On Wed, Mar 19, 2014 at 8:38 AM, Kevin Modzelewski <k...@dropbox.com> wrote: > 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/fijall%40gmail.com >
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