Implement string join in cudf-polars (#17755)

A small new string feature.

Authors:
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - Matthew Murray (https://github.com/Matt711)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #17755

python/cudf_polars/cudf_polars/dsl/expressions/string.py

@@ -112,6 +112,7 @@ def __init__(
 
     def _validate_input(self):
         if self.name not in (
+            StringFunction.Name.ConcatVertical,
             StringFunction.Name.Contains,
             StringFunction.Name.EndsWith,
             StringFunction.Name.Lowercase,
@@ -125,7 +126,7 @@ def _validate_input(self):
             StringFunction.Name.StripCharsEnd,
             StringFunction.Name.Uppercase,
         ):
-            raise NotImplementedError(f"String function {self.name}")
+            raise NotImplementedError(f"String function {self.name!r}")
         if self.name is StringFunction.Name.Contains:
             literal, strict = self.options
             if not literal:
@@ -205,7 +206,20 @@ def do_evaluate(
         mapping: Mapping[Expr, Column] | None = None,
     ) -> Column:
         """Evaluate this expression given a dataframe for context."""
-        if self.name is StringFunction.Name.Contains:
+        if self.name is StringFunction.Name.ConcatVertical:
+            (child,) = self.children
+            column = child.evaluate(df, context=context, mapping=mapping)
+            delimiter, ignore_nulls = self.options
+            if column.obj.null_count() > 0 and not ignore_nulls:
+                return Column(plc.Column.all_null_like(column.obj, 1))
+            return Column(
+                plc.strings.combine.join_strings(
+                    column.obj,
+                    plc.interop.from_arrow(pa.scalar(delimiter, type=pa.string())),
+                    plc.interop.from_arrow(pa.scalar(None, type=pa.string())),
+                )
+            )
+        elif self.name is StringFunction.Name.Contains:
             child, arg = self.children
             column = child.evaluate(df, context=context, mapping=mapping)
 

python/cudf_polars/tests/expressions/test_stringfunction.py

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES.
+# SPDX-FileCopyrightText: Copyright (c) 2024-2025, NVIDIA CORPORATION & AFFILIATES.
 # SPDX-License-Identifier: Apache-2.0
 from __future__ import annotations
 
@@ -454,3 +454,10 @@ def test_string_to_numeric_invalid(numeric_type):
         polars_except=pl.exceptions.InvalidOperationError,
         cudf_except=pl.exceptions.ComputeError,
     )
+
+
+@pytest.mark.parametrize("ignore_nulls", [False, True])
+@pytest.mark.parametrize("delimiter", ["", "/"])
+def test_string_join(ldf, ignore_nulls, delimiter):
+    q = ldf.select(pl.col("a").str.join(delimiter, ignore_nulls=ignore_nulls))
+    assert_gpu_result_equal(q)