gistfile1.py

import json
from path import path
source = path('/home/whit/proj/workloadz/17c09d289f6e7c1f01f7/old.json')

data = json.loads(source.text())

from operator import itemgetter

pt  = data['db'][0]['data']['posts_tags']
ti = itemgetter('tag_id')
pi = itemgetter('post_id')
pi_to_tags = {}

[pi_to_tags.setdefault(pid, []).append(tid) for pid, tid in ((pi(x), ti(x)) for x in pt)]
print pi_to_tags