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https://issues.apache.org/jira/browse/ARROW-15643?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17490148#comment-17490148
]
&res commented on ARROW-15643:
------------------------------
Thanks for raising the issue.
I've noticed that you can't cast a struct array to a sub set of the struct. So
for example:
{code:python}
import pyarrow as pa
struct_type = pa.struct(
[pa.field("field1", pa.string()), pa.field("field2", pa.int32())]
)
sub_struct_type = pa.struct(
[
pa.field("field1", pa.string()),
]
)
struct_array = pa.array(
[
("ABC", 123),
("EFG", 456),
],
struct_type,
)
struct_array.cast(sub_struct_type)
{code}
Gives you:
{code}
return call_function("cast", [arr], options)
File "pyarrow/_compute.pyx", line 527, in pyarrow._compute.call_function
File "pyarrow/_compute.pyx", line 337, in pyarrow._compute.Function.call
File "pyarrow/error.pxi", line 143, in
pyarrow.lib.pyarrow_internal_check_status
File "pyarrow/error.pxi", line 120, in pyarrow.lib.check_status
pyarrow.lib.ArrowNotImplementedError: Unsupported cast from struct<field1:
string, field2: int32> to struct using function cast_struct
{code}
So one option would be to support this type of cast.
> [C++] Kernel to select subset of fields of a StructArray
> --------------------------------------------------------
>
> Key: ARROW-15643
> URL: https://issues.apache.org/jira/browse/ARROW-15643
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Joris Van den Bossche
> Priority: Major
>
> Triggered by
> https://stackoverflow.com/questions/71035754/pyarrow-drop-a-column-in-a-nested-structure.
> I thought there was already an issue about this, but don't directly find one.
> Assume you have a struct array with some fields:
> {code}
> >>> arr = pa.StructArray.from_arrays([[1, 2, 3]]*3, names=['a', 'b', 'c'])
> >>> arr.type
> StructType(struct<a: int64, b: int64, c: int64>)
> {code}
> We have a kernel to select a single child field:
> {code}
> >>> pc.struct_field(arr, [0])
> <pyarrow.lib.Int64Array object at 0x7ffa9e229940>
> [
> 1,
> 2,
> 3
> ]
> {code}
> But if you want to subset the StructArray to some of its fields, resulting in
> a new StructArray, that's not possible with {{struct_field}}, and doing this
> manually is a bit cumbersome:
> {code}
> >>> fields = ['a', 'c']
> >>> arrays = [arr.field(n) for n in fields]
> >>> arr_subset = pa.StructArray.from_arrays(arrays, names=fields)
> >>> arr_subset.type
> StructType(struct<a: int64, c: int64>)
> {code}
> (this is still OK, but if you had a ChunkedArray, it certainly gets annoying)
> One option could be to expand the existing {{struct_field}} to allow
> selecting multiple fields (although that probably gets ambigous/confusing
> with how you currently select a recursively nested field -> [0, 1] currently
> means "first child, second subchild" and not "first and second child").
> Or a new kernel like "struct_subset" or some other name.
> This might also overlap with general projection functionality? (cc
> [~westonpace])
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