【环境配置】Windows10-YOLOv8运行报错

问题如下：
Windows10 运行 YOLOv8 出现如下错误：
Traceback (most recent call last):
File “”, line 1, in
File “D:\anaconda3\envs\yolov8\Lib\multiprocessing\spawn.py”, line 116, in spawn_main
exitcode = _main(fd, parent_sentinel)
^^^^^^^^^^^^^^^^^^^^^^^^^
File “D:\anaconda3\envs\yolov8\Lib\multiprocessing\spawn.py”, line 125, in _main
prepare(preparation_data)
…

Traceback (most recent call last):
  File "", line 1, in <module>
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\spawn.py", line 122, in spawn_main
    exitcode = _main(fd, parent_sentinel)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\spawn.py", line 131, in _main
    prepare(preparation_data)
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\spawn.py", line 242, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\spawn.py", line 293, in _fixup_main_from_path
    main_content = runpy.run_path(main_path,
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "", line 291, in run_path
  File "", line 98, in _run_module_code
  File "", line 88, in _run_code
  File "F:\workspace\yolov8\train.py", line 8, in <module>
    results = model.train(data="fsd.yaml", epochs=200, batch=16) 
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\ultralytics\engine\model.py", line 377, in train
    self.trainer.train()
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\ultralytics\engine\trainer.py", line 192, in train
    self._do_train(world_size)
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\ultralytics\engine\trainer.py", line 294, in _do_train
    self._setup_train(world_size)
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\ultralytics\engine\trainer.py", line 259, in _setup_train
    self.train_loader = self.get_dataloader(self.trainset, batch_size=batch_size, rank=RANK, mode='train')
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\ultralytics\models\yolo\detect\train.py", line 40, in get_dataloader
    return build_dataloader(dataset, batch_size, workers, shuffle, rank)  # return dataloader
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\ultralytics\data\build.py", line 101, in build_dataloader
    return InfiniteDataLoader(dataset=dataset,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\ultralytics\data\build.py", line 29, in __init__
    self.iterator = super().__iter__()
                    ^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\torch\utils\data\dataloader.py", line 441, in __iter__
    return self._get_iterator()
           ^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\torch\utils\data\dataloader.py", line 388, in _get_iterator
    return _MultiProcessingDataLoaderIter(self)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\site-packages\torch\utils\data\dataloader.py", line 1042, in __init__
    w.start()
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\process.py", line 121, in start
    self._popen = self._Popen(self)
                  ^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\context.py", line 336, in _Popen
    return Popen(process_obj)
           ^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\popen_spawn_win32.py", line 45, in __init__
    prep_data = spawn.get_preparation_data(process_obj._name)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\spawn.py", line 160, in get_preparation_data
    _check_not_importing_main()
  File "D:\anaconda3\envs\yolov8\Lib\multiprocessing\spawn.py", line 140, in _check_not_importing_main
    raise RuntimeError('''
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.

修改

D:\anaconda3\envs\yolov8\Lib\site-packages\ultralytics\data 路径下的 build.py 文件里的函数 build_dataloader， 将 nw = min([os.cpu_count() // max(nd, 1), batch if batch > 1 else 0, workers]) # number of workers 直接修改为 nw=0

def build_dataloader(dataset, batch, workers, shuffle=True, rank=-1):
    """Return an InfiniteDataLoader or DataLoader for training or validation set."""
    batch = min(batch, len(dataset))
    nd = torch.cuda.device_count()  # number of CUDA devices
    # nw = min([os.cpu_count() // max(nd, 1), batch if batch > 1 else 0, workers])  # number of workers
    nw = 0 # update by jw 20230815
    .....

【参考】

解决RuntimeError: An attempt has been made to start a new process before…办法

python进程 - 调试报错 you are not using fork to start your child processes

WINDOWS FAQ-Usage (multiprocessing)