code: def get_access_token(self, request, callback=None):
          
          callback = request.build_absolute_uri(callback or request.path)
          if not self.check_application_state(request):
              logger.error('Application state check failed.')
              return None
          if 'code' in request.GET:
              args = {
                  'client_id': self.consumer_key,
                  'redirect_uri': callback,
                  'client_secret': self.consumer_secret,
                  'code': request.GET['code'],
                  'grant_type': 'authorization_code',
              }
          else:
              logger.error('No code returned by the provider')
              return None
          try:
              response = self.request('post', self.access_token_url, data=args)
              response.raise_for_status()
          except RequestException as e:
              logger.error('Unable to fetch access token: {0}'.format(e))
              return None
          else:
              return response.text
docs: Fetch access token from callback request.