ThreadPoolExecutor.map() prevents interpreter exit if there is a reference to the generator it returns. In the attached script:

- `python threadpool_map.py run1` exits as soon as the exception is raised on the main thread. This is the desired behavior in our case.

- `python threadpool_map.py run2` keeps running until the thread worker processes all queued work items. The only difference from `run1` is that the result of `ThreadPoolExecutor.map()` is assigned to a variable.

- `python threadpool_map.py run3` has a `finally` block that shuts down the executor without waiting. Still the worker thread keeps running even after the shutdown.

Initially it seemed like https://bugs.python.org/issue36780 but there is no change in the behavior after commenting out the `atexit.register(_python_exit)` call (for the `run2` case at least).