that condition isn't being checked by strcpy().



> Then I call the function "parseMask" (which I am trying to wrap in this
> extension) which returns a C array (integer array, elements are either 0
> or 1). Now I need to convert this integer array to python list and
> return it back to python. Here is the code for that:
>
>    result = (int *) malloc(sizeof(int) * nat);
>    parseMask(...) -> returns int array result
>
>    pylist = PyTuple_New(nat);
>    for (i = 0; i < nat; i++) {
>       /* convert resulting array [0/1] to PyObject */
>       if (result[i] == 0)
>          item = PyInt_FromLong(0);
>       else
>          item = PyInt_FromLong(1);
>
>       PyTuple_SetItem(pylist, i, item);
>       Py_DECREF(item);          /** <-- (dyoo): bug located */
>    }


Ah!  Got it.  Don't call Py_DECREF() here: PyTuple_SetItem will already
"steal"  the reference, according to the documentation:

    http://docs.python.org/api/tupleObjects.html#l2h-576

so there's no need to decrement the reference count here.


By the way, if you're really going to do more C extension stuff, take a
look at Pyrex or SWIG:

    http://nz.cosc.canterbury.ac.nz/~greg/python/Pyrex/
    http://www.swig.org/

Coding C wrappers by hand is painful: use these tools to make your life
easier.  *grin*


Best of wishes!