More a direction than a solution.
Use 'formatexpr' or 'formatprg'. When a line exceeds 'textwidth' and passes the criteria set by the 'formatoptions' these are used (if set) to break the line. The only real difference is that 'formatexpr' is a vimscript expression, while 'formatprg' filters the line through an exterior program.
So if you know of a formatter that can do this transformation to lines of python code, or are willing to write one, this will give you a hook to have it executed. And since vim supports python (see :help python) you can even write your python formatter in python.