def mean(x):
    t = 0.0
    N = len(x)
    for n in range(N):
        t += x[n]
    return t / N

def mean2(x):
    t = 0.0
    for xn in x:
        t += xn
    return t / len(x)

x = list(range(12))
mean(x), mean2(x)

(5.5, 5.5)