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 */...