lagrange.py

# -*- coding: utf-8 -*-
from sympy import *

init_printing()

def lagrange(matrix):
    size = len(matrix)
    x = Symbol('x')
    a = 0
    for k in range(0, size):
        b = 1
        for i in range(0, size):
            if i == k:
                continue
            b = b * (x - matrix[i][0]) / (matrix[k][0] - matrix[i][0])
        a = a + (matrix[k][1] * b)
    return simplify(a)

pprint(lagrange([(-1, 4),(0,1),(2,-1)]))