Pylint solves the undefined variable issue, but it is annoying as hell and always requires configuration. I wish there was a linter for python that didn't care about 80 character limit and was careful not to drown important errors with unimportant stylistic tics.
flake8 is a linter that doesn't need much config, and you can config it to give you only "serious" errors and not stylistic issues.
I gave a talk on these subjects last year at the Paris Open Source Summit: https://speakerdeck.com/sfermigier/python-quality-engineerin... (NB: Black didn't exist yet at the time).
Ideally instead of needing to be told whether to only show "serious" errors, for instance, it should just give you a project "score" (sum of the errors x their severity) and show the top 20 or so errors in order of importance and by default it should never exit with error code 1 with just stylistic errors unless explicitly configured to do so.
Or rather, I'd vaguely heard the name but I erroneously thought it was just another flake8 type thing.