feat: Expressify list.shift

crates/polars-core/src/chunked_array/list/iterator.rs

@@ -188,6 +188,47 @@ impl ListChunked {
         unsafe { self.amortized_iter().for_each(f) }
     }
 
+    /// Zip with a `ChunkedArray` then apply a binary function `F` elementwise.
+    #[must_use]
+    pub fn zip_and_apply_amortized<'a, T, I, F>(&'a self, ca: &'a ChunkedArray<T>, mut f: F) -> Self
+    where
+        T: PolarsDataType,
+        &'a ChunkedArray<T>: IntoIterator<IntoIter = I>,
+        I: TrustedLen<Item = Option<T::Physical<'a>>>,
+        F: FnMut(Option<UnstableSeries<'a>>, Option<T::Physical<'a>>) -> Option<Series>,
+    {
+        if self.is_empty() {
+            return self.clone();
+        }
+        let mut fast_explode = self.null_count() == 0;
+        // SAFETY: unstable series never lives longer than the iterator.
+        let mut out: ListChunked = unsafe {
+            self.amortized_iter()
+                .zip(ca)
+                .map(|(opt_s, opt_v)| {
+                    let out = f(opt_s, opt_v);
+                    match out {
+                        Some(out) if out.is_empty() => {
+                            fast_explode = false;
+                            Some(out)
+                        },
+                        None => {
+                            fast_explode = false;
+                            out
+                        },
+                        _ => out,
+                    }
+                })
+                .collect_trusted()
+        };
+
+        out.rename(self.name());
+        if fast_explode {
+            out.set_fast_explode();
+        }
+        out
+    }
+
     /// Apply a closure `F` elementwise.
     #[must_use]
     pub fn apply_amortized<'a, F>(&'a self, mut f: F) -> Self

crates/polars-ops/src/chunked_array/list/namespace.rs

@@ -259,10 +259,30 @@ pub trait ListNameSpaceImpl: AsList {
         ca.try_apply_amortized(|s| s.as_ref().diff(n, null_behavior))
     }
 
-    fn lst_shift(&self, periods: i64) -> ListChunked {
+    fn lst_shift(&self, periods: &Series) -> PolarsResult<ListChunked> {
         let ca = self.as_list();
-        let out = ca.apply_amortized(|s| s.as_ref().shift(periods));
-        self.same_type(out)
+        let periods_s = periods.cast(&DataType::Int64)?;
+        let periods = periods_s.i64()?;
+
+        match periods.len() {
+            1 => {
+                if let Some(periods) = periods.get(0) {
+                    Ok(ca.apply_amortized(|s| s.as_ref().shift(periods)))
+                } else {
+                    Ok(ListChunked::full_null_with_dtype(
+                        ca.name(),
+                        ca.len(),
+                        &ca.inner_dtype(),
+                    ))
+                }
+            },
+            _ => Ok(ca.zip_and_apply_amortized(periods, |opt_s, opt_periods| {
+                match (opt_s, opt_periods) {
+                    (Some(s), Some(periods)) => Some(s.as_ref().shift(periods)),
+                    _ => None,
+                }
+            })),
+        }
     }
 
     fn lst_slice(&self, offset: i64, length: usize) -> ListChunked {

crates/polars-plan/src/dsl/function_expr/list.rs

@@ -12,6 +12,7 @@ pub enum ListFunction {
     #[cfg(feature = "list_drop_nulls")]
     DropNulls,
     Slice,
+    Shift,
     Get,
     #[cfg(feature = "list_take")]
     Take(bool),
@@ -45,6 +46,7 @@ impl Display for ListFunction {
             #[cfg(feature = "list_drop_nulls")]
             DropNulls => "drop_nulls",
             Slice => "slice",
+            Shift => "shift",
             Get => "get",
             #[cfg(feature = "list_take")]
             Take(_) => "take",
@@ -104,6 +106,13 @@ fn check_slice_arg_shape(slice_len: usize, ca_len: usize, name: &str) -> PolarsR
     Ok(())
 }
 
+pub(super) fn shift(s: &[Series]) -> PolarsResult<Series> {
+    let list = s[0].list()?;
+    let periods = &s[1];
+
+    list.lst_shift(periods).map(|ok| ok.into_series())
+}
+
 pub(super) fn slice(args: &mut [Series]) -> PolarsResult<Option<Series>> {
     let s = &args[0];
     let list_ca = s.list()?;

crates/polars-plan/src/dsl/function_expr/mod.rs

@@ -555,6 +555,7 @@ impl From<FunctionExpr> for SpecialEq<Arc<dyn SeriesUdf>> {
                     #[cfg(feature = "list_drop_nulls")]
                     DropNulls => map!(list::drop_nulls),
                     Slice => wrap!(list::slice),
+                    Shift => map_as_slice!(list::shift),
                     Get => wrap!(list::get),
                     #[cfg(feature = "list_take")]
                     Take(null_ob_oob) => map_as_slice!(list::take, null_ob_oob),

crates/polars-plan/src/dsl/function_expr/schema.rs

@@ -103,6 +103,7 @@ impl FunctionExpr {
                     #[cfg(feature = "list_drop_nulls")]
                     DropNulls => mapper.with_same_dtype(),
                     Slice => mapper.with_same_dtype(),
+                    Shift => mapper.with_same_dtype(),
                     Get => mapper.map_to_list_inner_dtype(),
                     #[cfg(feature = "list_take")]
                     Take(_) => mapper.with_same_dtype(),

crates/polars-plan/src/dsl/list.rs

@@ -167,13 +167,13 @@ impl ListNameSpace {
     }
 
     /// Shift every sublist.
-    pub fn shift(self, periods: i64) -> Expr {
-        self.0
-            .map(
-                move |s| Ok(Some(s.list()?.lst_shift(periods).into_series())),
-                GetOutput::same_type(),
-            )
-            .with_fmt("list.shift")
+    pub fn shift(self, periods: Expr) -> Expr {
+        self.0.map_many_private(
+            FunctionExpr::ListExpr(ListFunction::Shift),
+            &[periods],
+            false,
+            false,
+        )
     }
 
     /// Slice every sublist.

py-polars/polars/expr/list.py

@@ -13,7 +13,12 @@
     from datetime import date, datetime, time
 
     from polars import Expr, Series
-    from polars.type_aliases import IntoExpr, NullBehavior, ToStructStrategy
+    from polars.type_aliases import (
+        IntoExpr,
+        IntoExprColumn,
+        NullBehavior,
+        ToStructStrategy,
+    )
 
 
 class ExprListNameSpace:
@@ -642,7 +647,7 @@ def diff(self, n: int = 1, null_behavior: NullBehavior = "ignore") -> Expr:
         """
         return wrap_expr(self._pyexpr.list_diff(n, null_behavior))
 
-    def shift(self, periods: int = 1) -> Expr:
+    def shift(self, periods: int | IntoExprColumn = 1) -> Expr:
         """
         Shift values by the given period.
 
@@ -663,6 +668,7 @@ def shift(self, periods: int = 1) -> Expr:
         ]
 
         """
+        periods = parse_as_expression(periods)
         return wrap_expr(self._pyexpr.list_shift(periods))
 
     def slice(

py-polars/polars/series/list.py

@@ -12,7 +12,12 @@
 
     from polars import Expr, Series
     from polars.polars import PySeries
-    from polars.type_aliases import IntoExpr, NullBehavior, ToStructStrategy
+    from polars.type_aliases import (
+        IntoExpr,
+        IntoExprColumn,
+        NullBehavior,
+        ToStructStrategy,
+    )
 
 
 @expr_dispatch
@@ -333,7 +338,7 @@ def diff(self, n: int = 1, null_behavior: NullBehavior = "ignore") -> Series:
 
         """
 
-    def shift(self, periods: int = 1) -> Series:
+    def shift(self, periods: int | IntoExprColumn = 1) -> Series:
         """
         Shift values by the given period.
 

py-polars/src/expr/list.rs

@@ -78,8 +78,8 @@ impl PyExpr {
         self.inner.clone().list().reverse().into()
     }
 
-    fn list_shift(&self, periods: i64) -> Self {
-        self.inner.clone().list().shift(periods).into()
+    fn list_shift(&self, periods: PyExpr) -> Self {
+        self.inner.clone().list().shift(periods.inner).into()
     }
 
     fn list_slice(&self, offset: PyExpr, length: Option<PyExpr>) -> Self {

py-polars/tests/unit/namespaces/test_list.py

@@ -141,6 +141,32 @@ def test_list_shift() -> None:
     expected = pl.Series("a", [[None, 1], [None, 3, 2]])
     assert s.list.shift().to_list() == expected.to_list()
 
+    df = pl.DataFrame(
+        {
+            "values": [
+                [1, 2, None],
+                [1, 2, 3],
+                [None, 1, 2],
+                [None, None, None],
+                [1, 2],
+            ],
+            "shift": [1, -2, 3, 2, None],
+        }
+    )
+    df = df.select(pl.col("values").list.shift(pl.col("shift")))
+    expected_df = pl.DataFrame(
+        {
+            "values": [
+                [None, 1, 2],
+                [3, None, None],
+                [None, None, None],
+                [None, None, None],
+                None,
+            ]
+        }
+    )
+    assert_frame_equal(df, expected_df)
+
 
 def test_list_drop_nulls() -> None:
     s = pl.Series("values", [[1, None, 2, None], [None, None], [1, 2], None])