A JSON request handler for Tornado.

jsonhandler.py

import json
import tornado.web


class JsonHandler(BaseHandler):
    """Request handler where requests and responses speak JSON."""
    def prepare(self):
        # Incorporate request JSON into arguments dictionary.
        if self.request.body:
            try:
                json_data = json.loads(self.request.body)
                self.request.arguments.update(json_data)
            except ValueError:
                message = 'Unable to parse JSON.'
                self.send_error(400, message=message) # Bad Request

        # Set up response dictionary.
        self.response = dict()

    def set_default_headers(self):
        self.set_header('Content-Type', 'application/json')

    def write_error(self, status_code, **kwargs):
        if 'message' not in kwargs:
            if status_code == 405:
                kwargs['message'] = 'Invalid HTTP method.'
            else:
                kwargs['message'] = 'Unknown error.'

        self.response = kwargs
        self.write_json()

    def write_json(self):
        output = json.dumps(self.response)
        self.write(output)

Also note, that BaseHandler needs to be defined. Instead one can probably use tornado.web.RequestHandler.

From documentation: "write() is used for non-template-based output; it accepts strings, bytes, and dictionaries (dicts will be encoded as JSON)." So, may be better send self.response to self.write()?
One moment more is about writing json_data into self.request.arguments. Also like in documentation, may be better write its in self.json_args and use tornado.escape.json_decode(self.request.body) instead json.loads(self.request.body)?