Error converting Webdataset

Hello, I am trying to convert a imagenet sharded as webdataset tar files into an ffcv dataset.

I downloaded the ffcv-imagnet repo https://github.com/libffcv/ffcv-imagenet

and modified it as best as I could tell from the webdataset tutorial

def pipeline(dataset):
    dataset.decode('rgb8').to_tuple('jpg;png;jpeg cls')

@section('cfg')
@param('dataset')
@param('split')
@param('data_dir')
@param('write_path')
@param('max_resolution')
@param('num_workers')
@param('chunk_size')
@param('subset')
@param('jpeg_quality')
@param('write_mode')
@param('compress_probability')
def main(dataset, split, data_dir, write_path, max_resolution, num_workers,
         chunk_size, subset, jpeg_quality, write_mode,
         compress_probability):

    my_shards = glob(path.join(data_dir, f'*{split}*'))

    writer = DatasetWriter(write_path, {
        'image': RGBImageField(write_mode=write_mode,
                               max_resolution=max_resolution,
                               compress_probability=compress_probability,
                               jpeg_quality=jpeg_quality),
        'label': IntField(),
    }, num_workers=num_workers)

    writer.from_webdataset(my_shards, pipeline=pipeline)

I am getting an error

Traceback (most recent call last):
  File "/home/cc/ffcv-imagenet/write_imagenet_web.py", line 67, in <module>
    main()
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/site-packages/fastargs/decorators.py", line 41, in __call__
    raise e
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/site-packages/fastargs/decorators.py", line 35, in __call__
    return self.func(*args, **filled_args)
  File "/home/cc/ffcv-imagenet/write_imagenet_web.py", line 58, in main
    writer.from_webdataset(my_shards, pipeline=pipeline)
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/site-packages/ffcv/writer.py", line 311, in from_webdataset
    lengths = thread_map(counter, shards, max_workers=self.num_workers)
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/site-packages/tqdm/contrib/concurrent.py", line 94, in thread_map
    return _executor_map(ThreadPoolExecutor, fn, *iterables, **tqdm_kwargs)
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/site-packages/tqdm/contrib/concurrent.py", line 76, in _executor_map
    return list(tqdm_class(ex.map(fn, *iterables, **map_args), **kwargs))
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/site-packages/tqdm/std.py", line 1195, in __iter__
    for obj in iterable:
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/concurrent/futures/_base.py", line 609, in result_iterator
    yield fs.pop().result()
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/concurrent/futures/_base.py", line 446, in result
    return self.__get_result()
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/concurrent/futures/_base.py", line 391, in __get_result
    raise self._exception
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/cc/miniconda3/envs/ffcv/lib/python3.9/site-packages/ffcv/writer.py", line 36, in count_samples_in_shard
    for _ in from_shard(shard, pipeline):
TypeError: 'NoneType' object is not iterable

it looks like an error with either the WebDataset or the pipeline (from writer.py)

def from_shard(shard, pipeline):
    # We import webdataset here so that it desn't crash if it's not required
    # (Webdataset is an optional depdency)
    from webdataset import WebDataset

    dataset = WebDataset(shard)
    dataset = pipeline(dataset)
    return dataset

def count_samples_in_shard(shard, pipeline):
    #
    # We count the length of the dataset
    # We are not using __len__ since it might not be implemented
    count = 0
    print(shard)
    for _ in from_shard(shard, pipeline):
        count += 1

    return count

what could cause the dataset to return None? I double checked that the glob is correct and returns the right paths to the tar files when called. I'm kinda at a loss here.

libffcv / ffcv

Error converting Webdataset #212