JSON and JSONP Rendering

JSON Renderer

TurboGears always provided builtin support for JSON Rendering, this is provided by the JSONRenderer and the json.encode() function.

The first is what empowers the @expose('json') feature while the second is an utility function you can call whenever encoding to json is needed. Both rely on on tg.jsonify.JSONEncoder which is able to handle more types than the standard one provided by the python json module and can be extended to support more types.

Using it is as simple as:

def jp(self, **kwargs):
    return dict(hello='World')

Which, when calling /jp would result in:

{"hello": "World"}

Customizing JSON Encoder

While you can create your own encoder, turbogears has a default instance of JSONEncoder which is used for all encoding performed by the framework itself. Behavior of this encoder can be driven by providing a __json__ method inside objects for which you want to customize encoding and can be configured using through following options:

  • json.isodates -> Whenever to encode dates in ISO8601 or not, the default is False
  • json.custom_encoders -> Dictionary of type: function mappings which can specify custom encoders for specific types. Custom encoders are functions that are called to get a basic object the json encoder knows how to handle.

For example to configure a custom encoder for dates your project app_cfg.py would look like:

from datetime import date

def dmy_encoded_date(d):
    return d.strftime('%d/%m/%Y')

base_config['json.custom_encoders'] = {date: dmy_encoded_date}

That would cause all datetime.date instances to be encoded using dmy_encode_date function.

If the encoded object provides a __json__ method this is considered the custom encoder for the object itself and it is called to get a basic type the json encoder knows how to handle (usually a dict).


json.custom_encoders take precedence over __json__, this is made so that users can override behavior for third party objects that already provide a __json__ method.

Per method customization

The same options available inside the json. configuration namespace are available as render_params for the expose decorator. So if you want to turn on/off iso formatted dates for a single method you can do that using:

from datetime import datetime

@expose('json', render_params=dict(isodates=True))
def now(self, **kwargs):
    return dict(now=datetime.utcnow())

JSONP Renderer

Since version 2.3.2 TurboGears provides built-in support for JSONP rendering.

JSONP works much like JSON output, but instead of providing JSON response it provides an application/javascript response with a call to a javascript function providing all the values returned by the controller as function arguments.

To enable JSONP rendering you must first append it to the list of required engines inside your application config/app_cfg.py:


Then you can declare a JSONP controller by exposing it as:

def jp(self, **kwargs):
    return dict(hello='World')

When accessing /jp?callback=callme you should see:

callme({"hello": "World"});

If you omit the callback parameter an error will be returned as it is required to know the callback name when using JSONP.

Custom callback parameter

By default TurboGears will expect the callback name to be provided in a callback parameter. This parameter has to be accepted by your controller (otherwise you can use **kwargs like the previous examples).

If you need to use a different name for the callback parameter just provide it in the render_params of your exposition:

@expose('jsonp', render_params={'callback_param': 'call'})
def jp(self, **kwargs):
    return dict(hello='World')

Then instead of opening /jp?callback=callme to get the JSONP response you will need to open /jp?call=callme as stated by the callback_param option provided in the render_params.

Exposing both JSON and JSONP

If you want to expose a controller as both JSON and JSONP, just provide both expositions. You can then use TurboGears request extensions support to choose which response you need:

def jp(self, **kwargs):
    return dict(hello='World')

To get the JSON response simply open /jp.json while to get the JSONP response go to /jp.js?callback=callme. If no extension is provided the first exposition will be returned (in this case JSON).