[jira] [Commented] (ARROW-2515) Errors with DictionaryArray inside of ListArray or other DictionaryArray

["Rok Mihevc (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-2515?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17659541#comment-17659541 ] 

Rok Mihevc commented on ARROW-2515:
-----------------------------------

This issue has been migrated to [issue #18791|https://github.com/apache/arrow/issues/18791] on GitHub. Please see the [migration documentation|https://github.com/apache/arrow/issues/14542] for further details.

> Errors with DictionaryArray inside of ListArray or other DictionaryArray
> ------------------------------------------------------------------------
>
>                 Key: ARROW-2515
>                 URL: https://issues.apache.org/jira/browse/ARROW-2515
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: Python
>    Affects Versions: 0.9.0
>            Reporter: Brent Kerby
>            Assignee: Brent Kerby
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 0.10.0
>
>   Original Estimate: 1h
>          Time Spent: 2h
>  Remaining Estimate: 0h
>
> An exception ("KeyError: 26") is raised when .as_py() is called on elements of a ListArray over a DictionaryArray, or of a DictionaryArray with values in a DictionaryArray. Here are a couple tests that currently fail:
>  
> {code:java}
> import pyarrow as pa
> def test_dictionary_array_1():
>     dict_arr = pa.DictionaryArray.from_arrays([0, 1, 0], ['a', 'b'])
>     list_arr = pa.ListArray.from_arrays([0, 2, 3], dict_arr)
>     assert list_arr.to_pylist() == [['a', 'b'], ['a']]
> def test_dictionary_array_2():
>     dict_arr = pa.DictionaryArray.from_arrays([0, 1, 0], ['a', 'b'])
>     dict_arr2 = pa.DictionaryArray.from_arrays([0, 1, 2, 1, 0], dict_arr)
>     assert dict_arr2.to_pylist() == ['a', 'b', 'a', 'b', 'a']
> {code}
> It appears that the problem is caused by the fact that the function box_scalar in scalar.pxi does not handle the case of dictionary array, as we currently have no DictionaryValue type. 
> DictionaryArray.__getitem__ currently works around the lack of DictionaryValue type by dereferencing the index and constructing a scalar based on the value in the underlying dictionary. In other words, if we have a dictionary with int8 indices and string values, then the result of __getitem__ will be a StringValue (rather than a DictionaryValue). This works in simple cases but not in the more complex scenarios illustrated above.
> I have a patch ready, which would add a DictionaryValue type similar to other scalar types, resolving these bugs and removing the need for a special-cased implementation of DictionaryArray.__getitem__. This DictionaryValue would contain a couple accessor properties, "indices_value" and "dictionary_value" to allow access to both the index in the dictionary as well as the looked-up value. Then DictionaryValue.as_py() would simply call .as_py() on the underlying dictionary_value. 



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