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0 pointssimonw3y ago0 comments

I used to have the exact same problem with Python, until Python 3.7 made maintaining sort order a feature of the language: https://softwaremaniacs.org/blog/2020/02/05/dicts-ordered/

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c2h5oh3y ago

Go actually went in the other direction for a bunch of reasons (e.g. hash collision dos) and made key order quasi-random when iterating. Small maps used to maintain order, but a change was made to randomize that so people didn't rely on that and get stung when their maps got larger: https://github.com/golang/go/issues/6719

tialaramex3y ago

Right, the startling thing about Python's previous dict was that it was so terrible that the ordered dict was actually significantly faster.

It's like if you did such a bad job making a drag racer that the street legal model of the same car was substantially faster over a quarter mile despite also having much better handling and reliability.

In some communities the reaction would have been to write a good unordered dict which would obviously be even faster, but since nobody is exactly looking for the best possible performance from Python, they decided that ordered behaviour was worth the price, and it's not as though existing Python programmers could complain since it was faster than what they'd been tolerating previously.

Randomizing is the other choice if you actually want your maps to be fast and want to resist Hyrum's law, but see the absl experience - they initially didn't bother to randomize tiny maps but then the order of those tiny maps changed for technical reasons and... stuff broke. Because hey, in testing I made six of this tiny map, they always had the same order therefore (ignoring the documentation imploring me not to) I shall assume the order is always the same...

chippiewill3y ago

> In some communities the reaction would have been to write a good unordered dict which would obviously be even faster

Actually an ordered dictionary has improved performance over an unordered dictionary for the kinds of common Python workloads you encounter in the real world. The reason why is that the design is only incidentally ordered, the design arises from trying to improve memory efficiency and iteration speed. The dict ends up ordered because they stash the real k/v pairs in a regular array which is indexed by the hash table, populating the array is most efficient in insertion order. For pure "unordered map" type operations the newer implementation is actually a tiny bit slower.

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alecthomas3y ago

> Right, the startling thing about Python's previous dict was that it was so terrible that the ordered dict was actually significantly faster.

I've never heard that before and it would be really surprising, given that Python's builtin dict is used for everything from local symbol to object field lookup. Do you have more information?

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Groxx3y ago

`select{..}` cases with multiple valid channel operations also select randomly.

I really like it, it helps you discover (and fix) order-dependent logic WAY earlier. Though I would really like some way to influence how long it blocks before selecting one (to simulate high load scenarios, and trigger more logical races).

gabereiser3y ago

you'll need an interrupt chan for that if you're in a select{..}

hoppla3y ago

This burnt me when I wrote an algorithm. I depended on the order of keys in dicts as it allowed me reference the value both by index and key.

I wrote the code in python 3.7+, and ended up spending a good amount of time debugging it when I ran it in a earlier python version.

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