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bpo-46906: Mention PY_BIG_ENDIAN in PyFloat_Pack8() doc#31832

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bpo-46906: Mention PY_BIG_ENDIAN in PyFloat_Pack8() doc #31832

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6 changes: 4 additions & 2 deletions Doc/c-api/float.rst
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Expand Up@@ -109,7 +109,8 @@ Pack functions
The pack routines write 2, 4 or 8 bytes, starting at *p*. *le* is an
:c:type:`int` argument, non-zero if you want the bytes string in little-endian
format (exponent last, at ``p+1``, ``p+3``, or ``p+6`` ``p+7``), zero if you
want big-endian format (exponent first, at *p*).
want big-endian format (exponent first, at *p*). Pass :c:data:`PY_BIG_ENDIAN`

@JelleZijlstraJelleZijlstraMar 12, 2022

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So if your platform is little-endian, then passing PY_BIG_ENDIAN uses a little-endian format? That seems confusing.

@vstinnervstinnerMar 13, 2022

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PY_BIG_ENDIAN is 1 on big endian platform, and 0 on little endian platform.

int.to_bytes() uses the native endian by default (if the byte_order parameter is not set). Maybe we could accept -1 to mean "use native endian"?

@mdickinsonmdickinsonMar 13, 2022

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int.to_bytes() uses the native endian by default

It uses big-endian by default.

@vstinnervstinnerMar 13, 2022

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Oops. You're right. I now recall a long discussion about the default :-)

@mdickinsonmdickinsonMar 13, 2022

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That seems confusing.

Agreed, but it's primarily a problem with the naming of PY_BIG_ENDIAN, which would perhaps be better spelled as something like PY_PLATFORM_ENDIAN_IS_BIG_ENDIAN.

But usage of PY_BIG_ENDIAN is at least a little bit problematic, since there's nothing anywhere in C that guarantees that integer types and float types will use the same endianness (or even that float types have to follow one of big-endian or little-endian at all).

Maybe we could accept -1 to mean "use native endian"?

I'd suggest leaving it as-is - it's easy enough to pass PY_BIG_ENDIAN.

@vstinnervstinnerMar 14, 2022

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I proposed a different wording for the documentation: PR #31866. Is it better?

to *le* to use the native endian.

Return value: ``0`` if all is OK, ``-1`` if error (and an exception is set,
most likely :exc:`OverflowError`).
Expand DownExpand Up@@ -138,7 +139,8 @@ Unpack functions
The unpack routines read 2, 4 or 8 bytes, starting at *p*. *le* is an
:c:type:`int` argument, non-zero if the bytes string is in little-endian format
(exponent last, at ``p+1``, ``p+3`` or ``p+6`` and ``p+7``), zero if big-endian
(exponent first, at *p*).
(exponent first, at *p*). Pass :c:data:`PY_BIG_ENDIAN` to *le* to use the
native endian.

Return value: The unpacked double. On error, this is ``-1.0`` and
:c:func:`PyErr_Occurred` is true (and an exception is set, most likely
Expand Down