How can I write a C function that takes either an int or a float?

Question

I want to create a function in C that extends Python that can take inputs of either float or int type. So basically, I want f(5) and f(5.5) to be acceptable inputs.

I don't think I can use if (!PyArg_ParseTuple(args, "i", $value)) because it only takes only int or only float.

How can I make my function allow inputs that are either ints or floats?

I'm wondering if I should just take the input and put it into a PyObject and somehow take the type of the PyObject - is that the right approach?

Answer 1

If you declare a C function to accept floats, the compiler won't complain if you hand it an int. For instance, this program produces the answer 2.000000:

#include <stdio.h>

float f(float x) {
  return x+1;
}

int main() {
  int i=1;
  printf ("%f", f(i));
}

A python module version, iorf.c:

#include <Python.h>

static PyObject *IorFError;

float f(float x) {
  return x+1;
}


static PyObject *
fwrap(PyObject *self, PyObject *args) {
  float in=0.0;
  if (!PyArg_ParseTuple(args, "f", &in))
    return NULL;
  return Py_BuildValue("f", f(in));
}

static PyMethodDef IorFMethods[] = {
    {"fup",  fwrap, METH_VARARGS,
     "Arg + 1"},
    {NULL, NULL, 0, NULL}        /* Sentinel */
};


PyMODINIT_FUNC
initiorf(void)
{
  PyObject *m;

  m = Py_InitModule("iorf", IorFMethods);
  if (m == NULL)
    return;

  IorFError = PyErr_NewException("iorf.error", NULL, NULL);
  Py_INCREF(IorFError);
  PyModule_AddObject(m, "error", IorFError);
}

The setup.py:

from distutils.core import setup, Extension

module1 = Extension('iorf',
                    sources = ['iorf.c'])

setup (name = 'iorf',
       version = '0.1',
       description = 'This is a test package',
       ext_modules = [module1])

An example:

03:21 $ python
Python 2.7.10 (default, Jul 30 2016, 18:31:42)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.34)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import iorf
>>> print iorf.fup(2)
3.0
>>> print iorf.fup(2.5)
3.5

Answer 2

Floats are (usually) passed in via registers while ints are (usually) passed in via the stack. This means that you literally cannot, inside the function, check whether the argument is a float or an int.

The only workaround is to use variadic arguments, with the first argument specifying the type as either int or double (not float).

func_int_or_double (uint8_t type, ...) {
va_list ap;
va_start (ap, type);
int intarg;
double doublearg;
if (type==1) {
   intarg = va_arg (ap, int);
}
if (type==2) {
   doublearg = va_arg (ap, double);
}
va_end (ap);
// Your code goes here
}

Although, I'm not really sure if python can handle calling variadic functions, so YMMV. As a last ditch-effort you can always sprintf the value into a buffer and let your function sscanf float/int from the buffer.

Answer 3

You can check type of input value like this:

    PyObject* check_type(PyObject*self, PyObject*args) {
    PyObject*any;

    if (!PyArg_ParseTuple(args, "O", &any)) {
        PyErr_SetString(PyExc_TypeError, "Nope.");
        return NULL;
    }

    if (PyFloat_Check(any)) {
        printf("indeed float");
    }
    else {
        printf("\nint\n");
    }

    Py_INCREF(Py_None);

    return Py_None;
}

You can extract float value from object using:

double result=PyFloat_AsDouble(any);

But in this particular situation probably no need to do this, no matter what you parsing int or float you can grab it as a float and check on roundness:

    float target;
    if (!PyArg_ParseTuple(args, "f", &target)) {
                PyErr_SetString(PyExc_TypeError, "Nope.");
                return NULL;
    }

    if (target - (int)target) {
        printf("\n input is float \n");
    }
    else {
        printf("\n input is int \n");
    }