Copy nested types upon construction

python/cudf/cudf/core/column/column.py

@@ -40,7 +40,12 @@
 from cudf._typing import BinaryOperand, ColumnLike, Dtype, ScalarLike
 from cudf.core.abc import Serializable
 from cudf.core.buffer import Buffer
-from cudf.core.dtypes import CategoricalDtype, IntervalDtype
+from cudf.core.dtypes import (
+    CategoricalDtype,
+    IntervalDtype,
+    ListDtype,
+    StructDtype,
+)
 from cudf.utils import ioutils, utils
 from cudf.utils.dtypes import (
     check_cast_unsupported_dtype,
@@ -399,8 +404,7 @@ def from_arrow(cls, array: pa.Array) -> ColumnBase:
             "None"
         ]
 
-        if isinstance(result.dtype, cudf.Decimal64Dtype):
-            result.dtype.precision = array.type.precision
+        result = _copy_type_metadata_from_arrow(array, result)
         return result
 
     def _get_mask_as_column(self) -> ColumnBase:
@@ -2346,3 +2350,57 @@ def full(size: int, fill_value: ScalarLike, dtype: Dtype = None) -> ColumnBase:
     dtype: int8
     """
     return ColumnBase.from_scalar(cudf.Scalar(fill_value, dtype), size)
+
+
+def _copy_type_metadata_from_arrow(
+    arrow_array: pa.array, cudf_column: ColumnBase
+) -> ColumnBase:
+    """
+    Similar to `Column._copy_type_metadata`, except copies type metadata
+    from arrow array into a cudf column. Recursive for every level.
+    * When `arrow_array` is struct type and `cudf_column` is StructDtype, copy
+    field names.
+    * When `arrow_array` is decimal type and `cudf_column` is
+    Decimal64Dtype, copy precisions.
+    """
+    if pa.types.is_decimal(arrow_array.type) and isinstance(
+        cudf_column, cudf.core.column.DecimalColumn
+    ):
+        cudf_column.dtype.precision = arrow_array.type.precision
+    elif pa.types.is_struct(arrow_array.type) and isinstance(
+        cudf_column, cudf.core.column.StructColumn
+    ):
+        base_children = tuple(
+            _copy_type_metadata_from_arrow(arrow_array.field(i), col_child)
+            for i, col_child in enumerate(cudf_column.base_children)
+        )
+        cudf_column.set_base_children(base_children)
+        return cudf.core.column.StructColumn(
+            data=None,
+            size=cudf_column.base_size,
+            dtype=StructDtype.from_arrow(arrow_array.type),
+            mask=cudf_column.base_mask,
+            offset=cudf_column.offset,
+            null_count=cudf_column.null_count,
+            children=base_children,
+        )
+    elif pa.types.is_list(arrow_array.type) and isinstance(
+        cudf_column, cudf.core.column.ListColumn
+    ):
+        if arrow_array.values and cudf_column.base_children:
+            base_children = (
+                cudf_column.base_children[0],
+                _copy_type_metadata_from_arrow(
+                    arrow_array.values, cudf_column.base_children[1]
+                ),
+            )
+            return cudf.core.column.ListColumn(
+                size=cudf_column.base_size,
+                dtype=ListDtype.from_arrow(arrow_array.type),
+                mask=cudf_column.base_mask,
+                offset=cudf_column.offset,
+                null_count=cudf_column.null_count,
+                children=base_children,
+            )
+
+    return cudf_column

python/cudf/cudf/core/dtypes.py

@@ -143,6 +143,8 @@ def __init__(self, element_type: Any) -> None:
     def element_type(self) -> Dtype:
         if isinstance(self._typ.value_type, pa.ListType):
             return ListDtype.from_arrow(self._typ.value_type)
+        elif isinstance(self._typ.value_type, pa.StructType):
+            return StructDtype.from_arrow(self._typ.value_type)
         else:
             return np.dtype(self._typ.value_type.to_pandas_dtype()).name
 
@@ -176,10 +178,10 @@ def __eq__(self, other):
         return self._typ.equals(other._typ)
 
     def __repr__(self):
-        if isinstance(self.element_type, ListDtype):
-            return f"ListDtype({self.element_type.__repr__()})"
+        if isinstance(self.element_type, (ListDtype, StructDtype)):
+            return f"{type(self).__name__}({self.element_type.__repr__()})"
         else:
-            return f"ListDtype({self.element_type})"
+            return f"{type(self).__name__}({self.element_type})"
 
     def __hash__(self):
         return hash(self._typ)

python/cudf/cudf/tests/test_dtypes.py

@@ -6,14 +6,16 @@
 import pytest
 
 import cudf
+from cudf.core.column import ColumnBase
 from cudf.core.dtypes import (
     CategoricalDtype,
     Decimal64Dtype,
+    IntervalDtype,
     ListDtype,
     StructDtype,
-    IntervalDtype,
 )
 from cudf.tests.utils import assert_eq
+from cudf.utils.dtypes import np_to_pa_dtype
 
 
 def test_cdt_basic():
@@ -155,3 +157,105 @@ def test_interval_dtype_pyarrow_round_trip(fields, closed):
     expect = pa_array
     got = IntervalDtype.from_arrow(expect).to_arrow()
     assert expect.equals(got)
+
+
+def assert_column_array_dtype_equal(column: ColumnBase, array: pa.array):
+    """
+    In cudf, each column holds its dtype. And since column may have child
+    columns, child columns also holds there datatype. This method tests
+    that every level of `column` matches the type of the given `array`
+    recursively.
+    """
+
+    if isinstance(column.dtype, ListDtype):
+        return array.type.equals(
+            column.dtype.to_arrow()
+        ) and assert_column_array_dtype_equal(
+            column.base_children[1], array.values
+        )
+    elif isinstance(column.dtype, StructDtype):
+        return array.type.equals(column.dtype.to_arrow()) and all(
+            [
+                assert_column_array_dtype_equal(child, array.field(i))
+                for i, child in enumerate(column.base_children)
+            ]
+        )
+    elif isinstance(column.dtype, Decimal64Dtype):
+        return array.type.equals(column.dtype.to_arrow())
+    elif isinstance(column.dtype, CategoricalDtype):
+        raise NotImplementedError()
+    else:
+        return array.type.equals(np_to_pa_dtype(column.dtype))
+
+
+@pytest.mark.parametrize(
+    "data",
+    [
+        [[{"name": 123}]],
+        [
+            [
+                {
+                    "IsLeapYear": False,
+                    "data": {"Year": 1999, "Month": 7},
+                    "names": ["Mike", None],
+                },
+                {
+                    "IsLeapYear": True,
+                    "data": {"Year": 2004, "Month": 12},
+                    "names": None,
+                },
+                {
+                    "IsLeapYear": False,
+                    "data": {"Year": 1996, "Month": 2},
+                    "names": ["Rose", "Richard"],
+                },
+            ]
+        ],
+        [
+            [None, {"human?": True, "deets": {"weight": 2.4, "age": 27}}],
+            [
+                {"human?": None, "deets": {"weight": 5.3, "age": 25}},
+                {"human?": False, "deets": {"weight": 8.0, "age": 31}},
+                {"human?": False, "deets": None},
+            ],
+            [],
+            None,
+            [{"human?": None, "deets": {"weight": 6.9, "age": None}}],
+        ],
+        [
+            {
+                "name": "var0",
+                "val": [
+                    {"name": "var1", "val": None, "type": "optional<struct>"}
+                ],
+                "type": "list",
+            },
+            {},
+            {
+                "name": "var2",
+                "val": [
+                    {
+                        "name": "var3",
+                        "val": {"field": 42},
+                        "type": "optional<struct>",
+                    },
+                    {
+                        "name": "var4",
+                        "val": {"field": 3.14},
+                        "type": "optional<struct>",
+                    },
+                ],
+                "type": "list",
+            },
+            None,
+        ],
+    ],
+)
+def test_lists_of_structs_dtype(data):
+    got = cudf.Series(data)
+    expected = pa.array(data)
+
+    assert_column_array_dtype_equal(got._column, expected)
+    # TODO: use assert_eq after
+    # https://github.com/pandas-dev/pandas/issues/41466 resolved
+    assert expected.equals(got._column.to_arrow())