Property to return the default value.

        If the default value is callable and call_default is True, return
        the result of default(). Else return default.

        Returns:
            object: the default value.

def default_value(self):
        """
        Property to return the default value.

        If the default value is callable and call_default is True, return
        the result of default(). Else return default.

        Returns:
            object: the default value.
        """
        if callable(self.default) and self.call_default:
            return self.default()
        return self.default