Converting Python list to a char **

A common problem in many C programs is the processing of command line arguments, which are usually passed in an array of NULL terminated strings. The following SWIG interface file allows a Python list object to be used as a char ** object.

%module argv

// This tells SWIG to treat char ** as a special case
%typemap(python,in) char ** {
  /* Check if is a list */
  if (PyList_Check($source)) {
    int size = PyList_Size($source);
    int i = 0;
    $target = (char **) malloc((size+1)*sizeof(char *));
    for (i = 0; i < size; i++) {
      PyObject *o = PyList_GetItem($source,i);
      if (PyString_Check(o))
        $target[i] = PyString_AsString(PyList_GetItem($source,i));
      else {
        PyErr_SetString(PyExc_TypeError,"list must contain strings");
        free($target);
        return NULL;
      }
    }
    $target[i] = 0;
  } else {
    PyErr_SetString(PyExc_TypeError,"not a list");
    return NULL;
  }
}

// This cleans up the char ** array we malloced before the function call
%typemap(python,freearg) char ** {
  free((char *) $source);
}

// This allows a C function to return a char ** as a Python list
%typemap(python,out) char ** {
  int len,i;
  len = 0;
  while ($source[len]) len++;
  $target = PyList_New(len);
  for (i = 0; i < len; i++) {
    PyList_SetItem($target,i,PyString_FromString($source[i]));
  }
}

// Now a few test functions
%inline %{
int print_args(char **argv) {
    int i = 0;
    while (argv[i]) {
         printf("argv[%d] = %s\n", i,argv[i]);
         i++;
    }
    return i;
}

// Returns a char ** list 

char **get_args() {
    static char *values[] = { "Dave", "Mike", "Susan", "John", "Michelle", 0};
    return &values[0];
}
%}
 

When this module is compiled, our wrapped C functions now operate as follows :

>>> from argv import *
>>> print_args(["Dave","Mike","Mary","Jane","John"])
argv[0] = Dave
argv[1] = Mike
argv[2] = Mary
argv[3] = Jane
argv[4] = John
5
>>> get_args()
[`Dave', `Mike', `Susan', `John', `Michelle']
>>>
 

Our type-mapping makes the Python interface to these functions more natural and easy to use.