fix up to_arrow for datatype and add some tests

python/cudf/cudf/_lib/pylibcudf/interop.pyx

@@ -209,15 +209,28 @@ def to_arrow(cudf_object, metadata=None):
 
 
 @to_arrow.register(DataType)
-def _to_arrow_datatype(cudf_object):
-    if cudf_object.id() == type_id.DECIMAL128:
-        return pa.decimal128(-cudf_object.scale)
-    elif cudf_object.id() in {type_id.LIST, type_id.STRUCT}:
-        # TODO: need column metadata
-        raise ValueError(
-            f"Cannot convert {cudf_object} to PyArrow type"
-        )
-    return ARROW_TO_PYLIBCUDF_TYPES.get(cudf_object.id())
+def _to_arrow_datatype(cudf_object, **kwargs):
+    if cudf_object.id() in {type_id.DECIMAL32, type_id.DECIMAL64, type_id.DECIMAL128}:
+        if not (precision := kwargs.get("precision")):
+            raise ValueError(
+                "Precision must be provided for decimal types"
+            )
+            # no pa.decimal32 or pa.decimal64
+        return pa.decimal128(precision, -cudf_object.scale())
+    elif cudf_object.id() == type_id.STRUCT:
+        if not (fields := kwargs.get("fields")):
+            raise ValueError(
+                "Fields must be provided for struct types"
+            )
+        return pa.struct(fields)
+    elif cudf_object.id() == type_id.LIST:
+        if not (value_type := kwargs.get("value_type")):
+            raise ValueError(
+                "Value type must be provided for list types"
+            )
+        return pa.list_(value_type)
+    else:
+        return ARROW_TO_PYLIBCUDF_TYPES.get(cudf_object.id())
 
 
 @to_arrow.register(Table)

python/cudf/cudf/pylibcudf_tests/test_interop.py

@@ -0,0 +1,66 @@
+# Copyright (c) 2024, NVIDIA CORPORATION.
+
+import pyarrow as pa
+import pytest
+
+import cudf._lib.pylibcudf as plc
+
+
+def test_list_dtype_roundtrip():
+    list_type = pa.list_(pa.int32())
+    plc_type = plc.interop.from_arrow(list_type)
+
+    assert plc_type == plc.types.DataType(plc.types.TypeId.LIST)
+
+    with pytest.raises(ValueError):
+        plc.interop.to_arrow(plc_type)
+
+    arrow_type = plc.interop.to_arrow(
+        plc_type, value_type=list_type.value_type
+    )
+    assert arrow_type == list_type
+
+
+def test_struct_dtype_roundtrip():
+    struct_type = pa.struct([("a", pa.int32()), ("b", pa.string())])
+    plc_type = plc.interop.from_arrow(struct_type)
+
+    assert plc_type == plc.types.DataType(plc.types.TypeId.STRUCT)
+
+    with pytest.raises(ValueError):
+        plc.interop.to_arrow(plc_type)
+
+    arrow_type = plc.interop.to_arrow(plc_type, fields=struct_type)
+    assert arrow_type == struct_type
+
+
+def test_decimal128_roundtrip():
+    decimal_type = pa.decimal128(10, 2)
+    plc_type = plc.interop.from_arrow(decimal_type)
+
+    assert plc_type.id() == plc.types.TypeId.DECIMAL128
+
+    with pytest.raises(ValueError):
+        plc.interop.to_arrow(plc_type)
+
+    arrow_type = plc.interop.to_arrow(
+        plc_type, precision=decimal_type.precision
+    )
+    assert arrow_type == decimal_type
+
+
+@pytest.mark.parametrize(
+    "data_type",
+    [
+        plc.types.DataType(plc.types.TypeId.DECIMAL32),
+        plc.types.DataType(plc.types.TypeId.DECIMAL64),
+    ],
+)
+def test_decimal_other(data_type):
+    precision = 3
+
+    with pytest.raises(ValueError):
+        plc.interop.to_arrow(data_type)
+
+    arrow_type = plc.interop.to_arrow(data_type, precision=precision)
+    assert arrow_type == pa.decimal128(precision, 0)