fix: Align fill_null w/ polars

Author: dangotbanned

narwhals/_plan/dummy.py

@@ -124,6 +124,10 @@ def over(
             order = by, options
         return self._from_ir(Over().to_window_expr(self._ir, partition, order))
 
+    def sort(self, *, descending: bool = False, nulls_last: bool = False) -> Self:
+        options = SortOptions(descending=descending, nulls_last=nulls_last)
+        return self._from_ir(expr.Sort(expr=self._ir, options=options))
+
     def sort_by(
         self,
         by: DummyExpr | t.Iterable[DummyExpr],
@@ -174,12 +178,11 @@ def fill_null(
         strategy: FillNullStrategy | None = None,
         limit: int | None = None,
     ) -> Self:
-        node: F.FillNullWithStrategy | F.FillNull
-        if strategy is not None:
-            node = F.FillNullWithStrategy(strategy=strategy, limit=limit)
-        else:
-            node = F.FillNull(value=parse.parse_into_expr_ir(value, str_as_lit=True))
-        return self._from_ir(node.to_function_expr(self._ir))
+        if strategy is None:
+            ir = parse.parse_into_expr_ir(value, str_as_lit=True)
+            return self._from_ir(F.FillNull().to_function_expr(self._ir, ir))
+        fill = F.FillNullWithStrategy(strategy=strategy, limit=limit)
+        return self._from_ir(fill.to_function_expr(self._ir))
 
     def shift(self, n: int) -> Self:
         return self._from_ir(F.Shift(n=n).to_function_expr(self._ir))

narwhals/_plan/functions.py

@@ -8,7 +8,6 @@
 
 from typing import TYPE_CHECKING
 
-from narwhals._plan.common import ExprIR
 from narwhals._plan.common import Function
 from narwhals._plan.options import FunctionFlags
 from narwhals._plan.options import FunctionOptions
@@ -96,10 +95,6 @@ def __repr__(self) -> str:
 
 
 class FillNull(Function):
-    __slots__ = ("value",)
-
-    value: ExprIR
-
     @property
     def function_options(self) -> FunctionOptions:
         return FunctionOptions.elementwise()