[jira] [Assigned] (ARROW-1940) [Python] Extra metadata gets added after multiple conversions between pd.DataFrame and pa.Table

["Phillip Cloud (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-1940?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Phillip Cloud reassigned ARROW-1940:
------------------------------------

    Assignee: Phillip Cloud

> [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
>            Assignee: Phillip Cloud
>            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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