distress signal

approach

each packet is parsed with eval, since the lines are already valid python lists. compare walks two lists side by side and returns -1, 0 or 1: two ints compare directly, two lists recurse, and a lone int is wrapped in a list before recursing. if one list runs out first, it's the smaller.

part 1 sums the indices of the pairs that compare as ordered. part 2 throws all the packets plus the two divider packets into one list, sorts it with cmp_to_key(compare), and multiplies the dividers' positions.

the trick

once the comparator returns -1/0/1, cmp_to_key turns it into a sort key and part 2 is a plain sort.

# p1
print(sum(i for i, ps in enumerate(pairs, 1) if compare(*ps) < 0))

# p2
l = [pp for p in pairs for pp in p] + [[[2]], [[6]]]
s = sorted(l, key = cmp_to_key(compare))
print((s.index([[2]]) + 1) * (s.index([[6]]) + 1))