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Python dicts are now ordered

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Re: Python dicts are now ordered

#91

Post by abetlen »

jhrmnn wrote: post #57
biddlesby wrote: post #26

Earlier quoted context omitted.

It doesn’t break existing code. Code written for Python 3.7 might break on older versions of Python

What you describe is forward compatibility, and Python (and most other programming language) doesn't have it.

TIL

I suppose Python doesn't even have backwards compatibility within the same major release as we saw with the addition of the async keyword in Python3.5. Many older Python 3 packages broke because they expected that to be a legal identifier for a variable name.

Re: Python dicts are now ordered

#93

Post by zackmorris »

I'm glad to see other languages finally catching up to PHP. I'm joking (kind of) but after a lot of years of doing this, I've begun de-prioritizing pure abstractions and favoring the way that humans tend to do things on their own.

Technically this is along the lines of the worse-is-better philosophy. The single biggest cost in software development is friction. Performance, size, etc are all less important, because they become less important with each passing year as computers grow more powerful. And along those lines, I think that silly concepts like assigning letter names to drives in Windows, or having a paltry few registers in x86, made those architectures more accessible to the masses than the more generalized Unix and Mac platforms. Not that I completely agree with that, just, it's all I can come up with (other than cost and familiarity) for why people are so attached to worse-is-better paradigms.

Preserving insertion order has some small cost associated with it, but that's going to be dwarfed by the cost of bugs introduced when junior devs are surprised by unordered dictionaries. And the pedantic people can just use an unordered dictionary manually since they are already converting pure abstractions to whatever imperfect implementations are provided by languages anyway.

Re: Python dicts are now ordered

#94

Post by mark-r »

Sean1708 wrote: post #40
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.

My main issue with it is that if someone figures out an even better way to do dictionaries but it doesn't preserve insertion order, then that technique can't be used. But I certainly wouldn't call it a stupid decision, the problem is that once a behaviour is observable then people absolutely will start relying on it.

All they have to do is give it a different name, such as unordered_dict.

Re: Python dicts are now ordered

#95

Post by andybak »

biddlesby wrote: post #26
setr wrote: post #20

Earlier quoted context omitted.

I'm not clear on how this break existing code. Code that assumed it was arbitrary, would expect to handle any arbitrary order, including a happens-to-be sorted order. Code that assumed it was random, like actually inserted by random(), was already broken, because that simply isn't the case. Code that assumed the order would stay constant was relying on implementation-specific behavior, and could potentially break on…

It doesn’t break existing code. Code written for Python 3.7 might break on older versions of Python

What versions of Python didn't have this behaviour? It was there but just not guaranteed.

I don't see how anything could break (unless there are alternative implementations with different behaviour I'm not aware of?)

Re: Python dicts are now ordered

#97

Post by quietbritishjim »

paulhodge wrote: post #60

Great decision IMO. I remember maintaining a bunch of Python code that we supported on both OSX and Windows. Out of all the platform-specific bugs we had (where it worked on one OS and not the other), one of the most common causes was code that relied on a certain key order. And we knew that relying on key order was bad, we're supposed to use things like OrderedDict, blah blah blah. It was still a really easy mistake…

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, these are quite a bit harder to read.

Re: Python dicts are now ordered

#100

Post by harryf »

zackmorris wrote: post #93

I'm glad to see other languages finally catching up to PHP. I'm joking (kind of) but after a lot of years of doing this, I've begun de-prioritizing pure abstractions and favoring the way that humans tend to do things on their own. Technically this is along the lines of the worse-is-better philosophy. The single biggest cost in software development is friction. Performance, size, etc are all less important, because th…

> I'm glad to see other languages finally catching up to PHP

Now if python could just implement PHP's date function the world would be a simpler place... ( https://simonwillison.net/2003/Oct/7/dateInPython/ )

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