I'm guessing interleaving is probably not possible because the `extend` call is passed the full list, and it holds the GIL until it's finished. But you'd have t

I'm guessing interleaving is probably not possible because the `extend` call is passed the full list, and it holds the GIL until it's finished. But you'd have to look at the bytecode to be sure, I suppose. If there were three append calls instead of a single extend, I would assume any interleaving would be valid. > or if the GIL is removed does the interpreter segfault (I'm pretty sure this latter will not be the case for GIL removal but I don't understand the gil remove plan very much yet). I haven't looked into the plan in detail either, but presumably not, that would be nuts. My understanding is that they're going to replace the GIL with locks on the objects themselves (your list `l` in this case). This is why in all the tests single-threaded performance suffer, you have to take and release more locks if you don't have a GIL, and the objects themselves grow larger as well.