The return type hint for SubsegmentContextManager.__enter__ is Subsegment, but the implementation can clearly return None. The type hint should therefore be Optional[Subsegment] or equivalent.

This has hit me when running tests on X-Ray-instrumented code, when there isn't a segment to derive from. I then end up with NREs as Mypy thinks that in_subsegment should have yielded a Subsegment.