fix(python): translate Polars fill_null with expression fills

vortex-python/python/vortex/polars_.py

@@ -159,6 +159,10 @@ def _polars_to_vortex(expr: dict[str, Any]) -> ve.Expr:
         expr = expr["Function"]
         _inputs = [_polars_to_vortex(e) for e in expr["input"]]
 
+        if expr["function"] == "FillNull":
+            if "Literal" in expr["input"][1]:
+                return ve.fill_null(_inputs[0], _inputs[1])
+            return ve.zip_(ve.is_null(_inputs[0]), _inputs[1], _inputs[0])
         fn = expr["function"]
         if "Boolean" in fn:
             fn = fn["Boolean"]

vortex-python/test/test_polars_.py

@@ -204,3 +204,41 @@ def test_polars_boolean_not(tmp_path):
     actual = vx.open(str(path)).to_polars().filter(expr).collect()
     assert_frame_equal(actual, expected_frame)
     assert actual["id"].to_list() == [2]
+
+
+def test_polars_fill_null(tmp_path):
+    frame = pl.DataFrame({"id": [0, 1, 2, 3], "x": [1, None, None, 5], "y": [4, 5, None, 6]})
+    expr = pl.col("x").fill_null(pl.col("y")) == 5
+    path = tmp_path / "fill_null.vortex"
+    vx.io.write(vx.array(frame.to_arrow()), str(path))
+    expected_frame = frame.lazy().filter(expr).collect()
+    actual = vx.open(str(path)).to_polars().filter(expr).collect()
+    assert_frame_equal(actual, expected_frame)
+    assert actual["id"].to_list() == [1, 3]
+
+
+@pytest.mark.parametrize("fill_value, expected_ids", [(5, [1, 2, 3]), (0, [1, 2]), (-1, [1, 2])])
+def test_polars_fill_null_literal(tmp_path, fill_value, expected_ids):
+    frame = pl.DataFrame({"id": [0, 1, 2, 3], "x": [1, None, None, 5]})
+    filled = pl.col("x").fill_null(fill_value)
+    assert polars_to_vortex(filled).serialize() == ve.fill_null(ve.column("x"), fill_value).serialize()
+    predicate = filled == fill_value
+    path = tmp_path / "fill_null_literal.vortex"
+    vx.io.write(vx.array(frame.to_arrow()), str(path))
+    expected_frame = frame.lazy().filter(predicate).collect()
+    actual = vx.open(str(path)).to_polars().filter(predicate).collect()
+    assert_frame_equal(actual, expected_frame)
+    assert actual["id"].to_list() == expected_ids
+
+
+def test_polars_fill_null_string_literal(tmp_path):
+    frame = pl.DataFrame({"id": [0, 1, 2], "x": ["a", None, "c"]})
+    filled = pl.col("x").fill_null("z")
+    assert polars_to_vortex(filled).serialize() == ve.fill_null(ve.column("x"), "z").serialize()
+    expr = filled == "z"
+    path = tmp_path / "fill_null_string.vortex"
+    vx.io.write(vx.array(frame.to_arrow()), str(path))
+    expected_frame = frame.lazy().filter(expr).collect()
+    actual = vx.open(str(path)).to_polars().filter(expr).collect()
+    assert_frame_equal(actual, expected_frame)
+    assert actual["id"].to_list() == [1]