Implement Py_BuildValue

[elbaro]

Py_BuildValue is not implemented.

func Py_BuildValue(format string, args ...interface{}) *PyObject {
	return nil
}

Example:
f(7,8,9) =

values := python.PyTuple_New(3)
python.PyTuple_SET_ITEM(values, 0, python.PyInt_FromLong(7))
python.PyTuple_SET_ITEM(values, 1, python.PyInt_FromLong(8))
python.PyTuple_SET_ITEM(values, 2, python.PyInt_FromLong(9))
f.CallObject(values)

should be possible with

values := python.Py_BuildValue("(i,i,i)", 7, 8, 9)
f.CallObject(values)

[sbinet]

@elbaro, unfortunately, cgo doesn't handle C functions that are variadic (IIRC).

so, essentially, one would need to implement Py_BuildValue in terms of e.g. PyTuple_New + PyTuple_SET_ITEM.
and, for this to work, one would need to also implement a parser for the Py_BuildValue(format) string to infer the tuple size and the various types for the elements of that tuple:

• https://docs.python.org/2/c-api/arg.html

that said that could be done. PR accepted :P

[sbinet]

(note that, according to Python2 C-API Py_BuildValue doesn't always return a slice, so this should also be taken care of...)

[icholy]

What about a hacky solution where there are a bunch of C wrapping functions with a fixed number of parameters. This code is obviously wrong, but it gets the idea across:

PyObject* Py_BuildValue_1(const char *format, PyObject* a1) {
  return Py_BuildValue(format, a1);
}

PyObject* Py_BuildValue_2(const char *format, PyObject* a1, PyObject* a2) {
  return Py_BuildValue(format, a1, a2);
}

PyObject* Py_BuildValue_3(const char *format, PyObject* a1, PyObject* a2, PyObject* a3) {
  return Py_BuildValue(format, a1, a2, a3);
}

func Py_BuildValue(format string, args ...*PyObject) *PyObject {
  switch len(args) {
  case 1:
    return C.Py_BuildValue_1(format, args[0])
  case 2:
    return C.Py_BuildValue_2(format, args[0], args[1])
  case 3:
    return C.Py_BuildValue_3(format, args[0], args[1], args[3])
  default:
    return Py_None
  }
}

[sbinet]

SGTM.
that's actually what I did there:
https://github.com/sbinet/go-python/blob/master/go-python.c#L16 for PyObject_CallFunction