def METHOD_NAME(self, config, fields_spec):
    """
    Set default values got from `fields_spec` into the `config`
    dictionary
    """
    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