code: def index(self, state, code=None, error=None):
          
          error = None
          if code:
              try:
                  auth_token = self.fetch_token(code, state)
              except MissingTokenError:
                  error = self._fmt_failure(
                      'Missing access token parameter.</br>Please check that '
                      'you are using the correct client_secret')
              except MismatchingStateError:
                  error = self._fmt_failure('CSRF Warning! Mismatching state')
          else:
              error = self._fmt_failure('Unknown error while authenticating')
          # Use a thread to shutdown cherrypy so we can return HTML first
          self._shutdown_cherrypy()
          return error if error else self.success_html % (auth_token)
docs: Receive a Exist response containing a verification code. Use the code
          to fetch the access_token.