> Isn't list(dict) atomic? At least in CPython.

I doubt it. There's no special code for list(dict), it uses regular iteration which could probably hit the GC and then release the GIL.

> I suspect that protecting *all* accesses to _data with a lock will hit the performance and invalidate the reason of using _data for caching.

I'm not convinced os.environ is performance-critical.  Perhaps getenv() is really expensive on some systems?