[C++] Cast dictionary of extension type to extension type

[asfimport]

We support casting a DictionaryArray to its dictionary values' type. For example:

>>> 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
]

However, if the type of the dictionary values is an ExtensionType, this cast is not supported:

>>> 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())

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Miles Granger / @milesgranger
Watchers: Rok Mihevc / @rok

Related issues:

• [C++] Support casting from storage type to extension type (relates to)
• [C++][Parquet] Writing DictionaryArrays with ExtensionType to Parquet (is depended upon by)
• [C++][Python] Support for pandas Categoricals with Intervals (is depended upon by)

PRs and other links:

• GitHub Pull Request #14106

_{Note: This issue was originally created as ARROW-15545. Please see the migration documentation for further details.}

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
Issue resolved by pull request 14106
#14106