code: def get_access_token(self, request, callback=None):
          "Fetch access token from callback request."
          raw_token = request.session.get(self.session_key, None)
          verifier = request.GET.get('oauth_verifier', None)
          if raw_token is not None and verifier is not None:
              data = {'oauth_verifier': verifier}
              callback = request.build_absolute_uri(callback or request.path)
              callback = force_text(callback)
              try:
                  response = self.request('post', self.provider.access_token_url,
                                          token=raw_token, data=data, oauth_callback=callback)
                  response.raise_for_status()
              except RequestException as e:
                  logger.error('Unable to fetch access token: {0}'.format(e))
                  return None
              else:
                  return response.text
          return Nonf get_access_token(self, request, callback=None):
          "Fetch access token from callback request."
          raw_token = request.session.get(self.session_key, None)
          verifier = request.GET.get('oauth_verifier', None)
          if raw_token is not None and verifier is not None:
              data = {'oauth_verifier': verifier}
              callback = request.build_absolute_uri(callback or request.path)
              callback = force_text(callback)
              try:
                  response = self.request('post', self.provider.access_token_url,
                                          token=raw_token, data=data, oauth_callback=callback)
                  response.raise_for_status()
              except RequestException as e:
                  logger.error('Unable to fetch access token: {0}'.format(e))
                  return None
              else:
                  return response.text
          return None
docs: Fetch access token from callback request.