From 7d7d56d8b1147a6b85e1c09d01b164df7c5c4942 Mon Sep 17 00:00:00 2001 From: Sergey B Kirpichev Date: Sat, 2 Nov 2024 20:11:12 +0300 Subject: [PATCH] gh-99880: document rounding mode for new-style formatting (GH-121481) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit * gh-99880: document rounding mode for new-style formatting The CPython uses _Py_dg_dtoa(), which does rounding to nearest with half to even tie-breaking rule. If that functions is unavailable, PyOS_double_to_string() fallbacks to system snprintf(). Since CPython 3.12, build requirements include C11 compiler *and* support for IEEE 754 floating point numbers (Annex F). This means that FE_TONEAREST macro is available and, per default, printf-like functions should use same rounding mode as _Py_dg_dtoa(). * Update Doc/library/string.rst Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com> --------- Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com> --- Doc/library/string.rst | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/Doc/library/string.rst b/Doc/library/string.rst index 9e8e44a8abe770..a000bb49f14800 100644 --- a/Doc/library/string.rst +++ b/Doc/library/string.rst @@ -589,6 +589,11 @@ The available presentation types for :class:`float` and | | as altered by the other format modifiers. | +---------+----------------------------------------------------------+ +The result should be correctly rounded to a given precision ``p`` of digits +after the decimal point. The rounding mode for :class:`float` matches that +of the :func:`round` builtin. For :class:`~decimal.Decimal`, the rounding +mode of the current :ref:`context ` will be used. + The available presentation types for :class:`complex` are the same as those for :class:`float` (``'%'`` is not allowed). Both the real and imaginary components of a complex number are formatted as floating-point numbers, according to the