[
https://issues.apache.org/jira/browse/ARROW-15545?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Miles Granger reassigned ARROW-15545:
-------------------------------------
Assignee: Miles Granger
> [C++] Cast dictionary of extension type to extension type
> ---------------------------------------------------------
>
> Key: ARROW-15545
> URL: https://issues.apache.org/jira/browse/ARROW-15545
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Joris Van den Bossche
> Assignee: Miles Granger
> Priority: Major
> Labels: good-first-issue, kernel
>
> We support casting a DictionaryArray to its dictionary values' type. For
> example:
> {code}
> >>> arr = pa.array([1, 2, 1]).dictionary_encode()
> >>> arr
> <pyarrow.lib.DictionaryArray object at 0x7f0c1aca46d0>
> -- dictionary:
> [
> 1,
> 2
> ]
> -- indices:
> [
> 0,
> 1,
> 0
> ]
> >>> arr.type
> DictionaryType(dictionary<values=int64, indices=int32, ordered=0>)
> >>> arr.cast(arr.type.value_type)
> <pyarrow.lib.Int64Array object at 0x7f0c19891dc0>
> [
> 1,
> 2,
> 1
> ]
> {code}
> However, if the type of the dictionary values is an ExtensionType, this cast
> is not supported:
> {code}
> >>> from pyarrow.tests.test_extension_type import UuidType
> >>> storage = pa.array([b"0123456789abcdef"], type=pa.binary(16))
> >>> arr = pa.ExtensionArray.from_storage(UuidType(), storage)
> >>> arr
> <pyarrow.lib.ExtensionArray object at 0x7f0c1875bc40>
> [
> 30313233343536373839616263646566
> ]
> >>> dict_arr = pa.DictionaryArray.from_arrays(pa.array([0, 0], pa.int32()),
> >>> arr)
> >>> dict_arr.type
> DictionaryType(dictionary<values=extension<arrow.py_extension_type<UuidType>>,
> indices=int32, ordered=0>)
> >>> dict_arr.cast(UuidType())
> ...
> ArrowNotImplementedError: Unsupported cast from
> dictionary<values=extension<arrow.py_extension_type<UuidType>>,
> indices=int32, ordered=0> to extension<arrow.py_extension_type<UuidType>> (no
> available cast function for target type)
> ../src/arrow/compute/cast.cc:119
> GetCastFunctionInternal(cast_options->to_type, args[0].type().get())
> {code}
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