[Python-Dev] Intricacies of calling eq (original) (raw)

Maciej Fijalkowski fijall at gmail.com
Wed Mar 19 17:27:29 CET 2014

On Wed, Mar 19, 2014 at 8:38 AM, Kevin Modzelewski <kmod at 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 <stephen at 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! :-(

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we're discussing builtin dicts

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