7-2.py

from anytree.importer import DictImporter
from anytree.exporter import DotExporter
from anytree import RenderTree
from itertools import chain

a = open('7.txt').read().split('\n')[:-1]
a = [x.split(' -> ') for x in a]
a = [x[0].split() + [x[1].split(', ') if len(x)>1 else []] for x in a]
a = {x[0]: (int(x[1][1:-1]), x[2]) for x in a}

root = (set(x for x in a) - set(chain(*[a[x][1] for x in a]))).pop()

def get_total_weight(node):
    children = a[node][1]
    if len(children) == 0:
        return a[node][0]
    else:
        return a[node][0] + sum([get_total_weight(x) for x in children])

def create_dict(node):
    children = a[node][1]
    if len(children) == 0:
        return {
            "a": a[node][0]
        }
    else:
        return {
            "a": (a[node][0], get_total_weight(node)),
            "children": [create_dict(x) for x in children]
        }

importer = DictImporter()
tree = importer.import_(create_dict(root))

print RenderTree(tree)