fix: Fix melt panic (#17088)

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

@@ -326,7 +326,10 @@ impl DataFrame {
             // self.column() is linear
             // together with this loop that would make it O^2 over value_vars
             let (pos, _name, _dtype) = schema.try_get_full(value_column_name)?;
-            let value_col = self.columns[pos].cast(&st).unwrap();
+            let col = &self.columns[pos];
+            let value_col = col.cast(&st).map_err(
+                |_| polars_err!(InvalidOperation: "'melt/unpivot' not supported for dtype: {}", col.dtype()),
+            )?;
             values.extend_from_slice(value_col.chunks())
         }
         let values_arr = concatenate_owned_unchecked(&values)?;

py-polars/tests/unit/operations/test_melt.py

@@ -1,3 +1,5 @@
+import pytest
+
 import polars as pl
 import polars.selectors as cs
 from polars.testing import assert_frame_equal
@@ -81,3 +83,10 @@ def test_melt_no_value_vars() -> None:
         schema={"a": pl.Int64, "variable": pl.String, "value": pl.Null}
     )
     assert_frame_equal(result, expected)
+
+
+def test_melt_raise_list() -> None:
+    with pytest.raises(pl.exceptions.InvalidOperationError):
+        pl.LazyFrame(
+            {"a": ["x", "y"], "b": [["test", "test2"], ["test3", "test4"]]}
+        ).melt().collect()