jrovegno

from math import sqrt

sum, sum2, n = 0.0, 0.0, 0.0
with open("numeros.txt") as infile:
    for line in infile:
        k = float(line)
        sum += k
        sum2 += k*k
        n += 1

jrovegno

import numpy as np

def std2(x):
    sum, sum2, n = 0.0, 0.0, 0.0
    for k in x:
        sum += k
        sum2 += k*k
        n += 1
    return sum2/n - sum*sum/(n*n)