gistfile1.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from twisted.internet defer
from functools import wraps
import json

def writeJSON(fn):
  @wraps(fn)
  def wrapped(request, *params, **kwargs):
    request.setResponseCode(200)
    d = fn(request, *params, **kwargs)
    d.addCallback(json.dumps)
    d.addCallback(request.write)
    return d
  return wrapped

@writeJSON
@defer.inlineCallbacks
def renderGET(request):
  data = yield xxx
  ...