The convention when flattening is exactly that I thought - an underscore per level of nesting. This is what I would expect to see too.

Agree. (What if the original users key has the double underscore?) maybe this can be defined as a param?

> What if the original users key has the double underscore?

Double it again.

some_key = 1

some__key = 2

some____key = 3

This is analogous to what C#11 does to allow raw string literal quotes containing those same quotes. https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...