Oh sorry, my mistake on having those values in the .arcconfig. It works fine, there was just an unrelated problem on my end.

Thanks for all the info, I have a much better idea of how/why things work now. I've implemented this using a .arclint with severity.rules and it works perfectly with little tweaking.

The idea is that we have some files that already have a bunch of lint errors. We've then modified the files, and ideally we only want 'new' errors to show up. My reading of --only-new made me think that it'd do exactly that. Essentially, run lint twice - once on a previous revision (for some definition of previous that we might have to specify), and once on the current head (including uncommited files). It'd then compare the two sets of errors, and only print ones that weren't there before.

Just some links to things that might be helpful:
.arcconfig: https://gist.github.com/dan-disqus/bd3de21c6e84bbbc6a62
Our custom arc stuff: https://github.com/disqus/disqus-arcanist/tree/master/src. It doesn't modify ArcanistLintWorkflow, so probably not the cause of the issues.

Sorry, I meant ArcanistPEP8Linter, not PyFlakes.