Python’s built-in structures are thread-safe for single operations, but it can sometimes be hard to see where a statement really becomes multiple operations.
Your code should be safe. Keep in mind: a lock here will add almost no overhead, and will give you peace of mind.
https://web.archive.org/web/20201108091210/http://effbot.org/pyfaq/what-kinds-of-global-value-mutation-are-thread-safe.htm has more details.