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by zbentley
622 days ago
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> It’s doing hash table lookups per variable access. That hasn't been true for many variable accesses for a very long time. LOAD_FAST, LOAD_CONST, and (sometimes) LOAD_DEREF provide references to variables via pointer offset + chasing, often with caches in front to reduce struct instantiations as well. No hashing is performed. Those access mechanisms account for the vast majority (in my experience; feel free to check by "dis"ing code yourself) of Python code that isn't using locals()/globals()/eval()/exec() tricks. The remaining small minority I've seen is doing weird rebinding/shadowing stuff with e.g. closures and prebound exception captures. https://github.com/python/cpython/blob/10094a533a947b72d01ed... https://github.com/python/cpython/blob/10094a533a947b72d01ed... So too for object field accesses; slotted classes significantly improve field lookup cost, though unlike LOAD_FAST users have to explicitly opt into slotting. Don't get me wrong, there are some pretty regrettably ordinary behaviors that Python makes much slower than they need to be (per-binding method refcounting comes to mind, though I hear that's going to be improved). But the old saw of "everything is a dict in python, even variable lookups use hashing!" has been incorrect for years. |
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