fix: Fix struct shift and list builder (#18189)

crates/polars-core/src/chunked_array/builder/list/anonymous.rs

@@ -63,12 +63,6 @@ impl<'a> AnonymousListBuilder<'a> {
             // Empty arrays tend to be null type and thus differ
             // if we would push it the concat would fail.
             DataType::Null if s.is_empty() => self.append_empty(),
-            #[cfg(feature = "dtype-struct")]
-            DataType::Struct(_) => {
-                let arr = &**s.array_ref(0);
-                self.builder.push(arr);
-                return Ok(());
-            },
             dt => self.inner_dtype.update(dt)?,
         }
         self.builder.push_multiple(s.chunks());
@@ -127,17 +121,9 @@ impl ListBuilderTrait for AnonymousOwnedListBuilder {
             self.append_empty();
         } else {
             unsafe {
-                match s.dtype() {
-                    #[cfg(feature = "dtype-struct")]
-                    DataType::Struct(_) => {
-                        self.builder.push(&*(&**s.array_ref(0) as *const dyn Array));
-                    },
-                    dt => {
-                        self.inner_dtype.update(dt)?;
-                        self.builder
-                            .push_multiple(&*(s.chunks().as_ref() as *const [ArrayRef]));
-                    },
-                }
+                self.inner_dtype.update(s.dtype())?;
+                self.builder
+                    .push_multiple(&*(s.chunks().as_ref() as *const [ArrayRef]));
             }
             // This make sure that the underlying ArrayRef's are not dropped.
             self.owned.push(s.clone());

crates/polars-core/src/chunked_array/ops/shift.rs

@@ -1,6 +1,7 @@
 use num_traits::{abs, clamp};
 
 use crate::prelude::*;
+use crate::series::implementations::null::NullChunked;
 
 macro_rules! impl_shift_fill {
     ($self:ident, $periods:expr, $fill_value:expr) => {{
@@ -183,6 +184,34 @@ impl<T: PolarsObject> ChunkShift<ObjectType<T>> for ObjectChunked<T> {
     }
 }
 
+#[cfg(feature = "dtype-struct")]
+impl ChunkShift<StructType> for StructChunked {
+    fn shift(&self, periods: i64) -> ChunkedArray<StructType> {
+        // This has its own implementation because a ArrayChunked cannot have a full-null without
+        // knowing the inner type
+        let periods = clamp(periods, -(self.len() as i64), self.len() as i64);
+        let slice_offset = (-periods).max(0);
+        let length = self.len() - abs(periods) as usize;
+        let mut slice = self.slice(slice_offset, length);
+
+        let fill_length = abs(periods) as usize;
+
+        // Go via null, so the cast creates the proper struct type.
+        let fill = NullChunked::new(self.name().into(), fill_length)
+            .cast(self.dtype(), Default::default())
+            .unwrap();
+        let mut fill = fill.struct_().unwrap().clone();
+
+        if periods < 0 {
+            slice.append(&fill).unwrap();
+            slice
+        } else {
+            fill.append(&slice).unwrap();
+            fill
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use crate::prelude::*;

crates/polars-core/src/series/implementations/struct__.rs

@@ -242,10 +242,7 @@ impl SeriesTrait for SeriesWrap<StructChunked> {
     }
 
     fn shift(&self, periods: i64) -> Series {
-        self.0
-            ._apply_fields(|s| s.shift(periods))
-            .unwrap()
-            .into_series()
+        self.0.shift(periods).into_series()
     }
 
     fn clone_inner(&self) -> Arc<dyn SeriesTrait> {

py-polars/tests/unit/datatypes/test_struct.py

@@ -996,3 +996,30 @@ def test_struct_outer_nullability_zip_18119() -> None:
             {"a": True, "b": False},
         ],
     }
+
+
+def test_struct_group_by_shift_18107() -> None:
+    df_in = pl.DataFrame(
+        {
+            "group": [1, 1, 1, 2, 2, 2],
+            "id": [1, 2, 3, 4, 5, 6],
+            "value": [
+                {"lon": 20, "lat": 10},
+                {"lon": 30, "lat": 20},
+                {"lon": 40, "lat": 30},
+                {"lon": 50, "lat": 40},
+                {"lon": 60, "lat": 50},
+                {"lon": 70, "lat": 60},
+            ],
+        }
+    )
+
+    assert df_in.group_by("group", maintain_order=True).agg(
+        pl.col("value").shift(-1)
+    ).to_dict(as_series=False) == {
+        "group": [1, 2],
+        "value": [
+            [{"lon": 30, "lat": 20}, {"lon": 40, "lat": 30}, None],
+            [{"lon": 60, "lat": 50}, {"lon": 70, "lat": 60}, None],
+        ],
+    }