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> Can we at least switch to PyLong_CheckExact?
+1
> I am not sure that converting to an exact int in low-level C API functions is the best option.
I am sure. :-) The number of naturally-occurring cases where we're actually passing a subtype of `int` that's not exactly `int` should be tiny. So long as there's a PyLong_CheckExact fast path, I don't think there are really any performance concerns here.
And we definitely shouldn't let performance concerns dictate API; get the API right first, _then_ see what can be done about performance without changing the API. It's clear to me that `operator.index(obj)` _should_ give the exact same results as `obj.__index__()`.
I'll split my PR up into two pieces, one for turning the deprecated behaviour into TypeErrors, and a second one that just makes the PyLong_CheckExact change. (I likely won't have time before feature freeze, though. OTOH, the PyLong_CheckExact change could be considered a bugfix.)