gistfile1.py

    def _iter_driver_output(self):
        while self.driver_stack:
            curdriver = self.driver
            # Iterate over the current driver until it changes.
            for output in curdriver.get_input():
                yield output
                
                # If the driver has changed, stop iterating over the old one.
                if self.driver is not curdriver:
                    break
            else:
                # Driver finished.
                if len(self.driver_stack) == 1:
                    # If it's the final one, finish iterating.
                    break
                else:
                    # If it's not the final one, pop it off the stack. 
                    self.driver_stack.pop()