Sorry, I definitely didn't mean to imply that this kind of optimization is valid on arbitrary subscript expressions; I thought we had restricted ourselves to talking about builtin dicts. If we do, I think this becomes a discussion about what subset of the semantics of CPython's builtins are language-specified vs implementation-dependent; my argument is that just because something results in an observable behavioral difference doesn't necessarily mean that it's a change in language semantics, if it's just a change in the implementation-dependent behavior.
On Tue, Mar 18, 2014 at 9:54 PM, Stephen J. Turnbull <step...@xemacs.org>wrote: > Kevin Modzelewski writes: > > > I think in this case, though, if we say for the sake of argument > > that the guaranteed semantics of a dictionary lookup are zero or > > I don't understand the point of that argument. It's simply false that > semantics are guaranteed, and all of the dunders might be user > functions. > > > more calls to __hash__ plus zero or more calls to __eq__, then two > > back-to-back dictionary lookups wouldn't have any observable > > differences from doing only one, unless you start to make > > assumptions about the behavior of the implementation. > > That's false. The inverse is true: you should allow the possibility of > observable differences, unless you make assumptions about the behavior > (implying there are none). > > > To me there seems to be a bit of a gap between seeing a dictionary > > lookup and knowing the exact sequence of user-functions that get > > called, far more than for example something like "a < b". > > The point here is that we *know* that there may be a user function > (the dunder that implements []) being called, and it is very hard to > determine that that function is pure. > > Your example of a caching hash is exactly the kind of impure function > that one would expect, but who knows what might be called -- there > could be a reference to a database on Mars involved (do we have a > vehicle on Mars at the moment? anyway...), which calls a pile of > Twisted code, and has latencies of many seconds. > > So Steven is precisely right -- in order to allow this optimization, > it would have to be explicitly allowed. > > Like Steven, I have no strong feeling against it, but then, I don't > have a program talking to a deep space vehicle in my near future. > Darn it! :-( > >
_______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
