"counter" dataset raises an error in normal mode, but not in streaming mode
Opened by severo (closed)

## Describe the bug

`counter` dataset raises an error on `load_dataset()`, but simply returns an empty iterator in streaming mode.

## Steps to reproduce the bug

```python
>>> import datasets as ds
>>> a = ds.load_dataset('counter', split="train", streaming=False)
Using custom data configuration default
Downloading and preparing dataset counter/default (download: 1.29 MiB, generated: 2.48 MiB, post-processed: Unknown size, total: 3.77 MiB) to /home/slesage/.cache/huggingface/datasets/counter/default/1.0.0/9f84962fa0f35bec5a34fe0bdff8681838d497008c457f7856c48654476ec0e9...
Traceback (most recent call last):
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.8/site-packages/datasets/builder.py", line 726, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.8/site-packages/datasets/builder.py", line 1124, in _prepare_split
    for key, record in utils.tqdm(
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.8/site-packages/tqdm/std.py", line 1185, in __iter__
    for obj in iterable:
  File "/home/slesage/.cache/huggingface/modules/datasets_modules/datasets/counter/9f84962fa0f35bec5a34fe0bdff8681838d497008c457f7856c48654476ec0e9/counter.py", line 161, in _generate_examples
    with derived_file.open(encoding="utf-8") as f:
  File "/home/slesage/.pyenv/versions/3.8.11/lib/python3.8/pathlib.py", line 1222, in open
    return io.open(self, mode, buffering, encoding, errors, newline,
  File "/home/slesage/.pyenv/versions/3.8.11/lib/python3.8/pathlib.py", line 1078, in _opener
    return self._accessor.open(self, flags, mode)
FileNotFoundError: [Errno 2] No such file or directory: '/home/slesage/.cache/huggingface/datasets/downloads/extracted/b57aa6db5601a738e57b95c1fd8cced54ff28fc540efcdaf0f6c4f1bb5dfe211/COUNTER/0032p.xml'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.8/site-packages/datasets/load.py", line 1112, in load_dataset
    builder_instance.download_and_prepare(
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.8/site-packages/datasets/builder.py", line 636, in download_and_prepare
    self._download_and_prepare(
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.8/site-packages/datasets/builder.py", line 728, in _download_and_prepare
    raise OSError(
OSError: Cannot find data file.
Original error:
[Errno 2] No such file or directory: '/home/slesage/.cache/huggingface/datasets/downloads/extracted/b57aa6db5601a738e57b95c1fd8cced54ff28fc540efcdaf0f6c4f1bb5dfe211/COUNTER/0032p.xml'
```

```python
>>> import datasets as ds
>>> b = ds.load_dataset('counter', split="train", streaming=True)
Using custom data configuration default
>>> list(b)
[]
```

## Expected results

An exception should be raised in streaming mode

## Actual results

No exception is raised in streaming mode: there is no way to tell if something has broken or if the dataset is simply empty.

## Environment info

- `datasets` version: 1.11.1.dev0
- Platform: Linux-5.11.0-1016-aws-x86_64-with-glibc2.29
- Python version: 3.8.11
- PyArrow version: 4.0.1


Comments:
Hi @severo, thanks for reporting.

Just note that currently not all canonical datasets support streaming mode: this is one case!

All datasets that use `pathlib` joins (using `/`) instead of `os.path.join` (as in this dataset) do not support streaming mode yet.

OK. Do you think it's possible to detect this, and raise an exception (maybe `NotImplementedError`, or a specific `StreamingError`)?

We should definitely support datasets using `pathlib` in streaming mode...

For non-supported datasets in streaming mode, we have already a request of raising an error/warning: see #2654.

Hi @severo, please note that "counter" dataset will be streamable (at least until it arrives at the missing file, error already in normal mode) once these PRs are merged:
- #2874
- #2876
- #2880

I have tested it. 😉 

Now (on master), we get:

```
import datasets as ds
ds.load_dataset('counter', split="train", streaming=False)
```

```
Using custom data configuration default
Downloading and preparing dataset counter/default (download: 1.29 MiB, generated: 2.48 MiB, post-processed: Unknown size, total: 3.77 MiB) to /home/slesage/.cache/huggingface/datasets/counter/default/1.0.0/9f84962fa0f35bec5a34fe0bdff8681838d497008c457f7856c48654476ec0e9...
Traceback (most recent call last):
  File "/home/slesage/hf/datasets/src/datasets/builder.py", line 726, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/slesage/hf/datasets/src/datasets/builder.py", line 1124, in _prepare_split
    for key, record in utils.tqdm(
  File "/home/slesage/hf/datasets/.venv/lib/python3.8/site-packages/tqdm/std.py", line 1185, in __iter__
    for obj in iterable:
  File "/home/slesage/.cache/huggingface/modules/datasets_modules/datasets/counter/9f84962fa0f35bec5a34fe0bdff8681838d497008c457f7856c48654476ec0e9/counter.py", line 161, in _generate_examples
    with derived_file.open(encoding="utf-8") as f:
  File "/home/slesage/.pyenv/versions/3.8.11/lib/python3.8/pathlib.py", line 1222, in open
    return io.open(self, mode, buffering, encoding, errors, newline,
  File "/home/slesage/.pyenv/versions/3.8.11/lib/python3.8/pathlib.py", line 1078, in _opener
    return self._accessor.open(self, flags, mode)
FileNotFoundError: [Errno 2] No such file or directory: '/home/slesage/.cache/huggingface/datasets/downloads/extracted/b57aa6db5601a738e57b95c1fd8cced54ff28fc540efcdaf0f6c4f1bb5dfe211/COUNTER/0032p.xml'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/slesage/hf/datasets/src/datasets/load.py", line 1112, in load_dataset
    builder_instance.download_and_prepare(
  File "/home/slesage/hf/datasets/src/datasets/builder.py", line 636, in download_and_prepare
    self._download_and_prepare(
  File "/home/slesage/hf/datasets/src/datasets/builder.py", line 728, in _download_and_prepare
    raise OSError(
OSError: Cannot find data file.
Original error:
[Errno 2] No such file or directory: '/home/slesage/.cache/huggingface/datasets/downloads/extracted/b57aa6db5601a738e57b95c1fd8cced54ff28fc540efcdaf0f6c4f1bb5dfe211/COUNTER/0032p.xml'
```

The error is now the same with or without streaming. I close the issue, thanks @albertvillanova and @lhoestq!


Note that we might want to open an issue to fix the "counter" dataset by itself, but I let it up to you.

Fixed here: https://github.com/huggingface/datasets/pull/2894. Thanks @albertvillanova 

On master, I get:

```python
>>> import datasets as ds
>>> iterable_dataset = ds.load_dataset('counter', split="train", streaming=True)
>>> rows = list(iterable_dataset.take(100))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/slesage/hf/datasets/src/datasets/iterable_dataset.py", line 341, in __iter__
    for key, example in self._iter():
  File "/home/slesage/hf/datasets/src/datasets/iterable_dataset.py", line 338, in _iter
    yield from ex_iterable
  File "/home/slesage/hf/datasets/src/datasets/iterable_dataset.py", line 273, in __iter__
    yield from islice(self.ex_iterable, self.n)
  File "/home/slesage/hf/datasets/src/datasets/iterable_dataset.py", line 78, in __iter__
    for key, example in self.generate_examples_fn(**self.kwargs):
  File "/home/slesage/.cache/huggingface/modules/datasets_modules/datasets/counter/b9e4378dbd3f5ce235d2302e48168c00196e67bbcd13cc7e1f6e69ef82c0cf2a/counter.py", line 153, in _generate_examples
    files = sorted(base_path.glob(r"[0-9][0-9][0-9][0-9].xml"))
TypeError: xpathglob() missing 1 required positional argument: 'pattern'
```

Associated to the above exception, if I create a test and run it with pytest, I get an awful traceback.

- create a file `test_counter.py`

```python
import pytest
from datasets import load_dataset, IterableDataset
from typing import Any, cast


def test_counter() -> Any:
    iterable_dataset = cast(IterableDataset, load_dataset("counter", split="train", streaming=True))
    with pytest.raises(TypeError):
        list(iterable_dataset.take(100))
```

- run the test with pytest

```bash
$ python -m pytest -x test_counter.py
============================================================================================================================= test session starts ==============================================================================================================================
platform linux -- Python 3.9.6, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: /home/slesage/hf/datasets-preview-backend, configfile: pyproject.toml
plugins: anyio-3.3.2, cov-2.12.1
collected 1 item

tests/test_counter.py .                                                                                                                                                                                                                                                  [100%]Traceback (most recent call last):
  File "/home/slesage/.pyenv/versions/3.9.6/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/slesage/.pyenv/versions/3.9.6/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pytest/__main__.py", line 5, in <module>
    raise SystemExit(pytest.console_main())
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/_pytest/config/__init__.py", line 185, in console_main
    code = main()
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/_pytest/config/__init__.py", line 162, in main
    ret: Union[ExitCode, int] = config.hook.pytest_cmdline_main(
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_hooks.py", line 265, in __call__
    return self._hookexec(self.name, self.get_hookimpls(), kwargs, firstresult)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_manager.py", line 80, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_callers.py", line 60, in _multicall
    return outcome.get_result()
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_result.py", line 60, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_callers.py", line 39, in _multicall
    res = hook_impl.function(*args)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/_pytest/main.py", line 316, in pytest_cmdline_main
    return wrap_session(config, _main)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/_pytest/main.py", line 304, in wrap_session
    config.hook.pytest_sessionfinish(
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_hooks.py", line 265, in __call__
    return self._hookexec(self.name, self.get_hookimpls(), kwargs, firstresult)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_manager.py", line 80, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_callers.py", line 55, in _multicall
    gen.send(outcome)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/_pytest/terminal.py", line 803, in pytest_sessionfinish
    outcome.get_result()
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_result.py", line 60, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/pluggy/_callers.py", line 39, in _multicall
    res = hook_impl.function(*args)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/_pytest/cacheprovider.py", line 428, in pytest_sessionfinish
    config.cache.set("cache/nodeids", sorted(self.cached_nodeids))
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/_pytest/cacheprovider.py", line 188, in set
    f = path.open("w")
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/utils/streaming_download_manager.py", line 199, in xpathopen
    return xopen(_as_posix(path), *args, **kwargs)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/utils/streaming_download_manager.py", line 117, in _as_posix
    path_as_posix = path.as_posix()
AttributeError: 'str' object has no attribute 'as_posix'
```


I opened a PR to fix these issues.
Also in your test you expect a TypeError but I don't know why. On my side it works fine without raising a TypeError

I had the issue (TypeError raised) on my branch, but it's fixed now. Thanks