bpo-45246: Document that sorted() only uses "<" comparisons (GH-28494)

(cherry picked from commit 9a0dcc5b2e04d9c51350107734f12a1cbc0284a7)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>

diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 39ce338d38871d7dcbeb0bbb918a98ad48d67f4b..15bfbf40462fc3d82a42ec150d1d841e43660c26 100644 (file)
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -1547,6 +1547,15 @@ are always available.  They are listed here in alphabetical order.
    compare equal --- this is helpful for sorting in multiple passes (for
    example, sort by department, then by salary grade).
 
+   The sort algorithm uses only ``<`` comparisons between items.  While
+   defining an :meth:`~object.__lt__` method will suffice for sorting,
+   :PEP:`8` recommends that all six :ref:`rich comparisons
+   <comparisons>` be implemented.  This will help avoid bugs when using
+   the same data with other ordering tools such as :func:`max` that rely
+   on a different underlying method.  Implementing all six comparisons
+   also helps avoid confusion for mixed type comparisons which can call
+   reflected the :meth:`~object.__gt__` method.
+
    For sorting examples and a brief sorting tutorial, see :ref:`sortinghowto`.
 
 .. decorator:: staticmethod