Compile time. There are preprocessor #if guards all over the code base to provide different implementations for core operations. Many of these are used to provide a thread-safe version (e.g. atomic refcount). Presumably, these should work fine single threaded (assuming correctness). But at the moment they are compile-time, yea.
> With these recommendations and the acceptance of this PEP, we as the Python developer community should broadly advertise that free-threading is a supported Python build option now and into the future, and that it will not be removed without a proper deprecation schedule.
It looks like it still needs to be enabled at compile-time.
> Any decision to transition to phase III, with free-threading as the default or sole build of Python is still undecided, and dependent on many factors both within CPython itself and the community. This decision is for the future.
