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Original issue. https://github.com/msgpack/msgpack-python/issues/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.

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.

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

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年01月26日):
* msgpack
* ddtrace
* bitstruct
* pickle5
* line_profiler
* zodbpickle
The 6 functions are used:
* _PyFloat_Pack2()
* _PyFloat_Pack4()
* _PyFloat_Pack8()
* _PyFloat_Unpack2()
* _PyFloat_Unpack4()
* _PyFloat_Unpack8()

The pack/unpack functions have been moved to the internal C API by this change:
commit 0a883a76cda8205023c52211968bcf87bd47fd6e
Author: Victor Stinner <vstinner@python.org>
Date: Thu Oct 14 23:41:06 2021 +0200
 bpo-35134: Add Include/cpython/floatobject.h (GH-28957)
 
 Split Include/floatobject.h into sub-files: add
 Include/cpython/floatobject.h and
 Include/internal/pycore_floatobject.h.

I prepared a pythoncapi_compat PR to provide these functions to Python 3.10 and older:
https://github.com/pythoncapi/pythoncapi_compat/pull/26
Note: bpo-11734 (commit 7c4e409d075fdb923807513353b18a75a4520eba) added _PyFloat_Pack2() and _PyFloat_Unpack2() to Python 3.6.0b1.

New changeset 882d8096c262a5945e0cfdd706e5db3ad2b73543 by Victor Stinner in branch 'main':
bpo-46906: Add PyFloat_Pack8() to the C API (GH-31657)
https://github.com/python/cpython/commit/882d8096c262a5945e0cfdd706e5db3ad2b73543

PR for msgpack: https://github.com/msgpack/msgpack-python/pull/499 

PR for bitstruct: https://github.com/eerimoq/bitstruct/pull/26 

zodbpickle issue: https://github.com/zopefoundation/zodbpickle/issues/67 

New changeset 11c25b87aeed162d422bc18530fe9699f311e586 by Victor Stinner in branch 'main':
bpo-46906: Mention native endian in PyFloat_Pack8() doc (GH-31866)
https://github.com/python/cpython/commit/11c25b87aeed162d422bc18530fe9699f311e586

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!