fix[python]: allow an apply that produces empty list values (#4437)

py-polars/src/apply/mod.rs

@@ -237,7 +237,16 @@ fn iterator_to_list(
     }
     builder.append_opt_series(first_value);
     for opt_val in it {
-        builder.append_opt_series(opt_val.as_ref())
+        match opt_val {
+            None => builder.append_null(),
+            Some(s) => {
+                if s.len() == 0 && s.dtype() != dt {
+                    builder.append_series(&Series::full_null("", 0, dt))
+                } else {
+                    builder.append_series(&s)
+                }
+            }
+        }
     }
     Ok(builder.finish())
 }

py-polars/tests/test_apply.py

@@ -223,3 +223,13 @@ def test_apply_type_propagation() -> None:
             ]
         )
     ).to_dict(False) == {"a": [1, 2, 3], "b": [1.0, 2.0, None]}
+
+
+def test_empty_list_in_apply() -> None:
+    df = pl.DataFrame(
+        {"a": [[1], [1, 2], [3, 4], [5, 6]], "b": [[3], [1, 2], [1, 2], [4, 5]]}
+    )
+
+    assert df.select(
+        pl.struct(["a", "b"]).apply(lambda row: list(set(row["a"]) & set(row["b"])))
+    ).to_dict(False) == {"a": [[], [1, 2], [], [5]]}