[needusername]

> Currently Python only allows certain thread interleavings, because those are the points where the GIL is released.

How is this currently avoided? My understanding is that anything that accepts a callback / lambda is potentially subject to interleavings as the code can call a C extension which releases the GIL. Moving code to C extensions is often recommended when Python performance is brought up. Am I misunderstanding something?

> It's also unclear how tractable it is to retain current garbage collection semantics without a GIL

Why is it important to retain current garbage collection semantics? Why does Python prescribe a certain garbage collection implementation and does not allow for others like Java?