Code:
Key: An additional identification that is not shared with any other peers. It is intended to allow a client to prove their identity should their IP address change.
Working: Keys are generated randomly by torrent clients. They are generated to different torrents you download or upload. Some clients use the same key for all the torrent while others use the same.
I cannot confirm whether trackers use it to track users. Maybe someone can tell you about this.
Don't change it randomly. Load couple of torrents with your torrent client and study their announces to the trackers. See whether it changes for different torrent. If it does your on the green light to change it with your cheating software (python one). However, if it remains the same, then you have to use the same one for all the torrents you plan to cheat on.