fix: Properly raise on mean_horizontal with wrong dtypes (#19472)

Author: ritchie46

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

@@ -2851,7 +2851,8 @@ impl DataFrame {
                         dtype.is_numeric() || matches!(dtype, DataType::Boolean)
                     })
                     .cloned()
-                    .collect();
+                    .collect::<Vec<_>>();
+                polars_ensure!(!columns.is_empty(), InvalidOperation: "'horizontal_mean' expected at least 1 numerical column");
                 let numeric_df = unsafe { DataFrame::_new_no_checks_impl(self.height(), columns) };
 
                 let sum = || numeric_df.sum_horizontal(null_strategy);

py-polars/tests/unit/test_errors.py

@@ -708,3 +708,15 @@ def test_raise_invalid_agg() -> None:
             .group_by("index")
             .agg(pl.col("foo").filter(pl.col("i_do_not_exist")))
         ).collect()
+
+
+def test_err_mean_horizontal_lists() -> None:
+    df = pl.DataFrame(
+        {
+            "experiment_id": [1, 2],
+            "sensor1": [[1, 2, 3], [7, 8, 9]],
+            "sensor2": [[4, 5, 6], [10, 11, 12]],
+        }
+    )
+    with pytest.raises(pl.exceptions.InvalidOperationError):
+        df.with_columns(pl.mean_horizontal("sensor1", "sensor2").alias("avg_sensor"))