Mean Squared error in Python

You are modifying the index for no reason. A for loop increments it anyways. Also, you are not using the index, for example, you are not using any y[i] - y_pred[i], hence you don't need the loop at all.

Use the arrays

mse = np.mean((y - y_pred)**2)

I would say :

def get_mse(y, y_pred):
d1 = y - y_pred
mse = (1/N)*d1.dot(d1) # N is int(len(y))
return mse

it would only work if y and y_pred are numpy arrays, but you would want them to be numpy arrays as long as you decide not to use other libraries so you can do math operations on it.

numpy dot() function is the dot product of 2 numpy arrays (you can also write np.dot(d1, d1) )