[issue30550] Document order-preserving dictionary output in json

[Raymond Hettinger]

Change by Raymond Hettinger :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue30550] Document order-preserving dictionary output in json

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset 4a40498ea96a3c606952712c7951b2ea4ab258e4 by Raymond Hettinger 
(Miss Islington (bot)) in branch '3.8':
bpo-30550: Clarify JSON ordering guarantees (GH-15397) (GH-15403)
https://github.com/python/cpython/commit/4a40498ea96a3c606952712c7951b2ea4ab258e4


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[issue30550] Document order-preserving dictionary output in json

[miss-islington]

Change by miss-islington :


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pull_requests: +15110
pull_request: https://github.com/python/cpython/pull/15403

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[issue30550] Document order-preserving dictionary output in json

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset 657008ea0336ff4f275ed3f0c2b6dd2e52de2bba by Raymond Hettinger in 
branch 'master':
bpo-30550: Clarify JSON ordering guarantees (GH-15397)
https://github.com/python/cpython/commit/657008ea0336ff4f275ed3f0c2b6dd2e52de2bba


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[issue30550] Document order-preserving dictionary output in json

[Raymond Hettinger]

Change by Raymond Hettinger :


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keywords: +patch
pull_requests: +15105
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/15397

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[issue30550] Document order-preserving dictionary output in json

[Kyle Stanley]

Kyle Stanley  added the comment:

> Please keep focused on the OP's thought that the JSON docs should explicitly 
> promised that
> key/value pairs are added in the order that they are encountered. The 
> existing JSON docs imply
> this pretty strongly, but it can be made more explicit.

I agree that the behavior could be more explicitly stated in the docs, as that 
could be potentially useful information for users to be aware of. 

Apologies for derailing the topic, I became too focused on the OrderedDict vs 
Dict topic, which was only a small part of the issue. Any further discussion on 
that topic or updating the OrderedDict docs should probably be moved into it's 
own issue, if at all.

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[issue30550] Document order-preserving dictionary output in json

[Raymond Hettinger]

Change by Raymond Hettinger :


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priority: normal -> low
versions:  -Python 3.7

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[issue30550] Document order-preserving dictionary output in json

[Raymond Hettinger]

Raymond Hettinger  added the comment:

Kyle, the link you gave already discusses performance to the extent that it 
matters.  Please keep focused on the OP's thought that the JSON docs should 
explicitly promised that key/value pairs are added in the order that they are 
encountered.  The existing JSON docs imply this pretty strongly, but it can be 
made more explicit.

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[issue30550] Document order-preserving dictionary output in json

[Raymond Hettinger]

Change by Raymond Hettinger :


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assignee: docs@python -> rhettinger

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[issue30550] Document order-preserving dictionary output in json

[Kyle Stanley]

Kyle Stanley  added the comment:

> OrderedDict is not recommended to preserve order any more.

> Please forget about OrderedDict unless you need additional feature 
> OrderedDict provides.  It is far inefficient than regular dict.

Thanks for the clarification. Should this recommendation be briefly mentioned 
in the documentation 
(https://docs.python.org/3.9/library/collections.html#ordereddict-objects)? 

Currently, the docs reference that as of 3.7, the order-preserving behavior of 
standard dictionaries is guaranteed, but there is no mention of the significant 
differences in efficiency. Based on the current description, a user might end 
up using OrderedDictionary on the basis that they *might* have a need for 
reordering elements. 

If the difference in performance is significant enough, it would be worth 
noting in the docs so that users can take it into consideration when deciding 
which data structure to use.

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[issue30550] Document order-preserving dictionary output in json

[Eric O. LEBIGOT]

Eric O. LEBIGOT  added the comment:

The essence of the original post is simply to document any order-preserving 
properties of the Python to JSON and JSON to Python transformation for 
mappings. This way users can rely on it. This is for instance useful if a 
program modifies JSON created by a user and wants to preserve the user ordering.

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[issue30550] Document order-preserving dictionary output in json

[Inada Naoki]

Inada Naoki  added the comment:

OrderedDict is not recommended to preserve order any more.

Note that mention about `object_pairs_hook=OrderedDict` is removed 
intentionally.
https://github.com/python/cpython/pull/5001

Please forget about OrderedDict unless you need additional feature OrderedDict 
provides.  It is far inefficient than regular dict.

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[issue30550] Document order-preserving dictionary output in json

[Kyle Stanley]

Kyle Stanley  added the comment:

>Thus, if a user gives an OrderedDict (or a dict with a known order, in Python 
>3.7+), it is useful that >he know that the order of its elements will not be 
>changed upon transformation into JSON.

I would agree that it could be helpful to document that if a user passes an 
OrderedDict, the order of the elements will not changed, I was just pointing 
out that a default dictionary implementation would not normally retain the 
order. 

However, after looking through the 3.7 changes and reading the discussions, it 
looks like dictionaries retaining insertion order was decided to be made an 
official part of the language. If the order matters though, it would make sense 
to recommend users to use OrderedDict. The order in the default dictionary 
implementation can end up becoming sorted if pprint is used.

Discussions: 
https://mail.python.org/pipermail/python-dev/2017-December/151283.html and 
https://mail.python.org/pipermail/python-dev/2017-December/151376.html

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[issue30550] Document order-preserving dictionary output in json

[Eric O. LEBIGOT]

Eric O. LEBIGOT  added the comment:

Kyle, what you're saying is correct but is unfortunately unrelated to any of 
the points in the original issue. In fact, the JSON encoder does preserve 
whatever order the dictionary elements are in, and it would be useful to 
document this. Thus, if a user gives an OrderedDict (or a dict with a known 
order, in Python 3.7+), it is useful that he know that the order of its 
elements will not be changed upon transformation into JSON.

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[issue30550] Document order-preserving dictionary output in json

[Kyle Stanley]

Kyle Stanley  added the comment:

> The JSON encoder for dictionaries preserves the order of the items in a 
> dictionary

>From what I can tell, the JSON encoder does nothing to preserve the order of 
>the keys. Although the default option is to not modify the existing 
>dictionary, the items() method returns the k,v pairs in a non-random arbitrary 
>manner, which may not match the order the values were entered in. See 
>https://docs.python.org/2/library/stdtypes.html#dict.items.

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[issue30550] Document order-preserving dictionary output in json

[Josh Rosenberg]

Josh Rosenberg  added the comment:

Is there a reason to document object_pairs_hook=OrderedDict rather than just 
making the decoder populate a regular dict in an order-preserving way? (No idea 
if it does this already)

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[issue30550] Document order-preserving dictionary output in json

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +rhettinger
versions: +Python 3.7, Python 3.8, Python 3.9 -Python 3.6

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[issue30550] Document order-preserving dictionary output in json

[Dima Tisnek]

Change by Dima Tisnek :


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nosy: +Dima.Tisnek

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[issue30550] Document order-preserving dictionary output in json

[Eric O. LEBIGOT]

Changes by Eric O. LEBIGOT :


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assignee:  -> docs@python
components: +Documentation -Library (Lib)
nosy: +docs@python

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[issue30550] Document order-preserving dictionary output in json

[Eric O. LEBIGOT]

Changes by Eric O. LEBIGOT :


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title: Document order-preserving dictionary output in son -> Document 
order-preserving dictionary output in json

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