On Mon, Aug 23, 2021 at 4:38 AM Mark Shannon <m...@hotpy.org> wrote: > Hi Nick, > > On 22/08/2021 4:51 am, Nick Coghlan wrote: > > > If Mark's claim that PyEval_GetLocals() could not be fixed was true then > > I would be more sympathetic to his proposal, but I know it isn't true, > > because it still works fine in the PEP 558 implementation (it even > > immediately sees changes made via proxies, and proxies see changes to > > extra variables). The only truly unfixable public API is > > PyFrame_LocalsToFast(). > > You are making claims that seem inconsistent with each other. > Namely, you are claiming that: > > 1. That the result of locals() is ephemeral. > 2. That PyEval_GetLocals() returns a borrowed reference. > > This seems impossible, as you can't return a borrowed reference to > an emphemeral object. That's just a pointer to freed memory. > > Do `locals()` and `PyEval_GetLocals()` behave differently? >
That is my understanding, yes. in PEP 558 locals() returns a snapshot dict, the Python-level f_locals property returns a fresh proxy that has no state except a pointer to the frame, and PyEval_GetLocals() returns a borrowed reference to the dict that's stored on the frame's C-level f_locals attribute. (In my "crazy" proposal all that is the same.) > Is the result of `PyEval_GetLocals()` cached, but `locals()` not? > I wouldn't call it a cache -- deleting it would affect the semantics, not just the performance. But yes, it returns a reference to an object that is owned by the frame, just as it does in 3.10 and before. > If that were the case, then it is a bit confusing, but could work. > Yes, see my "crazy" proposal. > Would PyEval_GetLocals() be defined as something like this? > > (add _locals_cache attribute to the frame which is initialized to NULL). > > def PyEval_GetLocals(): > frame._locals_cache attribute = locals() > return borrow(frame._locals_cache attribute) > Nah, the dict returned by PyEval_GetLocals() is stored in the frame's C-level f_locals attribute, which is consulted by the Python-level f_locals proxy -- primarily to store "extra" variables, but IIUC in Nick's latest version it is also still used to cache by that proxy. Nick's locals() just returns dict(sys._getframe().f_locals). > None of this is clear (at least not to me) from PEP 558. > One problem with PEP 558 is that it's got too many words, and it's lacking a section that crisply describes the semantics of the proposed implementation. I've suggested to Nick that he add a section with pseudo-code for the implementation, like you did in yours. (PS, did you read my PS about what locals() should do in class scope when __prepare__ returns a non-dict?) -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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