I have access to a pre-existing json.JSONEncoder subclass called
GpxJsonEncoder for serialising a specific type (called a GpxModel -
although that's not important). I can use this specialised JSONEncoder with
dumps() by specifying the 'cls' argument to json.dumps() and everything
works as expected:
import son
json_data = json.dumps(gpx_model, cls=GpxJsonEncoder)
I'd like to be able to use GpxJsonEncoder with Pyramid by configuring a
gpx_json renderer for a view which could then just return a GpxModel
instance. From reading the documentation I would expect to be able to do:
config.add_renderer('json_gpx', JSON(cls=GpxJsonEncoder))
since keyword arguments ('cls' in this case) are passed to the underlying
serializer which is json.dumps().
Unfortunately, this doesn't work as I would expect because Pyramid
intervenes and passes a 'default' callback argument to the serializer which
causes serialization to fail within the Pyramid defined default callback:
File
"/Users/rjs/dev/virtualenvs/sageroux/lib/python3.3/site-packages/pyramid/renderers.py",
line 273, in default
raise TypeError('%r is not JSON serializable' % (obj,))
TypeError: <trailer.model.gpx_model.GpxModel object at 0x10a13c310> is
not JSON serializable
There doesn't seem to be any way to prevent Pyramid interjecting with this
callback other than defining a customer serializer wrapping json.dumps()
which accepts, but then ignores, the default argument, like this:
def gpx_dumps(obj, default, **kwargs):
# This function is just a wrapper for json.dumps() which ignores the
# default parameter passed to the serializer by Pyramid, which would
# otherwise cause the JSONEncoder subclass to fail.
return json.dumps(obj, **kwargs)
config.add_renderer('json_gpx', JSON(serializer=gpx_dumps,
cls=GpxJsonEncoder))
Is there are better solution for using off-the-shelf JSONEncoder
subclasses? I think the API for JSON() needs to be slightly more flexible
to avoid the need for the serializer wrapper. Should this be considered a
bug or shortcoming in Pyramid?
Rob Smallshire
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