From 6aec84d757666cb4c8cb92de63ff6b611a596257 Mon Sep 17 00:00:00 2001
From: Ritchie Vink <ritchie46@gmail.com>
Date: Wed, 4 Jan 2023 14:04:55 +0100
Subject: [PATCH] fix(rust, python): deal with empty structs

---
 .../src/chunked_array/logical/struct_/mod.rs  |  3 +++
 .../src/chunked_array/ops/any_value.rs        |  1 +
 polars/polars-lazy/src/tests/queries.rs       | 21 -------------------
 py-polars/tests/unit/test_struct.py           | 12 +++++++++++
 4 files changed, 16 insertions(+), 21 deletions(-)

diff --git a/polars/polars-core/src/chunked_array/logical/struct_/mod.rs b/polars/polars-core/src/chunked_array/logical/struct_/mod.rs
index 8a56aa8b9798..eacba897fb96 100644
--- a/polars/polars-core/src/chunked_array/logical/struct_/mod.rs
+++ b/polars/polars-core/src/chunked_array/logical/struct_/mod.rs
@@ -79,6 +79,9 @@ impl StructChunked {
                 }
             }
             Ok(Self::new_unchecked(name, &new_fields))
+        } else if fields.is_empty() {
+            let fields = &[Series::full_null("", 1, &DataType::Null)];
+            Ok(Self::new_unchecked(name, fields))
         } else {
             Ok(Self::new_unchecked(name, fields))
         }
diff --git a/polars/polars-core/src/chunked_array/ops/any_value.rs b/polars/polars-core/src/chunked_array/ops/any_value.rs
index 6a5d4375a8f2..27bd1b4e89ec 100644
--- a/polars/polars-core/src/chunked_array/ops/any_value.rs
+++ b/polars/polars-core/src/chunked_array/ops/any_value.rs
@@ -113,6 +113,7 @@ pub(crate) unsafe fn arr_to_any_value<'a>(
             let arr = &*(arr as *const dyn Array as *const FixedSizeBinaryArray);
             PolarsExtension::arr_to_av(arr, idx)
         }
+        DataType::Null => AnyValue::Null,
         dt => panic!("not implemented for {dt:?}"),
     }
 }
diff --git a/polars/polars-lazy/src/tests/queries.rs b/polars/polars-lazy/src/tests/queries.rs
index aa75f1e6ddfa..f3610547d4b2 100644
--- a/polars/polars-lazy/src/tests/queries.rs
+++ b/polars/polars-lazy/src/tests/queries.rs
@@ -2009,24 +2009,3 @@ fn test_partitioned_gb_ternary() -> PolarsResult<()> {
 
     Ok(())
 }
-
-#[test]
-fn test_foo() -> PolarsResult<()> {
-    let df = df![
-        "a" => [2, 2],
-        "b" => [1, 2]
-    ]?;
-
-    let out = df
-        .lazy()
-        .groupby([col("a")])
-        .agg([
-            (col("a").filter(col("b").eq(0)).diff(1, Default::default()) * lit(100))
-                .diff(1, Default::default())
-                .alias("foo"),
-        ])
-        .collect();
-    dbg!(out);
-
-    Ok(())
-}
diff --git a/py-polars/tests/unit/test_struct.py b/py-polars/tests/unit/test_struct.py
index 3664a05db396..239edbaccdf7 100644
--- a/py-polars/tests/unit/test_struct.py
+++ b/py-polars/tests/unit/test_struct.py
@@ -718,3 +718,15 @@ def test_struct_categorical_5843() -> None:
             {"foo": "c", "counts": 1},
         ]
     }
+
+
+def test_struct_empty() -> None:
+    # List<struct>
+    df = pl.DataFrame({"a": [[{}]]})
+    assert df.to_dict(False) == {"a": [[{"": None}]]}
+    # Struct one not empty
+    df = pl.DataFrame({"a": [[{}, {"a": 10}]]})
+    assert df.to_dict(False) == {"a": [[{"a": None}, {"a": 10}]]}
+    # Empty struct
+    df = pl.DataFrame({"a": [{}]})
+    assert df.to_dict(False) == {"a": [{"": None}]}