Return a list of field names matching a field class

        :param field_class: field class object
        :return: list

def get_fields_by_class(cls, field_class):
        """ Return a list of field names matching a field class

        :param field_class: field class object
        :return: list
        """

        ret = []

        for key, val in getattr(cls, '_fields').items():
            if isinstance(val, field_class):
                ret.append(key)
        return ret