Add PyFloat_(Pack|Unpack)(2|4|8) to the public C API

[methane]

BPO	46906
Nosy	@malemburg, @rhettinger, @mdickinson, @vstinner, @methane
PRs	bpo-46906: Move _PyFloat_{Pack,Unpack}{4,8} from internal to cpython #31649 bpo-46906: Add PyFloat_Pack8() to the C API #31657 bpo-46906: Mention PY_BIG_ENDIAN in PyFloat_Pack8() doc #31832 bpo-46906: Mention native endian in PyFloat_Pack8() doc #31866

^{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 2022-03-14.15:58:36.669>
created_at = <Date 2022-03-03.03:09:32.087>
labels = ['expert-C-API', '3.11']
title = 'Add PyFloat_(Pack|Unpack)(2|4|8) to the public C API'
updated_at = <Date 2022-03-14.15:58:36.668>
user = 'https://github.com/methane'

bugs.python.org fields:

activity = <Date 2022-03-14.15:58:36.668>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-14.15:58:36.669>
closer = 'vstinner'
components = ['C API']
creation = <Date 2022-03-03.03:09:32.087>
creator = 'methane'
dependencies = []
files = []
hgrepos = []
issue_num = 46906
keywords = ['patch']
message_count = 12.0
messages = ['414401', '414410', '414411', '414416', '414419', '414427', '414944', '414952', '414953', '414954', '415151', '415152']
nosy_count = 6.0
nosy_names = ['lemburg', 'rhettinger', 'mark.dickinson', 'vstinner', 'stutzbach', 'methane']
pr_nums = ['31649', '31657', '31832', '31866']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46906'
versions = ['Python 3.11']

[methane]

Original issue. msgpack/msgpack-python#497

_PyFloat_(Pack|Unpack)(4|8) is very nice API for serializers like msgpack.
Converting double and float into char[] is not trivial and these APIs do it in very efficient way.

And these APIs don't reveal CPython internal strucutre. It just convert double and float into char[].

So please keep these APIs public for libraries like msgpack.

[vstinner]

I disagree. A function should be either fully public: tested, documented. Or fully internal.

In the past, many functions were in both, in the gray area.

If these functions are useful, please make them *public* and document them. I'm +1 to make these functions public.

[methane]

OK. By quick grepping, I found only msgpack and bitstruct use these API.
It is not enough number to make them public.

[vstinner]

I reopen the issue, I would like to make these functions public :-) I searched for the 6 functions moved to the internal C API and I found 6 projets using it in the top 5000 PyPI projects (at 2022-01-26):

• msgpack
• ddtrace
• bitstruct
• pickle5
• line_profiler
• zodbpickle

The 6 functions are used:

• _PyFloat_Pack2()
• _PyFloat_Pack4()
• _PyFloat_Pack8()
• _PyFloat_Unpack2()
• _PyFloat_Unpack4()
• _PyFloat_Unpack8()

[vstinner]

The pack/unpack functions have been moved to the internal C API by this change:

commit 0a883a7
Author: Victor Stinner <vstinner@python.org>
Date: Thu Oct 14 23:41:06 2021 +0200

bpo-35134: Add [Include/cpython/floatobject.h](https://github.com/python/cpython/blob/main/Include/cpython/floatobject.h) (GH-28957)

Split [Include/floatobject.h](https://github.com/python/cpython/blob/main/Include/floatobject.h) into sub-files: add
[Include/cpython/floatobject.h](https://github.com/python/cpython/blob/main/Include/cpython/floatobject.h) and
[Include/internal/pycore_floatobject.h](https://github.com/python/cpython/blob/main/Include/internal/pycore_floatobject.h).

[vstinner]

I prepared a pythoncapi_compat PR to provide these functions to Python 3.10 and older:
python/pythoncapi-compat#26

Note: bpo-11734 (commit 7c4e409) added _PyFloat_Pack2() and _PyFloat_Unpack2() to Python 3.6.0b1.

[vstinner]

New changeset 882d809 by Victor Stinner in branch 'main':
bpo-46906: Add PyFloat_Pack8() to the C API (GH-31657)
882d809

[vstinner]

PR for msgpack: msgpack/msgpack-python#499

[vstinner]

PR for bitstruct: eerimoq/bitstruct#26

[vstinner]

zodbpickle issue: zopefoundation/zodbpickle#67

[vstinner]

New changeset 11c25b8 by Victor Stinner in branch 'main':
bpo-46906: Mention native endian in PyFloat_Pack8() doc (GH-31866)
11c25b8

[vstinner]

msgpack and bitstruct use the newly added functions: my two PRs got merged. msgpack was my main motivation to add these functions :-)

Thanks to great reviews, the functions got a new better documentation!

I close the issue. Thanks again for reviews!