add !p to pprint.pformat() in str.format() an f-strings

[CharlesMachalow]

BPO	41312
Nosy	@rhettinger, @ericvsmith, @tirkarthi

^{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 2020-07-17.00:56:11.250>
created_at = <Date 2020-07-16.04:17:41.357>
labels = ['interpreter-core', 'type-feature', '3.10']
title = 'add !p to pprint.pformat() in str.format() an f-strings'
updated_at = <Date 2020-07-17.02:22:56.801>
user = 'https://bugs.python.org/CharlesMachalow'

bugs.python.org fields:

activity = <Date 2020-07-17.02:22:56.801>
actor = 'eric.smith'
assignee = 'none'
closed = True
closed_date = <Date 2020-07-17.00:56:11.250>
closer = 'rhettinger'
components = ['Interpreter Core']
creation = <Date 2020-07-16.04:17:41.357>
creator = 'Charles Machalow'
dependencies = []
files = []
hgrepos = []
issue_num = 41312
keywords = []
message_count = 8.0
messages = ['373738', '373739', '373741', '373786', '373787', '373788', '373792', '373793']
nosy_count = 4.0
nosy_names = ['rhettinger', 'eric.smith', 'Charles Machalow', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = 'later'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41312'
versions = ['Python 3.10']

[CharlesMachalow]

Right now in str.format(), we have !s, !r, and !a to allow us to call str(), repr(), and ascii() respectively on the given expression.

I'm proposing that we add a !p conversion to have pprint.pformat() be called to convert the given expression to a 'pretty' string.

Calling

print(f"My dict: {d!p}")

is a lot more concise than:

import pprint
print(f"My dict: {pprint.pformat(d)}")

We may even be able to have a static attribute stored to change the various default kwargs of pprint.pformat().

[tirkarthi]

This needs discussion on python-ideas/ideas category on discourse similar to f-string debug notation.

[CharlesMachalow]

Fair enough. Didn't really know that list existed. Sent this there. Awaiting moderator approval. Thanks.

[rhettinger]

If the python-ideas discussion is fruitful, go ahead and re-open this tracker item.

Personally, I don't see how this would work. The pretty printing routines rely on knowing their current level of indentation. Also, much of the "prettiness" comes from making the output span multiple lines, so this wouldn't fit well in the middle of an f-string. Lastly, the pprint module leaves a lot to be desired and likely needs a rewrite from scratch, so it wouldn't be wise to tie it directly to a core language feature.

[CharlesMachalow]

One of the key things for ppformat here is to format long spanning dicts/lists to multiple lines, that look easy to read in a log. I feel as though that feature/usefulness outweigh potential indentation weirdness.

A lot of the usage would probably be towards the end of an fstring.