Fix: expand dict transformation

marimo/_plugins/ui/_impl/dataframes/transforms/handlers.py

@@ -492,7 +492,12 @@ def handle_explode_columns(
     def handle_expand_dict(
         df: DataFrame, transform: ExpandDictTransform
     ) -> DataFrame:
-        return df.explode(transform.column_id)
+        # Convert to Polars for native unnest, which handles null values
+        # correctly (fixes: 'NoneType' cannot be converted to 'PyDict' issue #4583)
+        collected_df, undo = collect_and_preserve_type(df)
+        polars_df = collected_df.to_polars()
+        unnested = polars_df.unnest(transform.column_id)
+        return undo(nw.from_native(unnested))
 
     @staticmethod
     def handle_unique(df: DataFrame, transform: UniqueTransform) -> DataFrame:

marimo/_plugins/ui/_impl/dataframes/transforms/print_code.py

@@ -465,7 +465,7 @@ def generate_where_clause_polars(where: FilterCondition) -> str:
 
     elif transform.type == TransformType.EXPAND_DICT:
         column_id = _as_literal(transform.column_id)
-        return f"{df_name}.hstack(pl.DataFrame({df_name}.select({column_id}).to_series().to_list())).drop({column_id})"
+        return f"{df_name}.unnest({column_id})"
 
     elif transform.type == TransformType.UNIQUE:
         column_ids = transform.column_ids

tests/_plugins/ui/_impl/dataframes/test_handlers.py

@@ -47,8 +47,8 @@
 
 pytest.importorskip("ibis")
 pd = pytest.importorskip("pandas")
-pytest.importorskip("polars")
 pytest.importorskip("pyarrow")
+pl = pytest.importorskip("polars")
 
 
 def apply(df: DataFrameType, transform: Transform) -> DataFrameType:
@@ -1733,22 +1733,32 @@ def test_explode_columns(df: DataFrameType) -> None:
         assert nw_result.columns == ["A", "B", "C"]
 
     @staticmethod
-    @pytest.mark.skip(
-        reason="Dict/struct expansion not supported uniformly across backends"
-    )
     @pytest.mark.parametrize(
         ("df", "expected"),
         list(
             zip(
                 create_test_dataframes(
-                    {"A": [{"foo": 1, "bar": "hello"}], "B": [1]}
+                    {"A": [{"foo": 1, "bar": "hello"}], "B": [1]},
                 ),
                 create_test_dataframes(
-                    {"B": [1], "foo": [1], "bar": ["hello"]}
+                    {"B": [1], "foo": [1], "bar": ["hello"]},
                 ),
                 strict=False,
             )
-        ),
+        )
+        + [
+            (
+                pl.DataFrame(
+                    {
+                        "A": [{"foo": 1, "bar": "hello"}, None],
+                        "B": [1, 2],
+                    }
+                ),
+                pl.DataFrame(
+                    {"B": [1, 2], "foo": [1, None], "bar": ["hello", None]}
+                ),
+            )
+        ],
     )
     def test_expand_dict(df: DataFrameType, expected: DataFrameType) -> None:
         transform = ExpandDictTransform(