quick trick for changing exceptions to json

[Jonathan Vanasco]

Like many others, I've struggled with getting Pyramid to render JSON exceptions.  Thanks to a few recent updates to Pyramid and WebOb, things got a whole lot easier.

The new behavior allows for JSON to be generated based on the "accept" request headers.  Unfortunately, sending this header might not be possible (or easy) with low-level clients and utilities.  While testing some approaches, I came across the following trick:

    @view_config(context=HTTPException)

    def exception_view__upgrade(exc, request):

        """if we end with .json, serve json."""

        if (request.path[-5:]).lower() == '.json':

            request.environ['HTTP_ACCEPT'] = "application/json"

        return exc

I just used the `request.path` to ensure the test in `pyramid/httpexceptions.py` passes if a ".json" document is requested.  Depending on your needs, you may want to use another condition.  If you're already using a custom `context=HTTPException` view, this would need to be factored in.  This behavior works because of a few current implementation details on Pyramid, so it's not guaranteed to last... but for now it works and has saved me a lot of grief.

(i accidentally posted a version of this earlier with some code for customizing exceptions that wasn't properly abstracted out and didn't work outside my application. sorry if you saw that.)