Question

I am trying to replace MATLAB/MEX and switch to Python. I came across SWIG, ctypes & Cython as possible solutions and started trying out SWIG (which seems very simple).

My C functions have variable argument lengths of the form main(int argc, char *argv[]). I found solutions online, but getting this working with SWIG lead to a lot of issues.

1. Are the other methods (ctypes / Cython) any simpler?
2. Any example to do this task with SWIG will be helpful.

Answer 1

There’s actually an example in the SWIG documentation for exactly this sort of function with Python. I’ve quoted it here with a minor change:

%typemap(in) (int argc, char *argv[]) {
  int i;
  if (!PyList_Check($input)) {
    PyErr_SetString(PyExc_ValueError, "Expecting a list");
    return NULL;
  }
  $1 = PyList_Size($input);
  $2 = (char **) malloc(($1+1)*sizeof(char *));
  for (i = 0; i < $1; i++) {
    PyObject *s = PyList_GetItem($input,i);
    if (!PyString_Check(s)) {
        free($2);
        PyErr_SetString(PyExc_ValueError, "List items must be strings");
        return NULL;
    }
    $2[i] = PyString_AsString(s);
  }
  $2[i] = 0;
}

%typemap(freearg) (int argc, char *argv[]) {
   free($2); // If here is uneeded, free(NULL) is legal
}

This allows you in Python to do simply:

import test
test.foo(["a", "b", "c"])

Where test is the name of the module you gave SWIG and foo is a function that matches the signature int argc, char *argv[]. Simple and intuitive to use for a Python programmer and it encapsulates and reuses the complex bit.

---

What the documentation doesn’t seem to mention though is that there’s an interface file that does all this for you already:

%module test

%include <argcargv.i>

%apply (int ARGC, char **ARGV) { (int argc, char *argv[]) }

void foo(int argc, char *argv[]);

is sufficient.