import numpy as np
x = [1,2,3]
y = [0,0,1]

def mse(target_y, predicted_y):
    diff = np.array(target_y) - np.array(predicted_y) #diff = [1, 2, 2]
    square = diff ** 2 # square = [1, 4, 4]
    summation = sum(square) # summation = 9
    return summation / len(x) #3

mse(x,y)