<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def x_forwarded_for(self):
 """X-Forwarded-For header value. This is the amended header so that it contains the previous IP address in the forwarding change. """

        ip = self._request.META.get('REMOTE_ADDR')
        current_xff = self.headers.get('X-Forwarded-For')

        return '%s, %s' % (current_xff, ip) if current_xff else ip