Joris Van den Bossche created ARROW-10462:
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Summary: [Python] ParquetDatasetPiece's path broken when using
fsspec fs on Windows
Key: ARROW-10462
URL: https://issues.apache.org/jira/browse/ARROW-10462
Project: Apache Arrow
Issue Type: Bug
Components: Python
Reporter: Joris Van den Bossche
Fix For: 2.0.1
Dask reported some failures starting with the pyarrow 2.0 release, and
specifically on Windows: https://github.com/dask/dask/issues/6754
After some investigation, it seems that this is due to the
{{ParquetDatasetPiece}} its {{path}} attribute now returning a path with a
mixture of {{\\}} and {/}} in it.
It specifically happens when dask is passing a posix-style base path pointing
to the dataset base directory (so using all {{/}}), and passing an fsspec-based
(local) filesystem.
>From a debugging output during one of the dask tests:
{code}
(Pdb) dataset
<pyarrow.parquet.ParquetDataset object at 0x00000290D7506308>
(Pdb) dataset.paths
'C:/Users/joris/AppData/Local/Temp/pytest-of-joris/pytest-25/test_partition_on_pyarrow_0'
(Pdb) dataset.pieces[0].path
'C:/Users/joris/AppData/Local/Temp/pytest-of-joris/pytest-25/test_partition_on_pyarrow_0\\a1=A\\a2=X\\part.0.parquet'
{code}
So you can see that the result here has a mix of {{\\}} and {{/}}. Using
pyarrow 1.0, this was consistently using {{/}}.
The reason for the change is that in pyarrow 2.0 we started to replace fsspec
LocalFileSystem with our own LocalFileSystem (assuming for a local filesystem
that should be equivalent). But it seems that our own LocalFileSystem has a
{{pathsep}}} property that equals to {{os.path.sep}}, which is {{\\}} on
Windows
(https://github.com/apache/arrow/blob/9231976609d352b7050f5c706b86c15e8c604927/python/pyarrow/filesystem.py#L304-L306.
So note that while this started being broken in pyarrow 2.0 when using fsspec
filesystem, this was already "broken" before when using our own local
filesystem (or when not passing any filesystem). But, 1) dask always passes an
fsspec filesystem, and 2) dask uses the piece's path as dictionary key and is
thus especially sensitive to the change (using it as a file path to read
something in, it will probably still work even with the mixture of path
separators).
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