code: def _check_required(sbase, value, attribute):
      
  
      if (value is None) or (value == ""):
          msg = "Required attribute '%s' cannot be found or parsed in '%s'" % \
                (attribute, sbase)
          if hasattr(sbase, "getId") and sbase.getId():
              msg += " with id '%s'" % sbase.getId()
          elif hasattr(sbase, "getName") and sbase.getName():
              msg += " with name '%s'" % sbase.getName()
          elif hasattr(sbase, "getMetaId") and sbase.getMetaId():
              msg += " with metaId '%s'" % sbase.getName()
          raise CobraSBMLError(msg)
      return value
docs: Get required attribute from SBase.
  
      Parameters
      ----------
      sbase : libsbml.SBase
      value : existing value
      attribute: name of attribute
  
      Returns
      -------
      attribute value (or value if already set)