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

Maciej Fijalkowski fijall at gmail.com
Tue Mar 18 20:46:08 CET 2014

On Tue, Mar 18, 2014 at 4:21 PM, Steven D'Aprano <steve at pearwood.info> wrote:

On Tue, Mar 18, 2014 at 01:21:05PM +0200, Maciej Fijalkowski wrote:

note that this is specifically about dicts, where eq will be called undecided number of times anyway (depending on collisions in hash/buckets which is implementation specific to start with) Exactly. Using a eq method with side-effects is a good way to find out how many collisions your dict has :-) But specifically with your example, if x in d: return d[x] my sense of this is that it falls into the same conceptual area as the identity optimization for checking list or set containment: slightly unclean, but justified. Provided d is an actual built-in dict, and it hasn't been modified between one call and the next, I think it would be okay to optimize the second lookup d[x]. A question: how far away will this optimization apply? if x in d: dothis() dothat() dosomethingelse() spam = d[x]

it depends what those functions do. JIT will inline them and if they're small, it should work (although a modification of a different dict is illegal, since aliasing is not proven), but at some point it'll give up (Note that it'll also give up with a call to C releasing GIL since some other thread can modify it).

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