Jug puzzle

import collections

def jugPuzzle(jugs, goal, start=None):
    '''Given n jugs of specified capacities, use BFS to try to find a sequence
    of fillings, emptyings and transferings of water that will produce the goal
    amount from the start state. Otherwise return empty list.'''
    if not start:
        start = (0,) * len(jugs)

    if not (0 <= goal <= max(jugs)):
        return []

    if goal in start:
        return [('start', start)]

    paths = collections.deque([[('start', start)]])
    seen = set([start])

    while paths:
        path = paths.popleft()
        lastAction, lastState = path[-1]

        for action, state in successors(jugs, lastState):
            if state not in seen:
                currPath = path + [(action, state)]

                # goal reached
                if goal in state:
                    return currPath

                paths.append(currPath)
                seen.add(state)

    # no path found
    return []

def successors(jugs, levels):
    '''Compute all non-trivial successor states. Return as a list of (action, state) tuples.'''
    result = []
    L = len(jugs)

    for i in range(L):
        if levels[i] < jugs[i]:
            action = 'fill %d' % i
            state = tuple(jugs[j] if j == i else levels[j] for j in range(L))
            result.append((action, state))

        if levels[i] > 0:
            action = 'empty %d' % i
            state = tuple(0 if j == i else levels[j] for j in range(L))
            result.append((action, state))

            for j in range(L):
                if j != i and levels[j] < jugs[j]:
                    action = '%d -> %d' % (i, j)
                    total = levels[i] + levels[j]
                    iLevel = max(0, total - jugs[j])
                    jLevel = min(jugs[j], total)
                    state = tuple(iLevel if k == i else jLevel if k == j
                                                   else levels[k] for k in range(L))
                    result.append((action, state))

    return result

# demo
print(*jugPuzzle([100, 13, 43], 98), sep='\n')