def stringPropertyNames(self):
        r"""
        Returns a `set` of all keys in the `Properties` object and its
        `defaults` (and its `defaults`\ ’s `defaults`, etc.)

        :rtype: `set` of text strings
        """
        names = set(self.data)
        if self.defaults is not None:
            names.update(self.defaults.stringPropertyNames())
        return names