title: `DummyExecutor` for Python's `futures`
body: <p>Python's <code>futures</code> package allows us to enjoy <code>ThreadPoolExecutor</code> and <code>ProcessPoolExecutor</code> for doing tasks in parallel.</p>
  
  <p>However, for debugging it is sometimes useful to temporarily replace the true parallelism with a dummy one, which carries out the tasks in a serial way in the main thread, without spawning any threads or processes.</p>
  
  <p>Is there anywhere an implementation of a <code>DummyExecutor</code>?</p>