fix(rust, python): deal with empty structs (#6039)

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))
         }

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:?}"),
     }
 }

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(())
-}

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}]}