def __call__(self, selection):
        """Apply this read preference to Selection."""
        if selection.primary:
            return selection.primary_selection
        else:
            return secondary_with_tags_server_selector(
                self.tag_sets,
                max_staleness_selectors.select(
                    self.max_staleness, selection))
Apply this read preference to Selection.
stackoverflow/venv/lib/python3.6/site-packages/pymongo/read_preferences.py