You might have forgotten to include Python-dev in the reply.

Indeed, adding it back!

Thank you for the reply. I might have expressed the question poorely. I meant: I have a script that I know is not thread-safe but it doesn't matter because the test itself doesn't run any threads and the current tests are never(?) run in multiple threads (-j uses processes). Should this *new* test be fixed if e.g., there is a desire to be able to run (at least some) tests in multiple threads concurrently in the future?

The short answer is: no, you don't have to make you thread thread safe, as long as it can reliably run even in the presence of background threads (like the tkinter threads Victor mentions).