record "len" as simple aggregation

narwhals/pandas_like/group_by.py

@@ -102,7 +102,7 @@ def agg_pandas(
     simple_aggs = []
     complex_aggs = []
     for expr in exprs:
-        if is_simple_aggregation(expr, implementation="pandas"):
+        if is_simple_aggregation(expr):
             simple_aggs.append(expr)
         else:
             complex_aggs.append(expr)
@@ -173,7 +173,7 @@ def agg_generic(  # noqa: PLR0913
     dfs: list[Any] = []
     to_remove: list[int] = []
     for i, expr in enumerate(exprs):
-        if is_simple_aggregation(expr, implementation):
+        if is_simple_aggregation(expr):
             dfs.append(evaluate_simple_aggregation(expr, grouped))
             to_remove.append(i)
     exprs = [expr for i, expr in enumerate(exprs) if i not in to_remove]

narwhals/pandas_like/utils.py

@@ -232,7 +232,7 @@ def evaluate_simple_aggregation(expr: PandasExpr, grouped: Any) -> Any:
     Returns naive DataFrame.
     """
     if expr._depth == 0:
-        return grouped.size()["size"].rename(expr._output_names[0])
+        return grouped.size()["size"].rename(expr._output_names[0])  # type: ignore[index]
     if expr._root_names is None or expr._output_names is None:
         msg = "Expected expr to have root_names and output_names set, but they are None. Please report a bug."
         raise AssertionError(msg)