code: def set_defaults(self, config, fields_spec):
          
          defaults = dict([(f, d)
                           for f, d in fields_spec.items()
                           if not isinstance(d, type)])
          for field, default_value in defaults.items():
              if field not in config:
                  config[field] = default_value
docs: Set default values got from `fields_spec` into the `config`
          dictionary