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Python dicts are now ordered

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Re: Python dicts are now ordered

#21

Post by dfox »

I had firsthand experience with this otherwise useful feature masking an obvious bug. In one codebase there was function along the lines of (for calling into MSSQL with it's "peculiar" stored procedures):

    def call_stored_procedure(name, **args):
        cursor.call_proc(name, *args.values())
Of course this is obviously wrong and on Python 3.6 this will break horribly on first use, while in Python 3.7 this worked as long as the keyword arguments were in the correct order.

Re: Python dicts are now ordered

#22

Post by [deleted] »

setr wrote: post #20
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 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…

[deleted]

Re: Python dicts are now ordered

#23

Post by pmart123 »

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 agree, but more from the point of view on what if a new unordered implement if discovered 10 years from now that’s better? Would a new type have to be added?

Re: Python dicts are now ordered

#24

Post by doubleunplussed »

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.

3.5 is EOL soon, and it's already uncommon to write code using a new interpreter that targets an older interpreter - there's more than just ordered dicts that you'll be missing if you do so.

People will just do what they've always done - they'll be aware of the differences, or they'll test.

There's nowhere I have seen that mentions dicts are ordered without mentioning in the same paragraph the version since which this has been the case, so anyone who knows they're ordered will be aware.

Tldr its fine

Re: Python dicts are now ordered

#25

Post by oefrha »

Not sure why this is posted and upvoted to front page now. After all this is a major bullet point in py37 What’s New, and even py38 has been out for a while.

Anyway, I’ll keep using collections.OrderedDict (except for personal scripts) until py35 EOL.

Re: Python dicts are now ordered

#26

Post by biddlesby »

setr wrote: post #20
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 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

Re: Python dicts are now ordered

#27

Post by [deleted] »

pfdietz wrote: post #4
dmitriid wrote: post #3

Javascript taught me to never assume any ordering in a dict/map/hashmap (whatever the term)

Is the ordering there at least fixed, or can it vary from run to run even on the same data, or between implementations or versions of the same implementation? EDIT: to clarify, I was asking about JavaScript there.

[deleted]

Re: Python dicts are now ordered

#28

Post by _bxg1 »

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 would consider changing how a builtin works to be a major breaking change.

This one is unusual because it won't break old code being brought forward, only the other way around, and theoretically there are lots of things going the other direction that would break (though most of them are explicit, not silent).

Re: Python dicts are now ordered

#29

Post by doubleunplussed »

dfox wrote: post #21

I had firsthand experience with this otherwise useful feature masking an obvious bug. In one codebase there was function along the lines of (for calling into MSSQL with it's "peculiar" stored procedures): def call_stored_procedure(name, **args): cursor.call_proc(name, *args.values()) Of course this is obviously wrong and on Python 3.6 this will break horribly on first use, while in Python 3.7 this worked as long as t…

On 3.6 it will work, it's just relying on an implementation detail (dicts are ordered in 3.6, it's just that it is considered an implementation detail, not a guarantee)

Re: Python dicts are now ordered

#30

Post by iiodiiod »

nobleach wrote: post #9
pfdietz wrote: post #4

Earlier quoted context omitted.

Is the ordering there at least fixed, or can it vary from run to run even on the same data, or between implementations or versions of the same implementation? EDIT: to clarify, I was asking about JavaScript there.

It is deterministic, but it's an odd rule-set. For example: `var o = { 3: 'foo', 1: 'bar', b: 'baz', a: 'quux' }` Will yield: `{1: "bar", 3: "foo", b: "baz", a: "quux"}`. Numeric keys get sorted. String keys are insertion-order. If key order is a priority, a Map should be used instead. Often when JavaScript's objects are used and a particular sort order is required, an accompanying (sorted) array is used.

(deleted--missed context)
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