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https://issues.apache.org/jira/browse/ARROW-1940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16300696#comment-16300696
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Wes McKinney commented on ARROW-1940:
-------------------------------------
Marked for 0.9.0 -- we can look into it. I will say that where bytes -> unicode
promotions are occurring it may be challenging to preserve a perfect round trip
on Python 2 in all cases.
> [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
> Fix For: 0.9.0
>
> 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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