build an engine
+-- weld metal
+-- screw bolts

build a car
+-- build an engine
|   +-- weld metal
|   +-- screw bolts
+-- build a frame
|   +-- weld metal
|   +-- screw bolts
|   +-- paint
+-- combine engine and frame

class Task:
    def do(self):
        raise NotImplementedError('abstract method')

class Job(Task):
    def __init__(self):
        self.tasks = []

    def do(self):
        for task in self.tasks:
            task.do()

class Task:
    def do(self):
        raise NotImplementedError('abstract method')

    def run(self):
        if self.block:
            self.do()
        else:
            threading.Thread(target=self.do).start()

class Task:
    def __init__(self):
        self._done = threading.Event()
        self.dependencies = []

    def wait_for_completion(self):
        self._done.wait()

    def do(self):
        raise NotImplementedError('abstract method')

    def do_async(self):
        for depends in self.dependencies:
            depends.wait_for_completion()

        self.do()
        self._done.set()

    def run(self):
        threading.Thread(target=self.do_async).start()


class Job(Task):
    def __init__(self):
        super(Job, self).__init__()
        self.tasks = []

    def do(self):
        started_tasks = []
        for task in self.tasks:
            if task.block:
                self._wait_for_tasks(started_tasks)
                started_tasks = []
            else:
                started_tasks.append(task)
            task.run()
            if task.block:
                task.wait_for_completion()
        self._wait_for_tasks(self.tasks)

    def _wait_for_tasks(self, task_list):
        for task in task_list:
            task.wait_for_completion()