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Python dicts are now ordered

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Re: Python dicts are now ordered

#151

Post by gnulinux »

laGrenouille wrote: post #132
quietbritishjim wrote: post #97

Earlier quoted context omitted.

One particular annoyance of OrderedDict objects was initialising them with literals. You couldn't just do: od = OrderedDict({ "y": "first", "x": "second", }) because, by the time the data got to OrderedDict's constructor, it had already been through a dict. Instead you had to supply a list of lists (or tuple of tuples etc.): od = OrderedDict([ ("y", "first"), ("x", "second"), ]) For hierarchically nested dictionaries…

But you can also just create them with named arguments, like this: collections.OrderedDict(y = "first", x = "second")

This also wasn't guaranteed to work since kwargs is a dict under the hood.

Re: Python dicts are now ordered

#152

Post by dagmx »

nkozyra wrote: post #141
OJFord wrote: post #125

Earlier quoted context omitted.

It's not breaking, it was implementation defined, now it's guaranteed order. If a new guarantee or behaviour were a breaking change, every non-patch release of a semversioned tool (which python isn't) would be major.

> It's not breaking And that's sort of the problem. If it was broken, you'd know it and you'd fix/rearchitect it. Instead, it will appear to work.

Sort of. You'd only know based on the interpreter you were using. You wouldn't know unless you tested other Python interpreters.

For example, if you wrote something in pypy it would be ordered versus cpython.

Re: Python dicts are now ordered

#153

Post by frosted-flakes »

mumblemumble wrote: post #32
jkbbwr wrote: post #15

Am I the only one that thinks this is a stupid decision? This will silently break code that starts to rely on this behaviour that gets executed on Python3.5 and lower. I would consider changing how a builtin works to be a major breaking change. It would have been fine if this was a change between 2 and 3 but on a minor version? Thats insane.

I'm of two minds. In principle, no, I don't love the idea of a change like this happening on a minor version. But, from a pragmatic angle, it strikes me as a genius move. As the article said, this was a natural by-product of a performance enhancement that was made in 3.6. In principle, that was fine, because there was officially no predictable ordering, so a change to how it was being ordered in practice shouldn't ma…

Python (the language) doesn't follow semver anyway.

Re: Python dicts are now ordered

#154

Post by alayne »

6gvONxR4sf7o wrote: post #98

I'd love to know more about where an ordered dict comes in handy. Anyone have use cases? Otherwise, guaranteeing this behavior just seems ‾\_(ツ)_/‾ If this is useful, I wonder if an ordered set is useful.

I use OrderedDict in code that generates JSON records so the key orders are preserved to help human readability. LRU caches are another use case.

Re: Python dicts are now ordered

#155

Post by rmdashrfstar »

gnulinux wrote: post #142
jakear wrote: post #82

Earlier quoted context omitted.

Usually languages don’t have it in a very explicit way though. If I try to use f-strings in python 3.5 I get a very explicit syntax error. If I rely on ordered insertions in Python3.5 I get a potentially difficult to diagnose bug.

Why does it matter if it's explicit or not. If python doesn't support forward compatibility, you should know that if you write code for 3.7, it's not gonna work in 3.5. Doesn't seem like a big deal to me.

Major version changes are for API compatibility. If there is a change which makes all other 3.* incompatible, then it should be a major version increase.

Re: Python dicts are now ordered

#156

Post by toyg »

jkbbwr wrote: post #15

Am I the only one that thinks this is a stupid decision? This will silently break code that starts to rely on this behaviour that gets executed on Python3.5 and lower. I would consider changing how a builtin works to be a major breaking change. It would have been fine if this was a change between 2 and 3 but on a minor version? Thats insane.

If you rely on this feature, surely you know it’s new to 3.7 and document your project accordingly...? It’s not like code magically appears, somebody has to write it.

Re: Python dicts are now ordered

#157

Post by alayne »

opportune wrote: post #133
6gvONxR4sf7o wrote: post #98

I'd love to know more about where an ordered dict comes in handy. Anyone have use cases? Otherwise, guaranteeing this behavior just seems ‾\_(ツ)_/‾ If this is useful, I wonder if an ordered set is useful.

Imagine a trie implemented as a tree of dictionaries (ignore thinking about whether a tree of arrays may actually be better for now). Let’s say you want to implement an autocomplete algorithm based on a dictionary of words from a corpus. The autocomplete algorithm is naive: if a user types in “abc” you recommend whichever word starting in “abc” has the most occurrences in the corpus. With ordered dicts you can use th…

This isn't a sorted dictionary, it's an insertion order dictionary.

Re: Python dicts are now ordered

#158

Post by bbrik »

_aleph2c_ wrote: post #144

This is an amazing contribution to the language. A mixture of speed and convenience, probably made by volunteers. As for people criticizing a change to what use to be a non-deterministic ordering of a dict iteration; I don't know what to say to them, other than, are you serious? There are people out there who are working for us, they work for free and they did some heavy lifting to give us this. They might read what…

As far as I know, it was implemented by Raymond Hettinger. This is a very interesting talk about the new tech:

https://www.youtube.com/watch?v=p33CVV29OG8&t=1202s

Re: Python dicts are now ordered

#159

Post by wutbrodo »

_aleph2c_ wrote: post #144

This is an amazing contribution to the language. A mixture of speed and convenience, probably made by volunteers. As for people criticizing a change to what use to be a non-deterministic ordering of a dict iteration; I don't know what to say to them, other than, are you serious? There are people out there who are working for us, they work for free and they did some heavy lifting to give us this. They might read what…

> As for people criticizing a change to what use to be a non-deterministic ordering of a dict iteration; I don't know what to say to them, other than, are you serious? There are people out there who are working for us, they work for free and they did some heavy lifting to give us this. They might read what you wrote and think, "Why bother? Maybe I should spend my weekends playing with my kids instead."

I don't agree with the complaints about ordering ducts, but this is a ludicrous response. Appreciation of volunteers' work on Python isn't diminished by criticism of language features or changes. In fact, it enhances it: if people have opinions about the project you work on, it's a good sign that it's important, significant work. I've contributed to OSS projects before, and the ones in use by more than just my friends and I were naturally the ones that felt the most meaningful.

Re: Python dicts are now ordered

#160

Post by throwaway894345 »

harikb wrote: post #61
hprotagonist wrote: post #47

Earlier quoted context omitted.

If you know you're supporting old code, use OrderedDict. you arguably ought to anyway, for explicitness.

The 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.

FWIW, when Go made that change, it was a much less-widely-used language (smaller blast radius).
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