[jira] [Updated] (ARROW-18099) [Python] Cannot create pandas categorical from table only with nulls

["Alenka Frim (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-18099?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Alenka Frim updated ARROW-18099:
--------------------------------
    Summary: [Python] Cannot create pandas categorical from table only with nulls  (was: Cannot create pandas categorical from table only with nulls)

> [Python] Cannot create pandas categorical from table only with nulls
> --------------------------------------------------------------------
>
>                 Key: ARROW-18099
>                 URL: https://issues.apache.org/jira/browse/ARROW-18099
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: Python
>    Affects Versions: 9.0.0
>         Environment: OSX 12.6
> M1 silicon
>            Reporter: Damian Barabonkov
>            Priority: Minor
>
> A pyarrow Table with only null values cannot be instantiated as a Pandas DataFrame with said column as a category. However, pandas does support "empty" categoricals. Therefore, a simple patch would be to load the pa.Table as an object first and convert, once in pandas, to a categorical which will be empty. However, that does not solve the pyarrow bug at its root.
>  
> Sample reproducible example
> {code:java}
> import pyarrow as pa
> pylist = [{'x': None, '__index_level_0__': 2}, {'x': None, '__index_level_0__': 3}]
> tbl = pa.Table.from_pylist(pylist)
>  
> # Errors
> df_broken = tbl.to_pandas(categories=["x"])
>  
> # Works
> df_works = tbl.to_pandas()
> df_works = df_works.astype({"x": "category"}) {code}



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