From 5bad66101779c020bce2b712626c0d5ada86d1b3 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Wed, 20 Mar 2024 18:29:38 +0000
Subject: [PATCH] never reset index

---
 narwhals/pandas_like/dataframe.py | 4 ++--
 narwhals/pandas_like/utils.py     | 9 +++------
 tests/test_common.py              | 4 ++--
 3 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/narwhals/pandas_like/dataframe.py b/narwhals/pandas_like/dataframe.py
index 1724a5395..c2a2c2796 100644
--- a/narwhals/pandas_like/dataframe.py
+++ b/narwhals/pandas_like/dataframe.py
@@ -8,9 +8,9 @@
 
 from narwhals.pandas_like.utils import evaluate_into_exprs
 from narwhals.pandas_like.utils import horizontal_concat
-from narwhals.pandas_like.utils import maybe_reset_indices
 from narwhals.pandas_like.utils import translate_dtype
 from narwhals.pandas_like.utils import validate_dataframe_comparand
+from narwhals.pandas_like.utils import validate_indices
 from narwhals.utils import flatten_str
 
 if TYPE_CHECKING:
@@ -87,7 +87,7 @@ def select(
         **named_exprs: IntoPandasExpr,
     ) -> Self:
         new_series = evaluate_into_exprs(self, *exprs, **named_exprs)
-        new_series = maybe_reset_indices(new_series)
+        new_series = validate_indices(new_series)
         df = horizontal_concat(
             [series._series for series in new_series],
             implementation=self._implementation,
diff --git a/narwhals/pandas_like/utils.py b/narwhals/pandas_like/utils.py
index 6c870506d..6c2c507b7 100644
--- a/narwhals/pandas_like/utils.py
+++ b/narwhals/pandas_like/utils.py
@@ -383,13 +383,10 @@ def reverse_translate_dtype(dtype: DType | type[DType]) -> Any:
     raise TypeError(msg)
 
 
-def maybe_reset_indices(series: list[PandasSeries]) -> list[PandasSeries]:
+def validate_indices(series: list[PandasSeries]) -> list[PandasSeries]:
     idx = series[0]._series.index
-    found_non_matching_index = False
     for s in series[1:]:
         if s._series.index is not idx and not (s._series.index == idx).all():
-            found_non_matching_index = True
-            break
-    if found_non_matching_index:
-        return [s._from_series(s._series.reset_index(drop=True)) for s in series]
+            msg = "Found implicit index alignment, which is not allowed by Narwhals."
+            raise RuntimeError(msg)
     return series
diff --git a/tests/test_common.py b/tests/test_common.py
index 011afaf11..e0f072888 100644
--- a/tests/test_common.py
+++ b/tests/test_common.py
@@ -328,8 +328,8 @@ def test_expr_min_max(df_raw: Any) -> None:
 @pytest.mark.parametrize("df_raw", [df_polars, df_pandas, df_mpd, df_lazy])
 def test_expr_sample(df_raw: Any) -> None:
     df = nw.LazyFrame(df_raw)
-    result_shape = nw.to_native(df.select(nw.col("a", "b").sample(n=2)).collect()).shape
-    expected = (2, 2)
+    result_shape = nw.to_native(df.select(nw.col("a").sample(n=2)).collect()).shape
+    expected = (2, 1)
     assert result_shape == expected