code: def check_config(self, config, fields_spec):
          
          for field, type_info in fields_spec.items():
              has_default = not isinstance(type_info, type)
              if field not in config and not has_default:
                  raise RuntimeError(
                      "Configuration not complete. %s missing" % field)
docs: Check that `config` has each field in `fields_spec` if a default
          has not been provided.