fix: Allow empty sort by columns

crates/polars-core/src/frame/mod.rs

@@ -1796,7 +1796,12 @@ impl DataFrame {
         slice: Option<(i64, usize)>,
     ) -> PolarsResult<Self> {
         if by_column.is_empty() {
-            polars_bail!(ComputeError: "No columns selected for sorting");
+            // If no columns selected, any order (including original order) is correct.
+            return if let Some((offset, len)) = slice {
+                Ok(self.slice(offset, len))
+            } else {
+                Ok(self.clone())
+            };
         }
         // note that the by_column argument also contains evaluated expression from
         // polars-lazy that may not even be present in this dataframe. therefore

crates/polars-expr/src/expressions/sortby.rs

@@ -201,6 +201,10 @@ impl PhysicalExpr for SortByExpr {
     }
     fn evaluate(&self, df: &DataFrame, state: &ExecutionState) -> PolarsResult<Series> {
         let series_f = || self.input.evaluate(df, state);
+        if self.by.is_empty() {
+            // Sorting by 0 columns returns input unchanged.
+            return series_f();
+        }
         let (series, sorted_idx) = if self.by.len() == 1 {
             let sorted_idx_f = || {
                 let s_sort_by = self.by[0].evaluate(df, state)?;

crates/polars-expr/src/planner.rs

@@ -338,7 +338,6 @@ fn create_physical_expr_inner(
             by,
             sort_options,
         } => {
-            polars_ensure!(!by.is_empty(), InvalidOperation: "'sort_by' got an empty set");
             let phys_expr = create_physical_expr_inner(*expr, ctxt, expr_arena, schema, state)?;
             let phys_by =
                 create_physical_expressions_from_nodes(by, ctxt, expr_arena, schema, state)?;

py-polars/tests/unit/dataframe/test_df.py

@@ -360,11 +360,8 @@ def test_sort_multi_output_exprs_01() -> None:
     ):
         df.sort("dts", "strs", nulls_last=[True, False, True])
 
-    with pytest.raises(
-        ComputeError,
-        match="No columns selected for sorting",
-    ):
-        df.sort(pl.col("^xxx$"))
+    # No columns selected - return original input.
+    assert_frame_equal(df, df.sort(pl.col("^xxx$")))
 
 
 @pytest.mark.parametrize(

py-polars/tests/unit/test_empty.py

@@ -1,7 +1,6 @@
 import pytest
 
 import polars as pl
-from polars.exceptions import InvalidOperationError
 from polars.testing import assert_frame_equal, assert_series_equal
 
 
@@ -52,9 +51,9 @@ def test_empty_count_window() -> None:
 
 
 def test_empty_sort_by_args() -> None:
-    df = pl.DataFrame([1, 2, 3])
-    with pytest.raises(InvalidOperationError):
-        df.select(pl.all().sort_by([]))
+    df = pl.DataFrame({"x": [2, 1, 3]})
+    assert_frame_equal(df, df.select(pl.col.x.sort_by([])))
+    assert_frame_equal(df, df.sort([]))
 
 
 def test_empty_9137() -> None: