...but yes, it's very obvious that exposing _PyType_Lookup() to pure Python is the right thing to do here.  This is a central part of the way Python works internally, after all.

Moreover, sorry about my previous note: if we started today to write PyPy, then it would be enough to have the pure Python version of operator.index(), based on the newly exposed _PyType_Lookup().  With PyPy's JIT, there is no real performance loss.  Some of my confusion came from the fact that there *would* be serious performance loss if we had to work with the pure Python looping-over-__mro__-and-fishing-in-__dict__ version.