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If you know you're supporting old code, use OrderedDict. you arguably ought to anyway, for explicitness.
by hprotagonist 7y ago
If you know you're supporting old code, use OrderedDict.
you arguably ought to anyway, for explicitness.
- ouid 7y agoright, so making it implicit is bad design
- mark-r 7y agoIt's part of the zen of Python: Explicit is better than implicit.
- skedaddle 7y agoPython is nothing like its guiding principles. That's why it's Zen -- the principles are a collection of contradictory statements, given what you will encounter in real world Python. You're meant to be confused and meditate on it.
- harikb 7y agoThe trouble is I publish a (new) code that advertises itself as working on 3.x and then it turns out it is being used by a person who only had the version prior to this change. That said, Go made a similar change (from insertion-order to explicitly-randomized) and world didn’t end. So there’s that.
- ryebit 7y agoIf your code relies on a minimum python version, you can add `python_requires=">=3.5"` to your setup.py [https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires https://packaging.python.org/guides/distributing-packages-us...] to ensure it's not installed on older releases. That field itself is kinda new; but if needing to block users with older versions, that shouldn't be an issue.
- hprotagonist 7y agopersonally, i just drop a f-string in setup.py and that'll filter out any python for which this issue pertains.
- baq 7y agoThis might not work if you’re distributing a wheel.
- AdamN 7y agoPython 3.4 is EOL anyway so there's no need to do this. Anybody running 3.4 is already unsupported.
- hprotagonist 7y agoand 3.5 dies in september. hurray!
- throwaway894345 7y agoFWIW, when Go made that change, it was a much less-widely-used language (smaller blast radius).
- strken 7y agoI thought Go made the change from undefined behaviour with an underlying implementation that was insertion order in a map with 8 or fewer entries, to similarly undefined behaviour with an implementation that randomised lookups in those cases. Any code that has ever relied on any kind of ordering in Go maps will almost certainly be wrong, even random ordering, because the distribution of the "random" ordering is biased. See https://medium.com/i0exception/map-iteration-in-go-275abb76f721 https://medium.com/i0exception/map-iteration-in-go-275abb76f...
- masklinn 7y agoOrderedDict is slow and expensive though: it maintains ordering through a doubly linked list. It has useful features for manipulating ordering but while I've regularly needed had use for maintaining insertion ordering I can't remember ever needing to move items around within a map.
- hencoappel 7y agoIf memory serves me correctly, ever since dicts became ordered the OrderedDict simply became a subclass of dict, so it will have exactly the same performance characteristics.
- edflsafoiewq 7y agoYup. >>> isinstance(OrderedDict(), dict) True
- masklinn 7y agoThat was already the case in python 2.7 or 3.1.
- masklinn 7y agoOrderedDict was always a subclass of dict maintaining additional information (which is not free, it has to store and manipulate two pointers per entry). It remains so today, ordereddict is not an alias or trivial facade to the normal dict because it has to maintain its doubly linked list and implement a bunch of additional operations based on that e.g. pop first or move to end.
- yurlungur 7y agoYeah I think this is probably my main issue. I don't think it's reasonable to ask users of your code to always use 3.7+ instead of 3.6 if they are usually expected to be compatible. And it's also unnecessary to break such compatibility for something like preferring dict over OrderedDict anyways. At least I would try to avoid any such issues by still using OrderedDict. That said, I have no idea about the internals of dict. I assume no performance was sacrificed for this change.
- TheFlyingFish 7y agoIt actually improves performance. Or at least, it comes along with a set of performance improvements that give you ordering for free. Raymond Hettinger has a great talk on it: https://www.youtube.com/watch?v=npw4s1QTmPg&t=1s https://www.youtube.com/watch?v=npw4s1QTmPg&t=1s
- viraptor 7y agoUsing OrderedDict is actually nice in this case, even if the default dict has the same ordering. That way you're explicitly saying you rely on that behaviour and it makes reading the code easier.