Wow - great stuff @chrisbillowsMO! Clearly written and a nice choice of tests.

I've tested on Jasmin and it runs cleanly.

Hi @alistairsellar - great! 😁 I'm so pleased you're pleased. @ehogan didn't want to miss out on all the fun so I'll await her final feedback and merge once she bequeaths her seal.

As an aside, I've warmed to @ehogan's suggestion of an esmvaltool config validate command: #3392 (comment). But makes sense I think to merge this to RTW now, and then discuss moving the functions to Core for a new command.

Great! I was working with this in mind with a view to having a crack at the issue raised against it in ESMValCore:

• Add esmvaltool config validate command? ESMValCore#2246

One point to consider: it may be a bigger job to implement this in core.

CFG/Configure() objects are specifically designed to throw an error on the first invalid value. I had to write a workaround to collect multiple errors (i.e. to save the user multiple runs/failures).

I wrote it up a little here: #3392 (comment) but the tldr; for core we'd probably want a different, more holistic solution than what I've written. Indeed, it might mean re-shaping a lot of the validation code in ESMValCore - probably not ideal! A decorator might offer a solution but I need to fully understand the validation code first. I'm hoping to continue exploring when/if time allows.

Also, you're probably already aware, but possibly to bear in mind are the broader changes afoot in the user config file arena:

• New configuration file plan ESMValCore#2371