Class ProcessPoolExecutor

Lib/concurrent/futures/process.py:652–953

class ProcessPoolExecutor(_base.Executor):
    def __init__(self, max_workers=None, mp_context=None,
                 initializer=None, initargs=(), *, max_tasks_per_child=None):
        """Initializes a new ProcessPoolExecutor instance.

        Args:
            max_workers: The maximum number of processes that can be used to
                execute the given calls. If None or not given then as many
                worker processes will be created as the machine has processors.
            mp_context: A multiprocessing context to launch the workers created
                using the multiprocessing.get_context('start method') API. This
                object should provide SimpleQueue, Queue and Process.
            initializer: A callable used to initialize worker processes.
            initargs: A tuple of arguments to pass to the initializer.
            max_tasks_per_child: The maximum number of tasks a worker process
                can complete before it will exit and be replaced with a fresh
                worker process. The default of None means worker process will
                live as long as the executor. Requires a non-'fork' mp_context
                start method. When given, we default to using 'spawn' if no
                mp_context is supplied.
        """
        _check_system_limits()

        if max_workers is None:
            self._max_workers = os.process_cpu_count() or 1
            if sys.platform == 'win32':
                self._max_workers = min(_MAX_WINDOWS_WORKERS,
                                        self._max_workers)
        else:
            if max_workers <= 0:
                raise ValueError("max_workers must be greater than 0")
            elif (sys.platform == 'win32' and
                max_workers > _MAX_WINDOWS_WORKERS):
                raise ValueError(
                    f"max_workers must be <= {_MAX_WINDOWS_WORKERS}")

            self._max_workers = max_workers

        if mp_context is None:
            if max_tasks_per_child is not None:
                mp_context = mp.get_context("spawn")
            else:
                mp_context = mp.get_context()
        self._mp_context = mp_context

        # https://github.com/python/cpython/issues/90622
        self._safe_to_dynamically_spawn_children = (
                self._mp_context.get_start_method(allow_none=False) != "fork")

        if initializer is not None and not callable(initializer):
            raise TypeError("initializer must be a callable")
        self._initializer = initializer
        self._initargs = initargs

        if max_tasks_per_child is not None:
            if not isinstance(max_tasks_per_child, int):
                raise TypeError("max_tasks_per_child must be an integer")
            elif max_tasks_per_child <= 0: