RuntimeError: An attempt has been made to start a new process before the current process has finished its bootstrapping phase.

pytorch 加载数据时报错：

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.

 

dataloaders = {x: torch.utils.data.DataLoader(image_datasets[x], batch_size=opt.batchsize,
shuffle=False, num_workers=0) for x in
原因：多进程需要在main函数中运行，

解决方法1：

加main函数，在main中调用

解决方法2：

num_workers改为0，单进程加载