https://docs.python.org/3/reference/datamodel.html#object.__ror__ has a note saying:

"If the right operand's type is a subclass of the left operand's type and that subclass provides the reflected method for the operation, this method will be called before the left operand's non-reflected method." The slightly unclear part (at least to me) is the "provides the reflected method."

What this actually means according to https://bugs.python.org/issue30140 is that the subclass **implements** the `__r*__` method, not just that the method is reachable on the subclass via getattr(). That wasn't clear to me when I initially read this.

Or to be more clear, the RHS subclass must provide a **different** implementation of the method than the LHS.

Yes, there is a corner case here. If A derives from B and B derives from C, A doesn't implement __rsub__ and B does, A()-C() will call B's __rsub__.

Sorry, of course I meant C()-A(). The point is that the class A _doesn't_ have to implement __rsub__.

Right, there is a `lhs.__rsub__ is not rhs.__rsub__` check which needs to be clearly communicated in that note.

New changeset d02d824e05e2cb86f4df381be18832e76e2c475f by Brett Cannon in branch 'master':
bpo-41584: clarify when the reflected method of a binary arithemtic operator is called (#22505)
https://github.com/python/cpython/commit/d02d824e05e2cb86f4df381be18832e76e2c475f

New changeset 31ceccb2c77854893f3a754aca04bedd74bedb10 by Miss Skeleton (bot) in branch '3.8':
bpo-41584: clarify when the reflected method of a binary arithemtic operator is called (GH-22505)
https://github.com/python/cpython/commit/31ceccb2c77854893f3a754aca04bedd74bedb10

New changeset 4aad1e5770fab72908f922a7876075d3d5a7c0d0 by Miss Skeleton (bot) in branch '3.9':
bpo-41584: clarify when the reflected method of a binary arithemtic operator is called (GH-22505)
https://github.com/python/cpython/commit/4aad1e5770fab72908f922a7876075d3d5a7c0d0