__init__.py

import json

def flatten(n):
    while True:
        changed = False
        for k, v in n.iteritems():
            if len(v.keys()) == 1:
                del n[k]
                vk = v.keys()[0]
                n[k + '/' + vk] = v[vk]
                changed = True
            else:
                flatten(v)

        if not changed:
            break

    return n

class Tree(object):
    def __init__(self):
        self.root = {}
        
    def add(self, url):
        r = self.root

        for p in url.split('/'):
            if p not in r:
                r[p] = {}

            r = r[p]


if __name__ == '__main__':
    urls = [
      'blog/a/a/a',
      'blog/a/a/b',
      'blog/a/b/a',
      'blog/a/c/a',
      'blog/a/d',
      'blog/a/d/e',
    ]

    t = Tree()
    map(t.add, urls)
    print(json.dumps(flatten(t.root), indent=2))