python threading ThreadPoolExecutor源码解析

future: 未来对象，或task的返回容器

1. 当submit后：

    def submit(self, fn, *args, **kwargs):
        with self._shutdown_lock: # lock是线程锁
            if self._shutdown:
                raise RuntimeError('cannot schedule new futures after shutdown')

            f = _base.Future() # 创建future对象
            w = _WorkItem(f, fn, args, kwargs) # 线程池执行基本单位

            self._work_queue.put(w) #实现的是queue
            self._adjust_thread_count() # 这里会进行判断当前执行线程的数量
            return f

2. _adjust_thread_count：

    def _adjust_thread_count(self):
        # When the executor gets lost, the weakref callback will wake up
        # the worker threads.
        def weakref_cb(_, q=self._work_queue):
            q.put(None)
        # TODO(bquinlan): Should avoid creating new threads if there are more
        # idle threads than items in the work queue.
        num_threads = len(self._threads)
        if num_threads < self._max_workers:
            thread_name = '%s_%d' % (self._thread_name_prefix or self,
                                     num_threads)
            t = threading.Thread(name=thread_name, target=_worker,
                                 args=(weakref.ref(self, weakref_cb),
                                       self._work_queue)) # 创建线程，并调用_worker方法，传入work queue
            t.daemon = True
            t.start()
            self._threads.add(t)
            _threads_queues[t] = self._work_queue

3. _worker:

def _worker(executor_reference, work_queue):
    try:
        while True:
            work_item = work_queue.get(block=True)
            if work_item is not None:
                work_item.run()
                # Delete references to object. See issue16284
                del work_item
                continue
            executor = executor_reference()
            # Exit if:
            #   - The interpreter is shutting down OR
            #   - The executor that owns the worker has been collected OR
            #   - The executor that owns the worker has been shutdown.
            if _shutdown or executor is None or executor._shutdown:
                # Notice other workers
                work_queue.put(None)
                return
            del executor
    except BaseException:
        _base.LOGGER.critical('Exception in worker', exc_info=True)

4. WorkItem

class _WorkItem(object):
    def __init__(self, future, fn, args, kwargs):
        self.future = future
        self.fn = fn
        self.args = args
        self.kwargs = kwargs

    def run(self):
        if not self.future.set_running_or_notify_cancel():
            return

        try:
            result = self.fn(*self.args, **self.kwargs)
        except BaseException as exc:
            self.future.set_exception(exc)
            # Break a reference cycle with the exception 'exc'
            self = None
        else:
            self.future.set_result(result)