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

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 0.8.0
• Fix Version/s: 0.9.0
• Component/s: Python
• Labels: pull-request-available
• External issue URL: https://github.com/apache/arrow/issues/17930
• Assignee: Phillip Cloud
• Reporter: Dima Ryazanov
• Created: 20/Dec/17 08:21
• Updated: 11/Jan/23 07:17
• Resolved: 09/Mar/18 04:11

Description

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):

"column_indexes": [{"metadata": null, "field_name": null, "name": null, "numpy_type": "object", "pandas_type": "bytes"}]

However, after converting it to pd.DataFrame and back into a pa.Table for the second time, the metadata gets an encoding field:

"column_indexes": [{"metadata": {"encoding": "UTF-8"}, "field_name": null, "name": null, "numpy_type": "object", "pandas_type": "unicode"}]

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.

Issue Links

• links to: GitHub Pull Request #1728