I'm curious why this one is a -1? Naively, I would have expected something like (|1), ie setting the lowest bit. Which would be equivalent to +1.

I assume there's something that goes horribly wrong with that native approach that I am missing, though?

(As far as I can tell, subtracting one should also work. Not sure what C does with null pointers in this context, though.)

_PyDictOrValues_SetValues mutates the pointer; _PyDictOrValues_GetValues restores the pointer. Think of it as $f(x) = y$ and $f^{-1}(y) = x$. Currently, $f(x) = x - 1$ and $f^{-1}(y) = y + 1$. If we'd used +1, as you suggest, we'd have to undo that using -1 iso. +1. AFAICS, either way is fine.

Thanks for explaining!

I am working on a prototype with a tagged pointer elsewhere in the code, I so I was looking for exiting examples to learn from.

Thanks for confirming that either way is fine.

I was just wrecking my brain about whether there's something more going on that I was missing. (And something that I would thus be bound to get horriby wrong in my code, where I want to store either a pointer directly, or an odd integer, in the same memory location.)