Python google appengine sessions using webapp2_extras confusion

Question

I'm trying to create user sessions as explained here http://www.essentialtech.co.nz/content/using_session_google_app_engine_and_python_27 . Below is the Login page handler class. Everything is fine, but I'm not able to understand what the 'self.session.get('referrer')' would do. I googled for it and found that 'HTTP_REFERER' refers to the site url where you are coming from. But why do we need it in the Login handler here? I feel glad if some one can explain it to me.

class LogIn(BaseHandler):
    def get(self):
        if self.session.get('user'):
            del self.session['user']
        if not self.session.get('referrer'):
            self.session['referrer'] = \
                self.request.environ['HTTP_REFERER'] \
                if 'HTTP_REFERER' in self.request.environ \
                else '/'
        template_values = {
            }
        template = jinja_environment.get_template('login.html')
        self.response.out.write(template.render(template_values))

    def post(self):
        user = self.request.get('user')
        self.session['user'] = user
        logging.info("%s just logged in" % user)
        self.redirect('/')

Answer 1

I presume it's used to know where to redirect the user after login. But the POST redirects to '/' so doesn't look like it's plugged in, at least not in this snippet. This doesn't make sense though, if you come in from a totally different website. So maybe it's just used for logging / tracking purposes. Again, not detailed in this snippet.