PEP 757: Answer open questions

[vstinner]

Add "Don't add PyLong_GetNativeLayout() function" to Rejected Ideas.

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📚 Documentation preview 📚: https://pep-previews--3980.org.readthedocs.build/

[skirpichev]

I think difference of PyLong_GetNativeLayout() vs PyLong_GetInfo() is a minor question and we don't need to rush here.

[skirpichev]

Suggested change

	Currently, all required information for :class:`int` import/export is
	already available via :c:func:`PyLong_GetInfo()` or
	:data:`sys.int_info`. Native endianness of "digits" and current order
	of digits (least significant digit first) --- is a common denominator of
	all libraries for aribitrary precision integer arithmetic.
	The problem is that this API is for the stable ABI and not only for
	CPython, but for any Python implementation. For example, the JVM based
	GraalPy uses Java's ``BigDecimal`` for "big" integers, which has
	compatible internal representation: 4 bytes per digit, big-endian, most
	significant byte is in the zeroth element.
	People relying on CPython implementation details may assume CPython's
	:c:struct:`PyLongLayout` without even querying it and will just use the
	other APIs. Providing a convenient :c:func:`PyLong_GetNativeLayout`
	function is a way to reduce the risk of relying on CPython
	implementation details. By the way, the CPython native layout can also
	evolve in future Python versions.
	Currently, most required information for :class:`int` import/export is
	already available via :c:func:`PyLong_GetInfo()` (and :data:`sys.int_info`).
	We also can add more (like order of digits), this interface doesn't poses
	any constraints on future evolution of the :type:`PyLongObject`.
	The problem is that the :c:func:`PyLong_GetInfo()` returns a Python object,
	:term:`named tuple`, not a convenient C structure and that might distract
	people from using it in favor e.g. of current semi-private macros like :c:macro:`!PyLong_SHIFT` and :c:macro:`!PyLong_BASE`.

[skirpichev]

I'm still not sure if we should rush with this change. But if so, please take look on my edits.

I think the only arguments is: it's easy to use. (BTW, it seems (I did GH search) the PyLong_GetInfo() isn't used outside of CPython.)

[vstinner]

I abandon my PR.