On Sun, 22 Aug 2021, 10:47 am Guido van Rossum, <gu...@python.org> wrote:
> Hopefully anyone is still reading python-dev. > > I'm going to try to summarize the differences between the two proposals, > even though Mark already did so in his PEP. But I'd like to start by > calling out the key point of contention. > > Everything here is about locals() and f_locals in *function scope*. (I use > f_locals to refer to the f_locals field of frame objects as seen from > Python code.) And in particular, it is about what I'll call "extra > variables": the current CPython feature that you can add *new* variables to > f_locals that don't exist in the frame, for example: > > def foo(): > x = 1 > locals()["y"] = 2 # or sys._getframe()["y"] = 2 > > My first reaction was to propose to drop this feature, but I realize it's > kind of important for debuggers to be able to execute arbitrary code in > function code -- assignments to locals should affect the frame, but it > should also be possible to create new variables (e.g. temporaries). So I > agree we should keep this. > I actually tried taking this feature out in one of the PEP 558 drafts, but actually doing so breaks the pdb test suite. > > So apparently the key difference of opinion between Mark and Nick is about > f_locals, and what to do with extras. In Nick's proposal when you reference > f.f_locals twice in a row (for the same frame object f), you get the same > proxy object, whereas in Mark's proposal you get a different object each > time, but it doesn't matter, because the proxy has no state other than a > reference to the frame. > If PEP 558 is still giving that impression, I need to fix the wording - the proxy objects are ephemeral in both PEPs (the 558 text is slightly behind the implementation on that point, as the fast refs mapping is now stored on the frame object, so it only needs to be built once) In Mark's proposal, if you assign a value to an extra variable, it gets > stored in a hidden dict field on the frame, and when you read the proxy, > the contents of that hidden dict field gets included. This hidden dict > lazily created on the first store to an extra variable. (Mark shows > pseudo-code to clarify this; the hidden dict is stored as _extra_locals on > the frame.) > PEP 558 works essentially the same way, the difference is that it uses the existing locals dict storage rather than adding new storage just for optimised frames. In Nick's proposal, there's a cache on the frame that stores both the > extras and the proper variables. This cache can get out of sync with the > contents of the proper variables when some bytecode is executed (for > performance reasons we don't want the bytecode to keep the cache up to date > on every store), so there's an operation to sync the frame cache > (sync_frame_cache(), it's not defined in which namespace this exists -- is > it a builtin or in sys?). > It's an extra method on the proxy objects. You only need it if you keep an old proxy object around - if you always retrieve a new proxy object after executing Python code, that proxy will refresh the cache when it needs to. > Frankly the description in Nick's PEP is hard to follow -- I am not 100% > sure what is meant by "the dynamic snapshot", and it's not quite clear > whether proper variables are copied into the cache (and if so, why). > Aye, Mark was a bit quicker with his PEP than I anticipated, so I've incorporated the implementation improvements arising from his last round of comments, but the PEP text hasn't been updated yet. Personally, I find Mark's proposed semantics for f_locals simpler -- > there's no cache, only storage for extras, so there's nothing that can get > out of sync. > The wording in PEP 667 undersells the cost of that simplification: "Code that uses PyEval_GetLocals() will continue to operate safely, but will need to be changed to use PyEval_Locals() to restore functionality." Code that uses PyEval_GetLocals() will NOT continue to operate safely under PEP 667: all such code will raise an exception at runtime, and need to be rewritten to use a new API with different refcounting semantics. That's essentially all code that accesses the frame locals from C, since we don't offer supported APIs for that other than PyEval_GetLocals() (directly accessing the f_locals field on the frame object is only "supported" in a very loose sense of the word, although PEP 558 mostly keeps that working, too) This means the real key difference between the two PEPs is that Mark is proposing a gratuitous compatibility break for PyEval_GetLocals() that also means that the algorithmic complexity characteristics of the proxy implementation will be completely off from those of a regular dict (e.g. len(proxy) will be O(n) in the number of variables defined on the frame rather than being O(1) after the proxy's initial cache update the way it is in PEP 558) 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(). On the code complexity front, while the cache management in PEP 558 does incur a bit of extra complexity, it also offers a lot of code simplification as many mutable mapping API operations can be delegated to the cache instead of needing to be implemented directly against the fast locals array (e.g. the keys(), values() and items() views all interact with the cache rather than the underlying frame storage, so the implementation doesn't need proxy-specific types for those). For O(n) operations, the cache is refreshed every time, while for less than O(n) operations, the cache is refreshed if it is the first time that particular proxy instance has needed it. While API clients *can* delve into the details of exactly when and how the cache gets refreshed, they can also adopt the simple principle of "if in doubt, request a new locals reference" and let the interpreter worry about the details. Cheers, Nick. >
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