Support constructing cudf.Scalar objects from host side lists

python/cudf/cudf/_lib/cpp/scalar/scalar.pxd

@@ -65,4 +65,5 @@ cdef extern from "cudf/scalar/scalar.hpp" namespace "cudf" nogil:
         # TODO: Figure out how to add an int32 overload of value()
 
     cdef cppclass list_scalar(scalar):
+        list_scalar(column_view col) except +
         column_view view() except +

python/cudf/cudf/_lib/scalar.pyx

@@ -2,7 +2,7 @@
 import decimal
 import numpy as np
 import pandas as pd
-
+import pyarrow as pa
 from libc.stdint cimport (
     int8_t,
     int16_t,
@@ -82,6 +82,9 @@ cdef class DeviceScalar:
         if isinstance(dtype, cudf.Decimal64Dtype):
             _set_decimal64_from_scalar(
                 self.c_value, value, dtype, valid)
+        elif isinstance(dtype, cudf.ListDtype):
+            _set_list_from_pylist(
+                self.c_value, value, dtype, valid)
         elif pd.api.types.is_string_dtype(dtype):
             _set_string_from_np_string(self.c_value, value, valid)
         elif pd.api.types.is_numeric_dtype(dtype):
@@ -295,6 +298,18 @@ cdef _set_decimal64_from_scalar(unique_ptr[scalar]& s,
         )
     )
 
+cdef _set_list_from_pylist(unique_ptr[scalar]& s,
+                           object value,
+                           object dtype,
+                           bool valid=True):
+    cdef Column col = cudf.core.column.as_column(
+        pa.array(value)
+    )
+    cdef column_view col_view = col.view()
+    s.reset(
+        new list_scalar(col_view)
+    )
+
 cdef _get_py_list_from_list(unique_ptr[scalar]& s):
 
     if not s.get()[0].is_valid():

python/cudf/cudf/core/scalar.py

@@ -7,7 +7,7 @@
 
 from cudf._lib.scalar import DeviceScalar, _is_null_host_scalar
 from cudf.core.column.column import ColumnBase
-from cudf.core.dtypes import Decimal64Dtype
+from cudf.core.dtypes import Decimal64Dtype, ListDtype
 from cudf.core.index import BaseIndex
 from cudf.core.series import Series
 from cudf.utils.dtypes import (
@@ -118,6 +118,15 @@ def _device_value_to_host(self):
     def _preprocess_host_value(self, value, dtype):
         valid = not _is_null_host_scalar(value)
 
+        if isinstance(value, list):
+            if dtype is not None:
+                raise TypeError("Lists may not be cast to a different dtype")
+            else:
+                dtype = ListDtype.from_arrow(
+                    pa.infer_type([value], from_pandas=True)
+                )
+                return value, dtype
+
         if isinstance(dtype, Decimal64Dtype):
             value = pa.scalar(
                 value, type=pa.decimal128(dtype.precision, dtype.scale)

python/cudf/cudf/tests/test_list.py

@@ -8,6 +8,7 @@
 
 import cudf
 from cudf import NA
+from cudf._lib.copying import get_element
 from cudf.tests.utils import assert_eq
 
 
@@ -336,25 +337,67 @@ def test_concatenate_list_with_nonlist():
 
 
 @pytest.mark.parametrize(
-    "indata,expect",
+    "data",
     [
-        ([1], [1]),
-        ([1, 2, 3], [1, 2, 3]),
-        ([[1, 2, 3], [4, 5, 6]], [[1, 2, 3], [4, 5, 6]]),
-        ([None], [NA]),
-        ([1, None, 3], [1, NA, 3]),
-        ([[1, None, 3], [None, 5, 6]], [[1, NA, 3], [NA, 5, 6]]),
+        [1],
+        [1, 2, 3],
+        [[1, 2, 3], [4, 5, 6]],
+        [NA],
+        [1, NA, 3],
+        [[1, NA, 3], [NA, 5, 6]],
     ],
 )
-def test_list_getitem(indata, expect):
-    list_sr = cudf.Series([indata])
-    # __getitem__ shall fill None with cudf.NA
-    assert list_sr[0] == expect
+def test_list_getitem(data):
+    list_sr = cudf.Series([data])
+    assert list_sr[0] == data
 
 
 @pytest.mark.parametrize(
-    "input_obj", [[[1, cudf.NA, 3]], [[1, cudf.NA, 3], [4, 5, cudf.NA]]]
+    "data",
+    [
+        [1, 2, 3],
+        [[1, 2, 3], [4, 5, 6]],
+        ["a", "b", "c"],
+        [["a", "b", "c"], ["d", "e", "f"]],
+        [1.1, 2.2, 3.3],
+        [[1.1, 2.2, 3.3], [4.4, 5.5, 6.6]],
+        [1, NA, 3],
+        [[1, NA, 3], [4, 5, NA]],
+        ["a", NA, "c"],
+        [["a", NA, "c"], ["d", "e", NA]],
+        [1.1, NA, 3.3],
+        [[1.1, NA, 3.3], [4.4, 5.5, NA]],
+    ],
 )
+def test_list_scalar_host_construction(data):
+    slr = cudf.Scalar(data)
+    assert slr.value == data
+
+
+@pytest.mark.parametrize(
+    "data",
+    [
+        [1, 2, 3],
+        [[1, 2, 3], [4, 5, 6]],
+        ["a", "b", "c"],
+        [["a", "b", "c"], ["d", "e", "f"]],
+        [1.1, 2.2, 3.3],
+        [[1.1, 2.2, 3.3], [4.4, 5.5, 6.6]],
+        [1, NA, 3],
+        [[1, NA, 3], [4, 5, NA]],
+        ["a", NA, "c"],
+        [["a", NA, "c"], ["d", "e", NA]],
+        [1.1, NA, 3.3],
+        [[1.1, NA, 3.3], [4.4, 5.5, NA]],
+    ],
+)
+def test_list_scalar_device_construction(data):
+    col = cudf.Series([data])._column
+    slr = get_element(col, 0)
+    assert slr.value == data
+
+
+@pytest.mark.parametrize("input_obj", [[[1, NA, 3]], [[1, NA, 3], [4, 5, NA]]])
 def test_construction_series_with_nulls(input_obj):
     expect = pa.array(input_obj, from_pandas=True)
     got = cudf.Series(input_obj).to_arrow()