Random choice with weighted probabilities

gistfile1.py

# typical to sum over your list of items perhaps 
# performing some calculation on each item
normalizing_constant = \
	sum([calc(item) for item in items if filter(item)])

r = random.random()

for item in items:

  prob = get_probability(item) / normalizing_constant

  if prob > r:
    # keep this guy 
    break

  r -= prob