pprint ignores the compact parameter for dicts

[NicolasHug]

BPO	34798
Nosy	@serhiy-storchaka, @posita, @miss-islington, @tirkarthi, @iritkatriel, @iansedano
PRs	bpo-34798: [doc] clearer presentation of pprint.PrettyPrinter constru… #26967 [3.10] bpo-34798: [doc] clearer presentation of pprint.PrettyPrinter constru… (GH-26967) #26990

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-07-02.10:08:55.398>
created_at = <Date 2018-09-25.14:45:41.804>
labels = ['3.11', 'type-bug', '3.10', 'docs']
title = 'pprint ignores the compact parameter for dicts'
updated_at = <Date 2021-12-11.16:41:58.483>
user = 'https://bugs.python.org/NicolasHug'

bugs.python.org fields:

activity = <Date 2021-12-11.16:41:58.483>
actor = 'iritkatriel'
assignee = 'docs@python'
closed = True
closed_date = <Date 2021-07-02.10:08:55.398>
closer = 'iritkatriel'
components = ['Documentation']
creation = <Date 2018-09-25.14:45:41.804>
creator = 'Nicolas Hug'
dependencies = []
files = []
hgrepos = []
issue_num = 34798
keywords = ['patch']
message_count = 14.0
messages = ['326358', '326379', '326385', '326386', '326387', '326391', '326394', '375805', '396777', '396851', '396852', '404952', '408307', '408317']
nosy_count = 8.0
nosy_names = ['docs@python', 'serhiy.storchaka', 'posita', 'miss-islington', 'xtreak', 'Nicolas Hug', 'iritkatriel', 'iansedano']
pr_nums = ['26967', '26990']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34798'
versions = ['Python 3.10', 'Python 3.11']

[NicolasHug]

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?

[tirkarthi]

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]

cpython/Lib/pprint.py

Line 350 in fdcb5ae

def _format_items(self, items, stream, indent, allowance, context, level):

[1]

cpython/Lib/pprint.py

Line 333 in fdcb5ae

def _format_dict_items(self, items, stream, indent, allowance, context,

[2] https://mail.python.org/pipermail/python-ideas/

Hope this helps!

[NicolasHug]

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]

[NicolasHug]

Sorry:

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

[serhiy-storchaka]

It is on purpose. See bpo-19132.

[tirkarthi]

Thanks much Serhiy for the details!

[NicolasHug]

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

[iritkatriel]

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.

[iritkatriel]

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.