chore: take reset_index(inplace=True) out of _select_results (#2828)

chore: take `reset_index(inplace=True)` out of `_select_results`, skip decimal test for polars pre 1.20

Author: MarcoGorelli

narwhals/_pandas_like/group_by.py

@@ -219,6 +219,9 @@ def agg(self, *exprs: PandasLikeExpr) -> PandasLikeDataFrame:
             raise empty_results_error()
         else:
             result = self._apply_aggs(exprs)
+        # NOTE: Keep `inplace=True` to avoid making a redundant copy.
+        # This may need updating, depending on https://github.com/pandas-dev/pandas/pull/51466/files
+        result.reset_index(inplace=True)  # noqa: PD002
         return self._select_results(result, agg_exprs)
 
     def _select_results(
@@ -228,9 +231,6 @@ def _select_results(
 
         See `ParseKeysGroupBy`.
         """
-        # NOTE: Keep `inplace=True` to avoid making a redundant copy.
-        # This may need updating, depending on https://github.com/pandas-dev/pandas/pull/51466/files
-        df.reset_index(inplace=True)  # noqa: PD002
         new_names = chain.from_iterable(e.aliases for e in agg_exprs)
         return (
             self.compliant._with_native(df, validate_column_names=False)

tests/frame/group_by_test.py

@@ -604,7 +604,7 @@ def test_group_by_no_preserve_dtype(
     if (
         "polars" in str(constructor_eager)
         and isinstance(low, Decimal)
-        and POLARS_VERSION < (1, 0, 0)
+        and POLARS_VERSION < (1, 21, 0)
     ):
         pytest.skip("Decimal support in group_by for polars didn't stabilize until 1.0.0")
     data = {