I have a Python program that calculates and prints various pieces of Linux memory information on a per-cgroup basis. In the beginning, its life was simple; cgroups had a total memory use that was split between 'user' and '(filesystem) cache', so the program only needed to display either one field or a primary field plus a secondary field. Then I discovered that there was additional important (ie, large) kernel memory use in cgroups and added the ability to report it as an additional option for the secondary field. However, this wasn't really ideal, because now I had a three-way split and I might want to see all three things at once.

A while back I wrote up my realization about flexible string formatting with named arguments . This sparked all sorts of thoughts about writing a general solution for my program that could show any number of fields. Recently I took a stab at implementing this and rapidly ran into problems figuring out how I wanted to do it. I had multiple things that could be calculated and presented, I had to print not just the values but also a header with the right field names, I'd need to think about how I structured argparse argument groups in light of argparse not supporting nested groups , and so on. At a minimum this wasn't going to be a quick change; I was looking at significantly rewriting how the program printed its output.

The other day, I had an obvious realization: while it would be nice to have a fully general solution that could print any number of additional fields, which would meet my needs now and in the future, all that I needed right now was an additional three-field version with the extra fields hard-coded and the whole thing selected through a new command line argument. And this command line argument could drop right into the existing argparse exclusive group for choosing the second field, even though this feels inelegant.

(The fields I want to show are added with '-c' and '-k' respectively in the two field display, so the morally correct way to select both at once would be '-ck', but currently they're exclusive options, which is enforced by argparse . So I added a third option, literally '-b' for 'both'.)

Actually implementing this hard-coded version was a bit annoying for structural reasons, but I put the whole thing together in not very long; certainly it was much faster than a careful redesign and rewrite (in an output pattern I haven't used before, no less). It's not necessarily the right answer for the long term, but it's definitely the right answer for now (and I'm glad I talked myself into doing it).

(I'm definitely tempted to go back and restructure the whole output reporting to be general. But now there's no rush to it; it's not blocking a feature I want, it's a cleanup.)