code: def do_authentication(environ, start_response, authn_context, key,
                        redirect_uri):
      
      logger.debug("Do authentication")
      auth_info = AUTHN_BROKER.pick(authn_context)
  
      if len(auth_info):
          method, reference = auth_info[0]
          logger.debug("Authn chosen: %s (ref=%s)", method, reference)
          return method(environ, start_response, reference, key, redirect_uri)
      else:
          resp = Unauthorized("No usable authentication method")
          return resp(environ, start_response)
docs: Display the login form