[GitHub] [arrow] wjones127 commented on pull request #37292: GH-20086: [C++] Cast between fixed size and variable size lists

["wjones127 (via GitHub)" <gi...@apache.org>]

wjones127 commented on PR #37292:
URL: https://github.com/apache/arrow/pull/37292#issuecomment-1687384784

   Python example:
   
   ```python
   >>> import pyarrow as pa
   >>> arr = pa.array([[1, 2, 3], [4, 5, 6]])
   >>> arr.type
   ListType(list<item: int64>)
   >>> arr.cast(pa.list_(pa.int64(), 3))
   <pyarrow.lib.FixedSizeListArray object at 0x132793e80>
   [
     [
       1,
       2,
       3
     ],
     [
       4,
       5,
       6
     ]
   ]
   >>> arr.cast(pa.list_(pa.int64(), 3)).cast(pa.list_(pa.int64()))
   <pyarrow.lib.ListArray object at 0x132793ee0>
   [
     [
       1,
       2,
       3
     ],
     [
       4,
       5,
       6
     ]
   ]
   >>> arr.cast(pa.list_(pa.int64(), 2))
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
     File "pyarrow/array.pxi", line 967, in pyarrow.lib.Array.cast
       return _pc().cast(self, target_type, safe=safe,
     File "/Users/willjones/Documents/arrow/python/pyarrow/compute.py", line 403, in cast
       return call_function("cast", [arr], options, memory_pool)
     File "pyarrow/_compute.pyx", line 572, in pyarrow._compute.call_function
       return func.call(args, options=options, memory_pool=memory_pool,
     File "pyarrow/_compute.pyx", line 367, in pyarrow._compute.Function.call
       result = GetResultValue(
     File "pyarrow/error.pxi", line 144, in pyarrow.lib.pyarrow_internal_check_status
       return check_status(status)
     File "pyarrow/error.pxi", line 100, in pyarrow.lib.check_status
       raise ArrowInvalid(message)
   pyarrow.lib.ArrowInvalid: ListType can only be casted to FixedSizeListType if the lists are all the expected size.
   /Users/willjones/Documents/arrow/cpp/src/arrow/compute/exec.cc:920  kernel_->exec(kernel_ctx_, input, &output)
   /Users/willjones/Documents/arrow/cpp/src/arrow/compute/function.cc:276  executor->Execute(input, &listener)
   ```


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