> I don't think this should be merged.  A lot of code of uses PyTuple_GET_ITEM(). Also, the "problem" your solving doesn't seem to exist in practice.

Sorry my comment on the PR, a friend contacted me because (...) he misused PyTuple_GET_ITEM() :-)

> Further, I worry that a compiler may choose not to inline on occasion, leading to performance regressions in code that has already been finely tuned.

Oh, maybe I didn't explain properly my change. I don't want to change the "default" API nor the "default" implementation. My change is enabled enabled if you compile Python in debug mode, if Py_DEBUG is defined.

In fact, it's an early attempt to see how we can introduce a new C API in Python. It seems like abusing Py_DEBUG was a bad idea, so I close my PR and I will close this issue as well. I will start a discussion on python-dev later, when I will have a plan.

See also the discussion on the PR:
https://github.com/python/cpython/pull/10435#issuecomment-437530430