The easiest way to see the carnage in action is to run it under gdb and break on PyInit__multiarray_umath. You'll see it gets called twice, the second time reentrantly (with a 276-frame backtrace ...

HMR means Hot Module Reload / Hot Module Replacement. It is a feature that allows part of your app to be updated at runtime without a full rerun. Imagine you're developing an ML service using FastAPI ...