Wrapping C structs/functions with Cython

Question

I have some c code that implements a polynomial as a struct and some functions to manipulate it. I would like to wrap these with cython for use with python.

Specifically, I would like to have a python class Polynomial that I can pass a list of floats by wrapping the ply_create_coef function. I would like this class to have the methods get_coef and set_coef by wrapping the ply_get_coef and ply_set_coef functions respectively. I would also like to have a function that I may pass an instance of the polynomial class and returns another instance of the class which is just a scalar multiple of the original. I would like to do this by wrapping the ply_scale function.

After reading some tutorials: https://cython.readthedocs.io/en/latest/src/tutorial/clibraries.html and Wrapping C library

I noticed there is a difference between how my and the tutorial's code is set up. In the tutorials, the creation functions return pointers to the created struct, which I am assuming is because memory was dynamically allocated at some point. Sine I am terrible at memory management, I decided to keep everything on the stack. So how would I go about wrapping these things in cython in this case?

Also I didn't see anything about dealing with the float to double conversion when going from python to c. I'm wondering if I have to loop inside the __cinit__ using ply_create function since towards the bottom of the docs, it says that you can't use pointers in the python functions.

Could someone explain how to proceed or point me to a reference? I'm new to cython, so feel free to point out other helpful tutorials as well.

// polynomial.h

#define MAX_DEG 200

typedef struct Polynomial Polynomial;

struct Polynomial {
    int deg;
    double coefs[MAX_DEG];
};

Polynomial ply_create(int deg);
Polynomial ply_create_coef(int deg, double *coefs);
double ply_get_coef(Polynomial p, int i);
void ply_set_coef(Polynomial *p, int i, double coef);
Polynomial ply_scale(double s; Polynomial p);

// polynomial.c

# include <stdio.h>

Polynomial ply_create(int deg) {
    Polynomial poly;
    poly.deg = deg;

    return poly;
}

Polynomial ply_create_coef(int deg, double *coefs) {
    Polynomial poly;
    poly.deg = deg;

    int i;
    for (i=0; i<=deg; i++) {
        double coef = coefs[i];
        poly.coefs[i] = coef;
    }

    return poly;
}

double ply_get_coef(Polynomial p, int i) {
    double coef = p.coefs[i];

    return coef;
}

void ply_set_coef(Polynomial *p, int i, double coef) {
    p->coefs[i] = coef;
}

Polynomial ply_scale(double s, Polynomial p) {
    int deg = p.deg;
    Polynomial scaled = ply_create(deg);
    int i;
    for (i=0; i<=deg; i++) {
        double coef = ply_get_coef(p, i);
        ply_set_coef(&scaled, i, s*coef);
    }
}

# cpolynomial.pxd

cdef extern from "polynomial.h":
    ctypedef struct Polynomial;

    Polynomial ply_create(int deg);
    Polynomial ply_create_coef(int deg, double *coefs);
    double ply_get_coef(Polynomial p, int i);
    void ply_set_coef(Polynomial *p, int i, double coef);
    Polynomial ply_scale(double s; Polynomial p);

# polynomial.pyx

cimport "cpolynomial.pxd"

cdef class Polynomial:
    def __cinit__(self, int deg, double *coefs):
        self.poly = cpolynomial.ply_create_coef(deg, coefs)