pytorch中Dataloader()中的num_workers设置问题

pytorch中Dataloader()中的num_workers设置问题：

如果num_workers的值大于0，要在运行的部分放进__main__()函数里，才不会有错：

import numpy as np
import torch
from torch.autograd import Variable
import torch.nn.functional
import matplotlib.pyplot as plt
import torch.utils.data as Data


BATCH_SIZE=5

x=torch.linspace(1,10,10)
y=torch.linspace(10,1,10)
torch_dataset=Data.TensorDataset(x,y)
loader=Data.DataLoader(
    dataset=torch_dataset,
    batch_size=BATCH_SIZE,
    shuffle=True,
    num_workers=2,
)


def main():
    for epoch in range(3):
        for step,(batch_x,batch_y) in enumerate(loader):
            # training....
            print('Epoch:',epoch,'| step:',step,'| batch x:',batch_x.numpy(),
                  '| batch y:',batch_y.numpy())


if __name__=="__main__":
    main()


'''

# 下面这样直接运行会报错：

 for epoch in range(3):
     for step,(batch_x,batch_y) in enumerate(loader):
         # training....
          print('Epoch:',epoch,'| step:',step,'| batch x:',batch_x.numpy(),
                  '| batch y:',batch_y.numpy()
'''