fix: support append/extend with null series (#11824)

Any `Series` can now be `append`-ed or `extend`-ed with
a series of null dtype.

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

@@ -3435,7 +3435,7 @@ fn ensure_can_extend(left: &Series, right: &Series) -> PolarsResult<()> {
         left.name(), right.name(),
     );
     polars_ensure!(
-        left.dtype() == right.dtype(),
+        left.dtype() == right.dtype() || *right.dtype() == DataType::Null,
         ShapeMismatch: "unable to vstack, dtypes for column {:?} don't match: `{}` and `{}`",
         left.name(), left.dtype(), right.dtype(),
     );

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

@@ -251,7 +251,12 @@ impl Series {
     ///
     /// See [`ChunkedArray::append`] and [`ChunkedArray::extend`].
     pub fn append(&mut self, other: &Series) -> PolarsResult<&mut Self> {
-        self._get_inner_mut().append(other)?;
+        if *other.dtype() == DataType::Null && *self.dtype() != DataType::Null {
+            let other_null = Series::full_null(other.name(), other.len(), self.dtype());
+            self._get_inner_mut().append(&other_null)?
+        } else {
+            self._get_inner_mut().append(other)?
+        }
         Ok(self)
     }
 
@@ -264,7 +269,12 @@ impl Series {
     ///
     /// See [`ChunkedArray::extend`] and [`ChunkedArray::append`].
     pub fn extend(&mut self, other: &Series) -> PolarsResult<&mut Self> {
-        self._get_inner_mut().extend(other)?;
+        if *other.dtype() == DataType::Null && *self.dtype() != DataType::Null {
+            let other_null = Series::full_null(other.name(), other.len(), self.dtype());
+            self._get_inner_mut().extend(&other_null)?
+        } else {
+            self._get_inner_mut().extend(other)?
+        }
         Ok(self)
     }
 

py-polars/tests/unit/dataframe/test_vstack.py

@@ -58,3 +58,16 @@ def test_vstack_column_name_mismatch(df1: pl.DataFrame) -> None:
 
     with pytest.raises(pl.ShapeError):
         df1.vstack(df2)
+
+
+def test_vstack_with_null_column() -> None:
+    df1 = pl.DataFrame({"x": [3.5]}, schema={"x": pl.Float64})
+    df2 = pl.DataFrame({"x": [None]}, schema={"x": pl.Null})
+
+    result = df1.vstack(df2)
+    expected = pl.DataFrame({"x": [3.5, None]}, schema={"x": pl.Float64})
+
+    assert_frame_equal(result, expected)
+
+    with pytest.raises(pl.ShapeError):
+        df2.vstack(df1)

py-polars/tests/unit/series/test_append.py

@@ -94,3 +94,15 @@ def test_struct_schema_on_append_extend_3452() -> None:
         ),
     ):
         housing1.extend(housing2)
+
+
+def test_append_null_series() -> None:
+    a = pl.Series("a", [1, 2], pl.Int64)
+    b = pl.Series("b", [None, None], pl.Null)
+
+    result = a.append(b)
+
+    expected = pl.Series("a", [1, 2, None, None], pl.Int64)
+    assert_series_equal(a, expected)
+    assert_series_equal(result, expected)
+    assert a.n_chunks() == 2

py-polars/tests/unit/series/test_extend.py

@@ -32,3 +32,15 @@ def test_extend_bad_input() -> None:
 
     with pytest.raises(AttributeError):
         a.extend(b)  # type: ignore[arg-type]
+
+
+def test_extend_with_null_series() -> None:
+    a = pl.Series("a", [1, 2], pl.Int64)
+    b = pl.Series("b", [None, None], pl.Null)
+
+    result = a.extend(b)
+
+    expected = pl.Series("a", [1, 2, None, None], pl.Int64)
+    assert_series_equal(a, expected)
+    assert_series_equal(result, expected)
+    assert a.n_chunks() == 1