PyTorch - Torchvision - BrokenPipeError: [Errno 32] Broken pipe - python-3.x

Question

I'm trying to carry out the tutorial named "Training a classifier" with PyTorch.
WHen trying to debug this part of the code :
import matplotlib.pyplot as plt
import numpy as np
# functions to show an image
def imshow(img):
img = img / 2 + 0.5 # unnormalize
npimg = img.numpy()
plt.imshow(np.transpose(npimg, (1, 2, 0)))
# get some random training images
dataiter = iter(trainloader)
images, labels = dataiter.next()
# show images
imshow(torchvision.utils.make_grid(images))
# print labels
print(' '.join('%5s' % classes[labels[j]] for j in range(4)))
I get this error message :
Files already downloaded and verified Files already downloaded and verified
Files already downloaded and verified Files already downloaded and verified Traceback (most recent call last):
File "<string>", line 1, in <module>
File "D:\Anaconda\lib\multiprocessing\spawn.py", line 105, in spawn_main
exitcode = _main(fd)
File "D:\Anaconda\lib\multiprocessing\spawn.py", line 114, in _main
prepare(preparation_data)
File "D:\Anaconda\lib\multiprocessing\spawn.py", line 225, in prepare
_fixup_main_from_path(data['init_main_from_path'])
File "D:\Anaconda\lib\multiprocessing\spawn.py", line 277, in
_fixup_main_from_path
run_name="__mp_main__")
File "D:\Anaconda\lib\runpy.py", line 263, in run_path
pkg_name=pkg_name, script_name=fname)
File "D:\Anaconda\lib\runpy.py", line 96, in _run_module_code
mod_name, mod_spec, pkg_name, script_name)
File "D:\Anaconda\lib\runpy.py", line 85, in _run_code
exec(code, run_globals)
File "d:\Yggdrasil\Programmation\PyTorch\TutorialCIFAR10.py", line 36, in <module>
dataiter = iter(trainloader)
File "D:\Anaconda\lib\site-packages\torch\utils\data\dataloader.py", line 451, in __iter__
return _DataLoaderIter(self)
File "D:\Anaconda\lib\site-packages\torch\utils\data\dataloader.py", line 239, in __init__
w.start()
File "D:\Anaconda\lib\multiprocessing\process.py", line 105, in start
self._popen = self._Popen(self)
File "D:\Anaconda\lib\multiprocessing\context.py", line 223, in _Popen
return _default_context.get_context().Process._Popen(process_obj)
File "D:\Anaconda\lib\multiprocessing\context.py", line 322, in _Popen
return Popen(process_obj)
File "D:\Anaconda\lib\multiprocessing\popen_spawn_win32.py", line 33, in __init__
prep_data = spawn.get_preparation_data(process_obj._name)
File "D:\Anaconda\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
_check_not_importing_main()
File "D:\Anaconda\lib\multiprocessing\spawn.py", line 136, in
_check_not_importing_main
is not going to be frozen to produce an executable.)
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.
Traceback (most recent call last):
File "d:\Yggdrasil\Programmation\PyTorch\TutorialCIFAR10.py", line 36, in <module>
dataiter = iter(trainloader)
File "D:\Anaconda\lib\site-packages\torch\utils\data\dataloader.py", line 451, in __iter__
return _DataLoaderIter(self)
File "D:\Anaconda\lib\site-packages\torch\utils\data\dataloader.py", line 239, in __init__
w.start()
File "D:\Anaconda\lib\multiprocessing\process.py", line 105, in start
self._popen = self._Popen(self)
File "D:\Anaconda\lib\multiprocessing\context.py", line 223, in _Popen
return _default_context.get_context().Process._Popen(process_obj) File "D:\Anaconda\lib\multiprocessing\context.py", line 322, in _Popen
return Popen(process_obj)
File "D:\Anaconda\lib\multiprocessing\popen_spawn_win32.py", line 65, in
__init__
reduction.dump(process_obj, to_child)
File "D:\Anaconda\lib\multiprocessing\reduction.py", line 60, in dump
ForkingPickler(file, protocol).dump(obj)
BrokenPipeError: [Errno 32] Broken pipe
All the previous lines in the tutorial are working perfectly.
Does someone know how to solve this, please ?
Thanks a lot in advance

Answer 1

The question happened because Windows cannot run this DataLoader in 'num_workers' more than 0.
You can see where the trainloader come from.we can see
trainloader = torch.utils.data.DataLoader(trainset, batch_size=4,
shuffle=True, num_workers=2)
We need to change the 'num_workers' to 0.like this:
trainloader = torch.utils.data.DataLoader(trainset, batch_size=4,
shuffle=True, num_workers=0)
Every trainloaders need to change like this.

Answer 2

Got the same error. The following workaround works for me:
def run():
# code goes here
if __name__ == '__main__':
run()

Answer 3

This doesn't look to be a PyTorch problem. Try executing the code in Jupyter notebooks and other environment troubleshooting.

Answer 4

you need to add a if-clause protection as stated in the pytorch docs:
https://pytorch.org/docs/stable/notes/windows.html#usage-multiprocessing