e1.py

def mean(Z):
    def sr(x, y):
        cnt = x[1] + 1
        return x[0]+y, cnt, (x[0]+y)/cnt
    return reduce(sr, Z, [0,0,0])[2]

assert mean([1,2,3]) == 2
assert mean([1.5, 2.5, 3.5]) == 2.5
assert mean([1]) == 1
assert mean([-2.0, 2.0, 0.0]) == 0