The documentation of sorted() lacks any reference to the comparison mechanism between items.

Compare with the documentation of list.sort(), which starts with:
    using only < comparisons between items

This is mentioned in the "Sorting HOW TO", under "Odd and Ends":
    The sort routines are guaranteed to use __lt__() when making comparisons between two objects.

However, the "Sorting HOW TO" is "a brief sorting tutorial", not the reference documentation. This property needs to be documented in the reference documentation of sorted().

-0 on this.  While it is true that __lt__ is used, we don't really want people to exploit that fact.  Doing so will get them into trouble elsewhere.  For example, max(seq) uses __gt__.  Also, when mixing types, a return of NotImplemented will trigger a call to the reflection method.  And PEP 8 recommends that all six rich comparison operators be defined to avoid hard-to-find bugs.

Then what about removing it from the list.sort() documentation too?

Note that maintainers of other PEPs insist that this is a known fact and base their code on that:
https://github.com/python/peps/pull/2077

I would recommend the wording in the "Sorting HOW TO" from:
    The sort routines are guaranteed to use __lt__() when making comparisons between two objects.
to:
    The sort routines happen to use __lt__() when making comparisons between two objects.

I've seen the new PR for sorted(), thank you for looking into this.

Would it make sense to change list.sort() in the same way?

> Thank you for looking into this.

You're welcome :-)


> Would it make sense to change list.sort() in the same way?

I think not.  This is a such a minor point and is a distractor.  Most users would be better off focusing on the other text.

New changeset 9a0dcc5b2e04d9c51350107734f12a1cbc0284a7 by Raymond Hettinger in branch 'main':
bpo-45246: Document that sorted() only uses "<" comparisons (GH-28494)
https://github.com/python/cpython/commit/9a0dcc5b2e04d9c51350107734f12a1cbc0284a7

New changeset aea92de9900ceb8d679a9324fc72621940b9c8a0 by Miss Islington (bot) in branch '3.9':
bpo-45246: Document that sorted() only uses "<" comparisons (GH-28494)
https://github.com/python/cpython/commit/aea92de9900ceb8d679a9324fc72621940b9c8a0

New changeset 0d41bf15702832540afc1b1c078bc4fde5baebd6 by Łukasz Langa in branch '3.10':
[3.10] bpo-45246: Document that sorted() only uses "<" comparisons (GH-28494) (GH-28502)
https://github.com/python/cpython/commit/0d41bf15702832540afc1b1c078bc4fde5baebd6

New changeset 2e4d66d2e71d36de394e0e7a4e60ed7b6a539933 by Pablo Galindo (Łukasz Langa) in branch '3.10':
[3.10] bpo-45246: Document that sorted() only uses "<" comparisons (GH-28494) (GH-28502)
https://github.com/python/cpython/commit/2e4d66d2e71d36de394e0e7a4e60ed7b6a539933