In [1]: from collections import defaultdict

In [2]: tree = lambda: defaultdict (tree)

In [3]: t = tree()

In [4]: t['name']['crow'] = '__crow__'

In [5]: t['name']['wlwang'] = '__wlwang__'

In [6]: t
Out[6]: defaultdict(<function <lambda> at 0x10bed7140>, {'name': defaultdict(<function <lambda> at 0x10bed7140>, {'crow': '__crow__', 'wlwang': '__wlwang__'})})

In [7]: import json

In [8]: print json.dumps(t)
{"name": {"crow": "__crow__", "wlwang": "__wlwang__"}}