Dima Ryazanov created ARROW-1940:
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Summary: [Python] Extra metadata gets added after multiple
conversions between pd.DataFrame and pa.Table
Key: ARROW-1940
URL: https://issues.apache.org/jira/browse/ARROW-1940
Project: Apache Arrow
Issue Type: Bug
Components: Python
Affects Versions: 0.8.0
Reporter: Dima Ryazanov
Priority: Minor
Attachments: fail.py
We have a unit test that verifies that loading a dataframe from a .parq file
and saving it back with no changes produces the same result as the original
file. It started failing with pyarrow 0.8.0.
After digging into it, I discovered that after the first conversion from
pd.DataFrame to pa.Table, the table contains the following metadata (among
other things):
{code}
"column_indexes": [{"metadata": null, "field_name": null, "name": null,
"numpy_type": "object", "pandas_type": "bytes"}]
{code}
However, after converting it to pd.DataFrame and back into a pa.Table for the
second time, the metadata gets an encoding field:
{code}
"column_indexes": [{"metadata": {"encoding": "UTF-8"}, "field_name": null,
"name": null, "numpy_type": "object", "pandas_type": "unicode"}]
{code}
See the attached file for a test case.
So specifically, it appears that dataframe->table->dataframe->table conversion
produces a different result from just dataframe->table - which I think is
unexpected.
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