Generate a Cumulative Distribution Function for each ID1 thus:
cdfs = defaultdict()
for id1,id2,val in d:
prevtotal = cdfs[id1][-1][0]
newtotal = prevtotal + val
cdfs[id1].append( (newtotal,id2) )
So you will have
cdfs = { 701 : [ (0.2,1), (0.5,2), (1.0,3) ],
702 : [ (0.2,1), (0.5,2) ],
703 : [ (0.5,3) ] }
Then generate a random number and search for it in the list.
def func(id1):
max = cdfs[id1][-1][0]
rand = random.random()*max
for upper,id2 in cdfs[id1]:
if upper>rand:
return id2
return None