def oauth2_callback(request):
    """ View that handles the user's return from OAuth2 provider.

    This view verifies the CSRF state and OAuth authorization code, and on
    success stores the credentials obtained in the storage provider,
    and redirects to the return_url specified in the authorize view and
    stored in the session.

    :param request: Django request
    :return: A redirect response back to the return_url
    """
    if 'error' in request.GET:
        reason = request.GET.get(
            'error_description', request.GET.get('error', ''))
        return http.HttpResponseBadRequest(
            'Authorization failed %s' % reason)

    try:
        encoded_state = request.GET['state']
        code = request.GET['code']
    except __HOLE__:
        return http.HttpResponseBadRequest(
            "Request missing state or authorization code")

    try:
        server_csrf = request.session[_CSRF_KEY]
    except KeyError:
        return http.HttpResponseBadRequest("No existing session for this flow.")

    try:
        state = json.loads(encoded_state)
        client_csrf = state['csrf_token']
        return_url = state['return_url']
    except (ValueError, KeyError):
        return http.HttpResponseBadRequest('Invalid state parameter.')

    if client_csrf != server_csrf:
        return http.HttpResponseBadRequest('Invalid CSRF token.')

    flow = _get_flow_for_token(client_csrf, request)

    if not flow:
        return http.HttpResponseBadRequest("Missing Oauth2 flow.")

    try:
        credentials = flow.step2_exchange(code)
    except client.FlowExchangeError as exchange_error:
        return http.HttpResponseBadRequest(
            "An error has occurred: {0}".format(exchange_error))

    storage.get_storage(request).put(credentials)

    signals.oauth2_authorized.send(sender=signals.oauth2_authorized,
                                   request=request, credentials=credentials)
    return shortcuts.redirect(return_url)
dataset/ETHPy150Open google/oauth2client/oauth2client/contrib/django_util/views.py/oauth2_callback