Klein produces decorator for setting the content type and optionally installing a content encoder.

produces.py

from functools import wraps

from twisted.internet.defer import maybeDeferred

def produces(contentType, encoder=None):
    def maybeEncode(result, request):
        request.setHeader("content-type", contentType)

        if encoder is not None:
            return encoder(result)

        return result

    def deco(f):
        @wraps(f)
        def inner(request, *args, **kwargs):
            d = maybeDeferred(f, request, *args, **kwargs)
            d.addCallback(maybeEncode, request)
            return d

        return inner

    return deco