The assertion error in question is triggered by this line: https://github.com/python-mode/python-mode/blob/develop/pymode/autopep8.py#L3035

It happens with for example this line in my .vimrc:

let g:pymode_lint_ignore = "E501"

which I got from the help file. Obviously the function where the assert takes place expects a list but gets a string instead, but I'm not sure where pymode_lint_ignore is or should be parsed into a list (from a comma-delimited string).