Passing and array argument to C from Python using SWIG

Question

I am using SWIG + Python + C for the first time, and I am having trouble passing an array from Python to C.

Here is a function signature in C.

my_setup(char * my_string, int my_count, int my_types[], double my_rate, int my_mode);

I would like to call this C function from Python as follows

my_array = [1, 2, 3, 4, 5, 6]
my_setup("My string", 6, my_array, 50, 0)

but I do not know how to construct the array my_array . The error I am getting is

Traceback (most recent call last):
  File "test_script.py", line 9, in <module>
    r = my_library.my_setup("My string", 6, my_types, 50, 0)
TypeError: in method 'my_setup', argument 3 of type 'int []'

I have tried unsuccessfully to use the SWIG interface file for numpy and ctypes .

I hope someone can help me pass an array as the third argument of the function my_setup .

Also, this is my first stack overflow post!

Answer 1

Parse the Python list inside my_setup() instead of trying to translate it in your SWIG .i file. Change

my_setup(char * my_string, int my_count, int my_types[], double my_rate, int my_mode);

to

my_setup(char * my_string, int my_count, PyObject *int_list, double my_rate, int my_mode);

and in my_setup

    int *array = NULL;
    if ( PyList_Check( int_list ) )
    {
        int nInts = PyList_Size( int_list );
        array = malloc( nInts * sizeof( *array ) );
        for ( int ii = 0; ii < nInts; ii++ )
        {
            PyObject *oo = PyList_GetItem( int_list, ii );
            if ( PyInt_Check( oo ) )
            {
                array[ ii ] = ( int ) PyInt_AsLong( oo );
            }
        }
    }

You'll have to add error checking. And from C, always return a PyObject * back to Python when you're using SWIG. That way, you can use PyErr_SetString() and return NULL to toss an exception.