Both the main function (offer) and the new function (process_interrupt) are python generators. So in fact you want to divide a generator into two generators. There is a special syntax for delegating to a subgenerator (PEP 380):

yield from <expr>

When

action = process_interrupt(action, simpy_car, simpy_tankcar)

is replaced by

action = yield from process_interrupt(action, simpy_car, simpy_tankcar) 

everything works fine.