fix: allow for fill_null with aggregate expression (#3105)

* fix: allow for `fill_null` with aggregate expression

* preserve str_as_lit=True

* extra test

Author: MarcoGorelli

narwhals/expr.py

@@ -1192,10 +1192,14 @@ def fill_null(
             raise ValueError(msg)
 
         return self.__class__(
-            lambda plx: self._to_compliant_expr(plx).fill_null(
-                value=plx.parse_into_expr(value, str_as_lit=True),
-                strategy=strategy,
-                limit=limit,
+            lambda plx: apply_n_ary_operation(
+                plx,
+                lambda *exprs: exprs[0].fill_null(
+                    exprs[1], strategy=strategy, limit=limit
+                ),
+                self,
+                value,
+                str_as_lit=True,
             ),
             self._metadata.with_orderable_window()
             if strategy is not None

tests/expr_and_series/fill_null_test.py

@@ -33,6 +33,17 @@ def test_fill_null(constructor: Constructor) -> None:
     assert_equal_data(result, expected)
 
 
+def test_fill_null_w_aggregate(constructor: Constructor) -> None:
+    data = {"a": [0.5, None, 2.0, 3.0, 4.5], "b": ["xx", "yy", "zz", None, "yy"]}
+    df = nw.from_native(constructor(data))
+
+    result = df.select(
+        nw.col("a").fill_null(nw.col("a").mean()), nw.col("b").fill_null("a")
+    )
+    expected = {"a": [0.5, 2.5, 2.0, 3.0, 4.5], "b": ["xx", "yy", "zz", "a", "yy"]}
+    assert_equal_data(result, expected)
+
+
 def test_fill_null_pandas_downcast() -> None:
     pytest.importorskip("pandas")
     import pandas as pd