def METHOD_NAME(self, request: HttpRequest, source_slug: str) -> HttpResponse:
    """Handles a POSTed SSO Assertion and logs the user in."""
    source: SAMLSource = get_object_or_404(SAMLSource, slug=source_slug)
    if not source.enabled:
        raise Http404
    processor = ResponseProcessor(source, request)
    try:
        processor.parse()
    except MissingSAMLResponse as exc:
        return bad_request_message(request, str(exc))
    except VerificationError as exc:
        return bad_request_message(request, str(exc))
    try:
        return processor.prepare_flow_manager().get_flow()
    except (UnsupportedNameIDFormat, ValueError) as exc:
        return bad_request_message(request, str(exc))