An apology to Paul

ftrain_treeify.py

def ftrain(l):
    '''Convert an iterable to a nested list based on ordered comparison'''
    i = iter(l)
    tree, seen = ftrain_helper(i, i.next())
    # This might not be the desired behavior, but it's not terrible
    # You could also emit a warning or an exception here
    while(not seen is None):
        rest_tree, seen = ftrain_helper(i, seen)
        tree = [tree] + rest_tree
    return tree

def ftrain_helper(i, curr_level):
    '''Iterate, call down when reach a larger level, return when level drops'''
    out = [curr_level]

    while(True):
        try:
            next = i.next()
        except StopIteration:
            # This is the only place we actually return to the ftrain function
            return out, None

        if next > curr_level:
            nested, seen = ftrain_helper(i, next)
            out.append(nested)
            if seen is None or seen < curr_level:
                return out, seen
            else:
                out.append(seen)
        elif next == curr_level:
            out.append(next)
        if next < curr_level:
            return out, next