Return list using SWIG from C to Python

Question

I have the following C function which I'm trying to SWIG-ify:

void    GetAttOrder(int node, DCE_ORDER order, float att[3]);

which I want to call in Python and access via:

node = 0; order = DCD_TYPR;
attitude = GetAttOrder(node, order);
attitude[0] // 45.232

Where I've previously implemented the DCE_ORDER type as

typedef enum
{
  DCD_TPYR = 0,
  DCD_TYPR,
  DCD_TYRP,
  ...
  DCD_PRYT
} DCE_ORDER;

I've found some documentation on similar problems in the SWIG documentation, but I haven't had any luck implementing a solution. I've also looked into some other stackoverflow questions (this one seems suspiciously close), also to no avail. I suspect that I should use a typemap here, but am young and foolish when it comes to SWIG.

Any suggestions or pointers?

Many thanks.

have you tried creating an overload float* GetAttOrder(int node, DCE_ORDER order) which calls your GetAttOrder? I believe SWIG knows how to treat a pointer return as a Python list. — Oliver
– Oliver, Commented May 3, 2014 at 11:29
No I hadn't tried that, but I did end up getting something to work. I'll go ahead and answer myself. — John Karasinski
– John Karasinski, Commented May 4, 2014 at 15:16

John Karasinski · Accepted Answer · 2014-05-04 15:27:18Z

Ended up solving this a couple days later. If you have a vector that you want to get out you can do something like:

%typemap(in, numinputs=0) float vec3out[3] (float temp[3]) {
  $1 = temp;
}

%typemap(argout) float vec3out[3] {
  int i;
  $result = PyList_New(3);
  for (i = 0; i < 3; i++) {
    PyObject *o = PyFloat_FromDouble((double) $1[i]);
    PyList_SetItem($result,i,o);
  }
}

And then can access this function through Python as I requested above. Additionally, if you have another function that you want to pass a list into (a getter/setter pair of functions), you can use the following code:

%typemap(in) float vec3in[3] (float temp[3]) {
  int i;
  if (!PySequence_Check($input)) {
    PyErr_SetString(PyExc_ValueError,"Expected a sequence");
    return NULL;
  }
  if (PySequence_Length($input) != 3) {
    PyErr_SetString(PyExc_ValueError,"Size mismatch. Expected 3 elements");
    return NULL;
  }
  for (i = 0; i < 3; i++) {
    PyObject *o = PySequence_GetItem($input,i);
    if (PyNumber_Check(o)) {
      temp[i] = (float) PyFloat_AsDouble(o);
    } else {
      PyErr_SetString(PyExc_ValueError,"Sequence elements must be numbers");
      return NULL;
    }
  }
  $1 = temp;
}

which would allow you to pass in a list.

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