Answer to this: http://blog.rapleaf.com/dev/2011/09/13/puzzle-coins-and-scales-part-12

coins.py

#!/usr/bin/python

from random import randint
from sys import exit

# list of coins containing their weight (2 grams)
coins = [2]*7
# insert a different sized coin (1 or 3 grams randomized) 
newcoin = randint(0,1)*2+1 
coins.insert(randint(0,7), newcoin)
print 'coins:', coins

# our weighing scale
def weight(list1, list2):
	if (type(list1) == type(1)):
		ret = list1 == list2
		if ret == True:
			return ret
		if list1 < list2:
			return 'less'
		return 'more'
	ret = sum(list1) == sum(list2)
	if ret == True:
		return ret
	if sum(list1) < sum(list2):
		return 'less'
	return 'more'


weight1 = weight(coins[2:5], coins[5:8])
if weight1 == True:
	weight2 = weight(coins[0], coins[1])
	weight3 = weight(coins[0], coins[2])
	if weight3 == True and weight2 == 'less':
		print 'coin 2 is heavier'
	elif weight3 == 'less' and weight2 == 'less':
		print 'coin 1 is lighter'
	elif weight3 == True:
		print 'coin 2 is lighter'
	else:
		print 'coin 1 is heavier'
	exit()

weight2 = weight(coins[4:6], coins[6:8])
if weight2 == True:
	weight3 = weight(coins[3], coins[4])
	if weight3 == True and weight1 == 'less':
		print 'coin 3 is lighter'
	elif weight3 == True and weight1 == 'more':
		print 'coin 3 is heavier'
	elif weight3 == 'less':
		print 'coin 4 is lighter'
	else:
		print 'coin 4 is heavier'
	exit()

if weight1 == 'more' and weight2 == 'less':
	print 'coin 6 is lighter'
	exit()
elif weight1 == 'less' and weight2 == 'more':
	print 'coin 6 is heavier'
	exit()

weight3 = weight(coins[6], coins[7])
if weight3 == True:
	if weight2 == 'less':
		print 'coin 5 is lighter'
	else:
		print 'coin 5 is heavier'
	exit()

if weight1 == weight2 and weight2 == weight3:
	if weight1 == 'less':
		print 'coin 8 is heavier'
	else:
		print 'coin 8 is lighter'
	exit()

if weight3 == 'less':
	print 'coin 7 is lighter'
else:
	print 'coin 7 is heavier'