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A “frozen” dictionary for Python

(lwn.net)
191 points jwilk | 2 comments | 11 Dec 25 09:51 UTC | HN request time: 0.033s | source
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drhagen ◴[11 Dec 25 11:45 UTC] No.46230206[source]▶
Great! Now make `set` have a stable order and we're done here.
replies(1): >>46230375 #
cr125rider ◴[11 Dec 25 12:06 UTC] No.46230375[source]▶
Aren’t sets unsorted by definition? Or do repeated accesses without modification yield different results?
replies(4): >>46230852 #>>46230876 #>>46230887 #>>46231151 #
sltkr ◴[11 Dec 25 13:02 UTC] No.46230852[source]▶
So are dictionary keys, but Python decided to make them insertion ordered (after having them be unordered just like set elements for decades). There is no fundamental reason sets couldn't have a defined order. That's what languages like JavaScript have done too.
replies(1): >>46231235 #
cpburns2009 ◴[11 Dec 25 13:41 UTC] No.46231235[source]▶
Python's decision to make dict keys ordered in the spec was a mistake. It may be the best implementation so far, but it eliminates potential improvements in the future.
replies(1): >>46232034 #
1. mrweasel ◴[11 Dec 25 14:48 UTC] No.46232034[source]▶
Agreed. The only reason to make them sorted is because people would wrongly assume that they where. You can argue that a programming language should not have unexpected behaviors, and apparently unsorted dictionary keys where a surprise to many, on the other hand I feel like it's a failure of education.

The problem was that assuming that keys would be sorted was frequently true, but not guaranteed. An alternative solution would have been to randomize them more, but that would probably break a lot of old code. Sorting the keys makes no difference if you don't expect them to be, but it will now be a greater surprise if you switch language.

replies(1): >>46257787 #
2. zahlman ◴[13 Dec 25 20:38 UTC] No.46257787[source]▶
"sorted" and "ordered" mean very different things in this context.

And the reason we have ordered dict keys now is because it's trivial with the new compact structure (the actual hash table contains indices to an auxiliary array, which can just be appended to with every insertion). It has nothing to do with any randomization of the hashing process.