The data model docs for __new__ say "If __new__() is invoked during object construction and it returns an instance or subclass of cls, then the new instance’s __init__() method will be invoked..."

"instance or subclass of cls" is incorrect - if for some reason __new__ returns a subclass of cls, __init__ will not be invoked, unless the subclass also happens to be an instance of cls (which can happen with metaclasses).

This should probably say something like "instance of cls (including subclass instances)", or "instance of cls or of a subclass of cls", or just "instance of cls".

The first variant seems best to me.

I would be happy with just "instance of cls".  Elsewhere in the docs, that phrasing almost always means "instance of a cls or instance of a subclass of cls".  Also, it is consistent with the meaning of isinstance(inst, cls) unless overridden by ABC logic.

I concur with Raymond. Originally it was "instance of cls". It was changed in issue15542 (6b16d938d6d1ccb443815e20e8812deed274dc09).

New changeset fa15df77f02ba4a66ba0b71989381a426038be01 by Raymond Hettinger in branch 'main':
bpo-45104: Clarify when __init__ is called (GH-28210)
https://github.com/python/cpython/commit/fa15df77f02ba4a66ba0b71989381a426038be01

New changeset 5a42a49477cd601d67d81483f9589258dccb14b1 by Miss Islington (bot) in branch '3.8':
bpo-45104: Clarify when __init__ is called (GH-28210) (GH-28213)
https://github.com/python/cpython/commit/5a42a49477cd601d67d81483f9589258dccb14b1

New changeset ef704137770b2e98da7880c828aaf921f0b45337 by Miss Islington (bot) in branch '3.10':
bpo-45104: Clarify when __init__ is called (GH-28210)
https://github.com/python/cpython/commit/ef704137770b2e98da7880c828aaf921f0b45337

New changeset b1f2fe777e1fe133d33cd1e714b8aa0f81fed1cc by Miss Islington (bot) in branch '3.9':
bpo-45104: Clarify when __init__ is called (GH-28210) (GH-28212)
https://github.com/python/cpython/commit/b1f2fe777e1fe133d33cd1e714b8aa0f81fed1cc

Thanks for the patch, Raymond!