code: def login():
      
  
      cas_token_session_key = current_app.config['CAS_TOKEN_SESSION_KEY']
  
      redirect_url = create_cas_login_url(
          current_app.config['CAS_SERVER'],
          current_app.config['CAS_LOGIN_ROUTE'],
          flask.url_for('.login', origin=flask.session.get('CAS_AFTER_LOGIN_SESSION_URL'), _external=True))
  
      if 'ticket' in flask.request.args:
          flask.session[cas_token_session_key] = flask.request.args['ticket']
  
      if cas_token_session_key in flask.session:
  
          if validate(flask.session[cas_token_session_key]):
              if 'CAS_AFTER_LOGIN_SESSION_URL' in flask.session:
                  redirect_url = flask.session.pop('CAS_AFTER_LOGIN_SESSION_URL')
              elif flask.request.args.get('origin'):
                  redirect_url = flask.request.args['origin']
              else:
                  redirect_url = flask.url_for(
                      current_app.config['CAS_AFTER_LOGIN'])
          else:
              del flask.session[cas_token_session_key]
  
      current_app.logger.debug('Redirecting to: {0}'.format(redirect_url))
  
      return flask.redirect(redirect_url)
docs: This route has two purposes. First, it is used by the user
      to login. Second, it is used by the CAS to respond with the
      `ticket` after the user logs in successfully.
  
      When the user accesses this url, they are redirected to the CAS
      to login. If the login was successful, the CAS will respond to this
      route with the ticket in the url. The ticket is then validated.
      If validation was successful the logged in username is saved in
      the user's session under the key `CAS_USERNAME_SESSION_KEY` and
      the user's attributes are saved under the key
      'CAS_USERNAME_ATTRIBUTE_KEY'