Sets default attributes when None.

        Args:
            default_attr: dict. Key-val of attr, default-value.

def _set_default_attr(self, default_attr):
        """Sets default attributes when None.

        Args:
            default_attr: dict. Key-val of attr, default-value.
        """
        for attr, val in six.iteritems(default_attr):
            if getattr(self, attr, None) is None:
                setattr(self, attr, val)