From 37d3d45edf72d91a226179f0b9ae85755ec68949 Mon Sep 17 00:00:00 2001
From: Weijie Guo <reswqa@163.com>
Date: Thu, 4 Apr 2024 14:12:03 +0800
Subject: [PATCH] fix: Don't prune alias in AnonymousFunction subtree (#15453)

---
 .../src/logical_plan/conversion.rs            | 13 ++++++++-----
 py-polars/tests/unit/test_schema.py           | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/crates/polars-plan/src/logical_plan/conversion.rs b/crates/polars-plan/src/logical_plan/conversion.rs
index d3fd74f6830a..211f69f89fda 100644
--- a/crates/polars-plan/src/logical_plan/conversion.rs
+++ b/crates/polars-plan/src/logical_plan/conversion.rs
@@ -210,11 +210,14 @@ fn to_aexpr_impl(expr: Expr, arena: &mut Arena<AExpr>, state: &mut ConversionSta
             function,
             output_type,
             options,
-        } => AExpr::AnonymousFunction {
-            input: to_aexprs(input, arena, state),
-            function,
-            output_type,
-            options,
+        } => {
+            state.prune_alias = false;
+            AExpr::AnonymousFunction {
+                input: to_aexprs(input, arena, state),
+                function,
+                output_type,
+                options,
+            }
         },
         Expr::Function {
             input,
diff --git a/py-polars/tests/unit/test_schema.py b/py-polars/tests/unit/test_schema.py
index 41d8c6777aef..e8d615d65857 100644
--- a/py-polars/tests/unit/test_schema.py
+++ b/py-polars/tests/unit/test_schema.py
@@ -643,3 +643,22 @@ def test_schema_boolean_sum_horizontal() -> None:
 def test_struct_alias_prune_15401() -> None:
     df = pl.DataFrame({"a": []}, schema={"a": pl.Struct({"b": pl.Int8})})
     assert df.select(pl.col("a").alias("c").struct.field("b")).columns == ["b"]
+
+
+def test_alias_prune_in_fold_15438() -> None:
+    df = pl.DataFrame({"x": [1, 2], "expected_result": ["first", "second"]}).select(
+        actual_result=pl.fold(
+            acc=pl.lit("other", dtype=pl.Utf8),
+            function=lambda acc, x: pl.when(x).then(pl.lit(x.name)).otherwise(acc),  # type: ignore[arg-type, return-value]
+            exprs=[
+                (pl.col("x") == 1).alias("first"),
+                (pl.col("x") == 2).alias("second"),
+            ],
+        )
+    )
+    expected = pl.DataFrame(
+        {
+            "actual_result": ["first", "second"],
+        }
+    )
+    assert_frame_equal(df, expected)