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https://issues.apache.org/jira/browse/ARROW-14500?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17470291#comment-17470291
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Will Ayd commented on ARROW-14500:
----------------------------------
Did a little research here. I see the casting from an extension type is handled
here:
[https://github.com/apache/arrow/blob/e64480db51fc9622d02613f3ec60bac34d765092/cpp/src/arrow/compute/kernels/scalar_cast_internal.cc#L190]
So the other way around could be pretty similar. However, I'm wondering if a
cast to an extension type is truly possible? When coming from an extension type
the built-in type can be cast from the extension's physical storage. But would
going the other way around would require a creation of a new ExtensionArray
instance that populates some of the additional methods?
> [C++] Support casting from storage type to extension type
> ---------------------------------------------------------
>
> Key: ARROW-14500
> URL: https://issues.apache.org/jira/browse/ARROW-14500
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Joris Van den Bossche
> Priority: Major
> Labels: good-first-issue, kernel
>
> We already support casting an extension array to its storage type
> (ARROW-7858):
> {code}
> >>> from pyarrow.tests.test_extension_type import IntegerType
> >>> arr = pa.array([1, 2, 3], pa.int64())
> >>> arr_ext = pa.ExtensionArray.from_storage(IntegerType(), arr)
> >>> arr_ext.cast(arr.type)
> Out[29]:
> <pyarrow.lib.Int64Array object at 0x7f7b0d4739a0>
> [
> 1,
> 2,
> 3
> ]
> {code}
> but we don't support the cast the other way around from storage type to
> extension type:
> {code}
> >>> arr.cast(arr_ext.type)
> ...
> ArrowNotImplementedError: Unsupported cast from int64 to
> extension<arrow.py_extension_type<IntegerType>> (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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