[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Irit Katriel  added the comment:

Don't discuss on a closed issue. Create a new one if there is still a problem.

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[issue34798] pprint ignores the compact parameter for dicts

[Matt Bogosian]

Matt Bogosian  added the comment:

Please consider highlighting that dicts are not included in the documentation. 
While *technically* true, this ...

> compact impacts the way that long sequences (lists, tuples, sets, etc) are 
> formatted. If compact is false (the default) then each item of a sequence 
> will be formatted on a separate line. If compact is true, as many items as 
> will fit within the width will be formatted on each output line.

... has several problems.

First, sequence is a term of art that also has a common meaning. This creates a 
potential ambiguity in the understanding of the reader. Resolving that 
ambiguity in this context requires that readers have already internalized that 
dicts are not Python sequences. Those new to Python may not understand the (to 
them, subtle) differences between Python's iterables and sequences. Second, the 
"etc" only expands that ambiguity and invites confusion. Third, the term 
"items" is strongly associated with dicts and is used several times throughout 
the paragraph.

This ...

> According to https://docs.python.org/3/library/pprint.html compact impacts 
> the way that sequences are displayed, and a dict is not a sequence.

... is unhelpfully pedantic, and ignorant of the needs of the newcomer (a key 
demographic of documentation). Documentation is a core product surface with a 
diverse audience. Rather than focus on technical correctness, documentation 
authors should focus on accessibility. Redundancy is a feature, not a bug. You 
can't predict how your reader got to that slice of the documentation. Imagine 
this as an opportunity to educate or reinforce concepts for readers, not as an 
opportunity to argue from a position of technical superiority.

The fact that there are now four developers who have taken their time to file 
patches, bugs, and comments is pretty strong signal that confusion exists among 
the audience and that the documentation is insufficient.

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[issue34798] pprint ignores the compact parameter for dicts

[Ian]

Ian  added the comment:

I came across this and was confused by it too. I also don't understand the 
justification with not having dicts to be affected by the `compact` parameter.

If the "compact form" is having separate entries or elements on one line, 
instead of having each element separated by a new line, then it seems like 
inconsistent behavior.

**If a dict is short enough, it will appear in "compact form", just like a 
list.**

If a dict is too long for the width, then each item will appear in "expanded 
form", also like a list.
However, the actual compact parameter only affects sequence items. Why is this?

There is no reason given in #19132. It does mention a review, but it doesn't 
seem to be available, or I don't know how to get to it, to understand the 
reason for that decision.

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[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Change by Irit Katriel :


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versions:  -Python 3.9

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[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Change by Irit Katriel :


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

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[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Irit Katriel  added the comment:


New changeset 42c26282a123d14591e1aa31107e566b302a19ac by Miss Islington (bot) 
in branch '3.10':
bpo-34798: [doc] clearer presentation of pprint.PrettyPrinter constru… 
(GH-26967) (GH-26990)
https://github.com/python/cpython/commit/42c26282a123d14591e1aa31107e566b302a19ac


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[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Irit Katriel  added the comment:


New changeset 943e77d42d3f84b581f32c05f1fc8c05366b8ed3 by Irit Katriel in 
branch 'main':
bpo-34798: [doc] clearer presentation of pprint.PrettyPrinter constru… 
(GH-26967)
https://github.com/python/cpython/commit/943e77d42d3f84b581f32c05f1fc8c05366b8ed3


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[issue34798] pprint ignores the compact parameter for dicts

[miss-islington]

Change by miss-islington :


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nosy: +miss-islington
nosy_count: 5.0 -> 6.0
pull_requests: +25552
pull_request: https://github.com/python/cpython/pull/26990

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[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Irit Katriel  added the comment:

I made PR26967 to break up the paragraph about the constructor params so that 
it's easier to read, and also added emphasis around the "sequence" point.

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versions: +Python 3.10, Python 3.11, Python 3.9 -Python 3.8

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[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Change by Irit Katriel :


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keywords: +patch
pull_requests: +25531
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/26967

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[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Change by Irit Katriel :


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assignee:  -> docs@python
components: +Documentation
nosy: +docs@python

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[issue34798] pprint ignores the compact parameter for dicts

[Irit Katriel]

Irit Katriel  added the comment:

> At the very least it should be made clear in the documentation that dicts are 
> not compacted.

According to https://docs.python.org/3/library/pprint.html compact impacts the 
way that sequences are displayed, and a dict is not a sequence.
So I'm not sure a documentation change is required.

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[issue34798] pprint ignores the compact parameter for dicts

[Nicolas Hug]

Nicolas Hug  added the comment:

Thanks for the link,

But I don't see any justification for this behavior*? Why should lists be 
compacted but not dicts (even when explicitly asked)?

At the very least it should be made clear in the documentation that dicts are 
not compacted.

* Maybe there's more content in the patch reviews but I am unable to see them

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[issue34798] pprint ignores the compact parameter for dicts

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

Thanks much Serhiy for the details!

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[issue34798] pprint ignores the compact parameter for dicts

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

It is on purpose. See issue19132.

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[issue34798] pprint ignores the compact parameter for dicts

[Nicolas Hug]

Nicolas Hug  added the comment:

Sorry:

[2] 
https://github.com/scikit-learn/scikit-learn/pull/11705/files#diff-f83e8d9362766b385472f1be7fed9482R96

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[issue34798] pprint ignores the compact parameter for dicts

[Nicolas Hug]

Nicolas Hug  added the comment:

Thank you for the feedback!

I'll try the python-ideas mail list. I posted a message on Python-list [1] a 
few weeks ago but it didn't get much traction.

I'm not sure about what the final solution could be (if any), but I had to hack 
pprint myself [2] for the scikit-learn project, and what I did was simply copy 
pasting _format_items into _format_dict_items, with some minimal changes.


[1] https://mail.python.org/pipermail/python-list/2018-September/737082.html
[2]

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[issue34798] pprint ignores the compact parameter for dicts

[Nicolas Hug]

Change by Nicolas Hug :


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versions: +Python 3.8 -Python 3.7

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[issue34798] pprint ignores the compact parameter for dicts

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

Thanks for the report Nicolas. I looked into the code and it seems that pprint 
for a dictionary now doesn't really take compact into account. List, sets, 
tuple and dequeue use _format_items [0] that honors value of compact but 
dictionary formatting uses _format_dict_items [1] that doesn't without using 
compact value. Unfortunately, I don't have any links over why dictionary 
doesn't take compact into account. I would suggest python-ideas [3] to get more 
feedback about formatting and implementation along with backwards compatibility 
so that you can proceed further. Your suggestion seems reasonable to me but I 
don't know if some program is using compact=True for a dictionary without 
knowing the internals in mind that might break for them. I would wait for 
others thoughts on this and I think this can be done only on 3.8 and not 3.7 
which is in bug fix mode.

[0] 
https://github.com/python/cpython/blob/fdcb5ae25c0b5c82a32955617d253810ef110cac/Lib/pprint.py#L350
[1] 
https://github.com/python/cpython/blob/fdcb5ae25c0b5c82a32955617d253810ef110cac/Lib/pprint.py#L333
[2] https://mail.python.org/pipermail/python-ideas/

Hope this helps!

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[issue34798] pprint ignores the compact parameter for dicts

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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[issue34798] pprint ignores the compact parameter for dicts

[Nicolas Hug]

New submission from Nicolas Hug :

Dict representations that exceed the line width are printed with one line per 
key-value pair, ignoring the compact=True parameter:

>>> pprint.pprint({i: 0 for i in range(15)}, compact=True)
{0: 0,
 1: 0,
 2: 0,
 3: 0,
 4: 0,
 5: 0,
 6: 0,
 7: 0,
 8: 0,
 9: 0,
 10: 0,
 11: 0,
 12: 0,
 13: 0,
 14: 0}

Expected behavior:

>>> pprint.pprint({i: 0 for i in range(15)}, compact=True)
{0: 0, 1: 0, 2: 0, 3: 0, 4: 0, 5: 0, 6: 0, 7: 0, 8: 0, 9: 0, 10: 0, 11: 0,
 12: 0, 13: 0, 14: 0}

Note that lists are correctly compacted, and that dicts that don't exceed line 
width are printed on a single line, regardless on the compact parameter.

I could try to work on that if needed?

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messages: 326358
nosy: Nicolas Hug
priority: normal
severity: normal
status: open
title: pprint ignores the compact parameter for dicts
type: behavior
versions: Python 3.7

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