I managed to achieve the implementation of precision without having to cannibalize the internals of _PyLong_Format, or (calc_number_widths, fill_number, NumberFieldWidths) in formatter_unicode.c, that's nice.

Note for myself: "PEP 786: Precision and Modulo-Precision Flag format specifiers for integer fields" is still a draft: python/peps#4416.

Can we avoid creating reference implementations on the main repo please? use a fork please. In addition, issues should be created once it's been discussed and accepted, not before.

I already thought of setting the PEP reference implementation link to a branch on my fork, but then I realized that the upstream python/cpython repo serves as a better permalink; that branch on my fork might get deleted once merged!

In this case, you can create your own repository. We usually avoid creating PRs for features that are still under discussions.

that branch on my fork might get deleted once merged!

You can create a pr in your fork and point to that, not to the branch. Then (unless you delete the fork) - you can re-create removed branches. And people still be able to see your patch even if the branch itself will be deleted. See e.g.: skirpichev#1

Though, as this pr was already opened - lets keep this work here (as a draft).