code: def username(self):
          
          token = self.session.params.get('access_token')
          if not token:
              raise errors.TokenError(
                  "session does not have a valid access_token param")
          data = token.split('.')[1]
          # replace url chars and add padding
          # (https://gist.github.com/perrygeo/ee7c65bb1541ff6ac770)
          data = data.replace('-', '+').replace('_', '/') + "==="
          try:
              return json.loads(base64.b64decode(data).decode('utf-8'))['u']
          except (ValueError, KeyError):
              raise errors.TokenError(
                  "access_token does not contain username")
docs: The username in the service's access token
  
          Returns
          -------
          str