standard deviation of the values

std-1.py

from math import sqrt

sum, sum2, n = 0.0, 0.0, 0.0
f = open('numeros.txt', 'r')
numbers = f.readlines()
f.close()

for num in numbers:
    k = float(num)
    sum += k
    sum2 += k*k
    n += 1

print sqrt(sum2/n - (sum/n)**2)