The Python function passes a reference to itself as the first argument, followed by real arguments given to the function. The PyArg_ParseTuple function is used to parse values from the Python function to local variables in the C function. In this function, we cast a value to a double, and hence we use d as the second argument. You can see a full list of strings that are accepted by this function at  https://docs.python.org/2/c-api/arg.html .

The final result of the computations is returned using Py_Buildvalue, which takes a similar type of format string to create a Python value from your answer. We use f here, which stands for float, to demonstrate that double and float are treated similarly:

/* 
Implementation of the actual C funtions 
*/ 
 
static PyObject* square_func(PyObject* self, PyObject* args) 
{ 
double value; 
double answer; 
 
/*  parse the input, from python float to c double */ 
if (!PyArg_ParseTuple(args, "d", &value)) 
return NULL; 
/* if the above function returns -1, an appropriate Python exception will 
* have been set, and the function simply returns NULL 
*/ 
 
answer = value*value; 
 
return Py_BuildValue("f", answer); 
}