Python script to interpolate with Lagrange method.

interpolate_lagrange.py

#! /usr/local/bin/python3.6
"""
Interpolation with Lagrange method
"""
import sys
import traceback


class InterpolateLagrange:
    X = [0.0, 2.0, 3.0, 5.0, 8.0]
    Y = [0.8, 3.2, 2.8, 4.5, 1.9]

    def __init__(self):
        self.n = len(self.X)

    def compute(self):
        """ Computation of interpolation with Lagrange method """
        try:
            print("      x      y")
            for a in range(int(self.X[-1]) * 2 + 1):
                t = 0.5 * a
                print("{:7.2f}{:7.2f}".format(t, self.__interpolate(t)))
        except Exception as e:
            raise

    def __interpolate(self, t):
        """ Interpoalation with Lagrange method
        
        :param float t
        """
        try:
            s = 0.0
            for i in range(0, self.n):
                p = self.Y[i]
                for j in range(0, self.n):
                    if i != j:
                        p *= (t - self.X[j]) / (self.X[i] - self.X[j])
                s += p
            return s
        except Exception as e:
            raise


if __name__ == '__main__':
    try:
        obj = InterpolateLagrange()
        obj.compute()
    except Exception as e:
        traceback.print_exc()
        sys.exit(1)

I made a few changes to this, so that it works as a function accepting the values as parameters.

def interpolate(t, xs, ys):
    s = 0.0
    n = len(xs)
    for i in range(n):
        p = ys[i]
        for j in range(n):
            if i != j:
                p *= (t - xs[j]) / (xs[i] - xs[j])
        s += p
    return s

def compute(xs, ys):
    for a in range(int(xs[-1]) * 2 + 1):
        t = 0.5 * a
        print("{:7.2f}{:7.2f}".format(t, interpolate(t, xs, ys)))

X = [0.0, 2.0, 3.0, 5.0, 8.0]
Y = [0.8, 3.2, 2.8, 4.5, 1.9]

compute(X, Y)

I needed this version for some homework. I know you have your reasons to do it with classes.
This was helpful. Thanks!