How do I call an object's method from C?

The PyObject_CallMethod function can be used to call an arbitrary method of an object. The parameters are the object, the name of the method to call, a format string like that used with Py_BuildValue, and the argument values:

PyObject *
PyObject_CallMethod(PyObject *object, char *method_name,
                    char *arg_format, ...);

This works for any object that has methods — whether built-in or user-defined. You are responsible for eventually Py_DECREF‘ing the return value.

To call, e.g., a file object’s “seek” method with arguments 10, 0 (assuming the file object pointer is “f”):

res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
if (res == NULL) {
        ... an exception occurred ...
}
else {
        Py_DECREF(res);
}

Note that since PyObject_CallObject always wants a tuple for the argument list, to call a function without arguments, pass “()” for the format, and to call a function with one argument, surround the argument in parentheses, e.g. “(i)”.

CATEGORY: extending