From 187b7c9752ee7a95621f1a0cc3332ff982a6cf5a Mon Sep 17 00:00:00 2001
From: Marco Edward Gorelli <marcogorelli@protonmail.com>
Date: Thu, 18 Jul 2024 19:13:29 +0100
Subject: [PATCH] chore: refactor agg_arrow (#549)

* chore: refactor agg_arrow

* less magic indexing
---
 narwhals/_arrow/group_by.py | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/narwhals/_arrow/group_by.py b/narwhals/_arrow/group_by.py
index 279d32b7c..59090c4d4 100644
--- a/narwhals/_arrow/group_by.py
+++ b/narwhals/_arrow/group_by.py
@@ -74,8 +74,8 @@ def agg_arrow(
 
     if all_simple_aggs:
         # Mapping from output name to
-        # (input_column_name, function_name, pyarrow_output_name)  # noqa: ERA001
-        simple_aggregations: dict[str, tuple[Any, str, str]] = {}
+        # (aggregation_args, pyarrow_output_name)  # noqa: ERA001
+        simple_aggregations: dict[str, tuple[tuple[Any, ...], str]] = {}
         for expr in exprs:
             if expr._depth == 0:
                 # e.g. agg(nw.len()) # noqa: ERA001
@@ -85,8 +85,7 @@ def agg_arrow(
                     msg = "Safety assertion failed, please report a bug to https://github.com/narwhals-dev/narwhals/issues"
                     raise AssertionError(msg)
                 simple_aggregations[expr._output_names[0]] = (
-                    keys[0],
-                    "count",
+                    (keys[0], "count", pc.CountOptions(mode="all")),
                     f"{keys[0]}_count",
                 )
                 continue
@@ -102,19 +101,18 @@ def agg_arrow(
             function_name = POLARS_TO_ARROW_AGGREGATIONS.get(function_name, function_name)
             for root_name, output_name in zip(expr._root_names, expr._output_names):
                 simple_aggregations[output_name] = (
-                    root_name,
-                    function_name,
+                    (root_name, function_name),
                     f"{root_name}_{function_name}",
                 )
 
         aggs: list[Any] = []
         name_mapping = {}
-        for output_name, named_agg in simple_aggregations.items():
-            if named_agg[1] == "count":
-                aggs.append((named_agg[0], named_agg[1], pc.CountOptions(mode="all")))
-            else:
-                aggs.append((named_agg[0], named_agg[1]))
-            name_mapping[named_agg[2]] = output_name
+        for output_name, (
+            aggregation_args,
+            pyarrow_output_name,
+        ) in simple_aggregations.items():
+            aggs.append(aggregation_args)
+            name_mapping[pyarrow_output_name] = output_name
         result_simple = grouped.aggregate(aggs)
         result_simple = result_simple.rename_columns(
             [name_mapping.get(col, col) for col in result_simple.column_names]