How can I return JSON output when repoze.who authentication fails?

Question

How can I return JSON output when repoze.who authentication fails?

112 Views Asked by Martin Thorsen Ranang At 17 May 2025 at 18:54

I'm writing a repoze.who plugin and want to return JSON from the repoze.who authentication middleware and still controll the HTTP status code. How can this be done?

Original Q&A

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**Martin Thorsen Ranang** · Accepted Answer

One way to accomplish this, is to implement a repoze.who Challenger interface. The following solution takes advantage of the fact that the WebOb exceptions, in webob.exc, can be used as a WSGI application. The following example shows how this can be used in a hypothetical Facebook plugin, where the 2.x API lets users not grant access to their email, which might be required for a successful registration/authentication:

import json
from webob.acceptparse import MIMEAccept
from webob.exc import HTTPUnauthorized, HTTPBadRequest


FACEBOOK_CONNECT_REPOZE_WHO_NOT_GRANTED = 'repoze.who.facebook_connect.not_granted'


class ExampleJSONChallengerPlugin(object):
    json_content_type = 'application/json'
    mime_candidates = ['text/html',
                       'application/xhtml+xml',
                       json_content_type,
                       'application/xml',
                       'text/xml']

    def is_json_request_env(self, environ):
        """Checks whether the current request is a json request as deemed by
        TurboGears (i.e. response_type is already set) or if the http
        accept header favours 'application/json' over html.
        """
        if environ['PATH_INFO'].endswith('.json'):
            return True

        if 'HTTP_ACCEPT' not in environ:
            return False

        # Try to mimic what Decoration.lookup_template_engine() does.
        return MIMEAccept(environ['HTTP_ACCEPT']) \
            .best_match(self.mime_candidates) is self.json_content_type

    def challenge(self, environ, status, app_headers, forget_headers):
        if FACEBOOK_CONNECT_REPOZE_WHO_NOT_GRANTED in environ:
            response = HTTPBadRequest(detail={
                'not_granted': 
                environ.pop(FACEBOOK_CONNECT_REPOZE_WHO_NOT_GRANTED),
            })
        elif status.startswith('401 '):
            response = HTTPUnauthorized()
        else:
            response = None

        if response is not None and self.is_json_request_env(environ):
            response.body = json.dumps({
                'code': response.code,
                'status': response.title,
                'explanation': response.explanation,
                'detail': response.detail,
            })
            response.content_type = self.json_content_type

        return response

A central point here is that response, an instance of a sub-class of webob.exc.WSGIHTTPException, is used as a WSGI application, but also that if response's body attribute is set, then it is not automatically generated, a fact we use to explicitly set the response's body to a JSON-formatted string representation of our dictionary. If the above challenger is invoked during the handling of a request to a URL ending in '.json' or the Accept header includes application/json, the body of the response might render as something like:

{
    "status": "Bad Request",
    "explanation": "The server could not comply with the request since it is either malformed or otherwise incorrect.",
    "code": 400, 
    "detail": {"not_granted": ["email"]}
}

and if not, then the body will be rendered as HTML:

<html>
 <head>
  <title>400 Bad Request</title>
 </head>
 <body>
  <h1>400 Bad Request</h1>
  The server could not comply with the request since it is either 
  malformed or otherwise incorrect.<br /><br />
{'not_granted': [u'email']}


 </body>
</html>

How can I return JSON output when repoze.who authentication fails?

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