Flake8 treats this equivalently to # flake8: noqa -- that is, it turns off all rules for the entire file. As of #5571, we emit a warning but don't respect it at all. I think better would be to just support it in the way the user expects (i.e., turn off F401 for the entire file). It deviates from Flake8, but #5571 deviates from Flake8 too, in a way that's less useful.