fix: Check funtion input len at expansion

crates/polars-plan/src/dsl/functions/horizontal.rs

@@ -204,7 +204,9 @@ pub fn all_horizontal<E: AsRef<[Expr]>>(exprs: E) -> PolarsResult<Expr> {
         input: exprs,
         function: FunctionExpr::Boolean(BooleanFunction::AllHorizontal),
         options: FunctionOptions {
-            flags: FunctionFlags::default() | FunctionFlags::INPUT_WILDCARD_EXPANSION,
+            flags: FunctionFlags::default()
+                | FunctionFlags::INPUT_WILDCARD_EXPANSION
+                | FunctionFlags::ALLOW_EMPTY_INPUTS,
             ..Default::default()
         },
     })
@@ -221,7 +223,9 @@ pub fn any_horizontal<E: AsRef<[Expr]>>(exprs: E) -> PolarsResult<Expr> {
         input: exprs,
         function: FunctionExpr::Boolean(BooleanFunction::AnyHorizontal),
         options: FunctionOptions {
-            flags: FunctionFlags::default() | FunctionFlags::INPUT_WILDCARD_EXPANSION,
+            flags: FunctionFlags::default()
+                | FunctionFlags::INPUT_WILDCARD_EXPANSION
+                | FunctionFlags::ALLOW_EMPTY_INPUTS,
             ..Default::default()
         },
     })

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

@@ -186,7 +186,15 @@ impl Expr {
 
     /// Drop null values.
     pub fn drop_nulls(self) -> Self {
-        self.apply_private(FunctionExpr::DropNulls)
+        Expr::Function {
+            input: vec![self],
+            function: FunctionExpr::DropNulls,
+            options: FunctionOptions {
+                collect_groups: ApplyOptions::GroupWise,
+                flags: FunctionFlags::default() | FunctionFlags::ALLOW_EMPTY_INPUTS,
+                ..Default::default()
+            },
+        }
     }
 
     /// Drop NaN values.

crates/polars-plan/src/plans/conversion/expr_expansion.rs

@@ -541,17 +541,22 @@ fn prepare_excluded(
 }
 
 // functions can have col(["a", "b"]) or col(String) as inputs
-fn expand_function_inputs(expr: Expr, schema: &Schema) -> Expr {
-    expr.map_expr(|mut e| match &mut e {
+fn expand_function_inputs(expr: Expr, schema: &Schema) -> PolarsResult<Expr> {
+    expr.try_map_expr(|mut e| match &mut e {
         Expr::AnonymousFunction { input, options, .. } | Expr::Function { input, options, .. }
             if options
                 .flags
                 .contains(FunctionFlags::INPUT_WILDCARD_EXPANSION) =>
         {
             *input = rewrite_projections(core::mem::take(input), schema, &[]).unwrap();
-            e
+            if input.is_empty() && !options.flags.contains(FunctionFlags::ALLOW_EMPTY_INPUTS) {
+                // Needed to visualize the error
+                *input = vec![Expr::Literal(LiteralValue::Null)];
+                polars_bail!(InvalidOperation: "expected at least 1 input in {}", e)
+            }
+            Ok(e)
         },
-        _ => e,
+        _ => Ok(e),
     })
 }
 
@@ -648,7 +653,7 @@ pub(crate) fn rewrite_projections(
         let result_offset = result.len();
 
         // Functions can have col(["a", "b"]) or col(String) as inputs.
-        expr = expand_function_inputs(expr, schema);
+        expr = expand_function_inputs(expr, schema)?;
 
         let mut flags = find_flags(&expr)?;
         if flags.has_selector {

crates/polars-plan/src/plans/options.rs

@@ -151,6 +151,8 @@ bitflags!(
             /// This can lead to recursively entering the engine and sometimes deadlocks.
             /// This flag must be set to handle that.
             const OPTIONAL_RE_ENTRANT = 1 << 6;
+            /// Whether this function allows no inputs.
+            const ALLOW_EMPTY_INPUTS = 1 << 7;
         }
 );
 

py-polars/tests/unit/test_empty.py

@@ -148,3 +148,14 @@ def test_empty_list_cat_16405() -> None:
 def test_empty_list_concat_16924() -> None:
     df = pl.DataFrame(schema={"a": pl.Int16, "b": pl.List(pl.String)})
     df.with_columns(pl.col("b").list.concat([pl.col("a").cast(pl.String)]))
+
+
+def test_empty_input_expansion() -> None:
+    df = pl.DataFrame({"A": [1], "B": [2]})
+
+    with pytest.raises(pl.exceptions.InvalidOperationError):
+        (
+            df.select("A", "B").with_columns(
+                pl.col("B").sort_by(pl.struct(pl.exclude("A", "B")))
+            )
+        )

py-polars/tests/unit/test_errors.py

@@ -542,7 +542,7 @@ def test_sort_by_err_9259() -> None:
 def test_empty_inputs_error() -> None:
     df = pl.DataFrame({"col1": [1]})
     with pytest.raises(
-        ComputeError, match="expression: 'sum_horizontal' didn't get any inputs"
+        pl.exceptions.InvalidOperationError, match="expected at least 1 input"
     ):
         df.select(pl.sum_horizontal(pl.exclude("col1")))