[jira] [Created] (ARROW-7066) [Python] support returning ChunkedArray from __arrow_array__ ?

["Joris Van den Bossche (Jira)" <ji...@apache.org>]

Joris Van den Bossche created ARROW-7066:
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             Summary: [Python] support returning ChunkedArray from __arrow_array__ ?
                 Key: ARROW-7066
                 URL: https://issues.apache.org/jira/browse/ARROW-7066
             Project: Apache Arrow
          Issue Type: Improvement
          Components: Python
            Reporter: Joris Van den Bossche
             Fix For: 1.0.0


The {{\_\_arrow_array\_\_}} protocol was added so that custom objects can define how they should be converted to a pyarrow Array (similar to numpy's {{\_\_array\_\_}}). This is then also used to support converting pandas DataFrames with columns using pandas' ExtensionArrays to a pyarrow Table (if the pandas ExtensionArray, such as nullable integer type, implements this {{\_\_arrow_array\_\_}} method).

This last use case could also be useful for fletcher (https://github.com/xhochy/fletcher/, a package that implements pandas ExtensionArrays that wrap pyarrow arrays, so they can be stored as is in a pandas DataFrame).  
However, fletcher stores ChunkedArrays in ExtensionArry / the columns of a pandas DataFrame (to have a better mapping with a Table, where the columns also consist of chunked arrays). While we currently require that the return value of {{\_\_arrow_array\_\_}} is a pyarrow.Array.

So I was wondering: could we relax this constraint and also allow ChunkedArray as return value? 
However, this protocol is currently called in the {{pa.array(..)}} function, which probably should keep returning an Array (and not ChunkedArray in certain cases).

cc [~uwe]



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