Calling function with optional arguments with PyObject_CallMethod

Question

Using Python C API, how can I call a function with optional arguments?

For example, assuming there is a Python module add_module.py with a function add that goes like:

def add(a=0, b=0):
  return a + b

add(1, 2) would correspond to

PyObject *add_module = PyImport_ImportModule("add_module");
PyObject *result = PyObject_CallMethod(add_module, "add", "ii", 1, 2);

How would I call something like add(b=5) using the Python API?

Thanks!

Answer 1

Calling with keyword arguments is not that straightforward from C, and you need to do PyObject_Call, which requires 3 arguments:

1. The first argument of PyObject_Call is the method object
2. The second argument of PyObject_Call is the empty tuple for *args
3. The third argument is the keyword dictionary

Thus we get

PyObject *function = PyObject_GetAttrString(add_module, "add");
PyObject *args = PyTuple_New(0);
PyObject *kwargs = Py_BuildValue("{s:i}", "b", 5)
result = PyObject_Call(function, args, kwargs);

Py_DECREF(kwargs);
Py_DECREF(args);
Py_DECREF(function);

except you must NULL-check the return values from the 4 first functions (omitted for brevity).