Keeping a running average with Python, see: http://invisibleblocks.com/2008/07/30/long-running-averages-without-the-sum-of-preceding-values/ for more information.

avg.py

#!/usr/bin/env python

from __future__ import division

import sys
import time
import random


class Averager(object):

    def __init__(self):
        self.total = 0.0
        self.count = 0

    def __call__(self, x):
        self.total += x
        self.count += 1
        return self.total / self.count

    @property
    def average(self):
        return self.total / self.count

    def __sizeof__(self):
        return sys.getsizeof(self.total) + sys.getsizeof(self.count)

class LightAverager(object):

    def __init__(self):
        self.average = 0.0
        self.count   = 0

    def __call__(self, x):
        self.count += 1
        self.average += (x - self.average) / self.count

    def __sizeof__(self):
        return sys.getsizeof(self.average) + sys.getsizeof(self.count)

if __name__ == "__main__":

    SEED = random.randint(1,1000)

    def timeit(func):
        def wrapper(*args, **kwargs):
            start  = time.time()
            result = func(*args, **kwargs)
            finish = time.time()
            return result, finish-start
        return wrapper

    @timeit
    def test(factory):
        random.seed(SEED)
        averager = factory()
        for x in xrange(1000000):
            averager(random.randint(100000,9999999999))

        return averager

    heavy, htime = test(Averager)
    light, ltime = test(LightAverager)

    stfmt = "%s computed an average of %0.6f in %0.3f seconds and weighs %i bytes"

    print stfmt % (heavy.__class__.__name__, heavy.average, htime, sys.getsizeof(heavy))
    print stfmt % (light.__class__.__name__, light.average, ltime, sys.getsizeof(light))