From 26c5cbcd91b4b3f3761b82c93f1e6d79ebcec2c0 Mon Sep 17 00:00:00 2001
From: raisadz <34237447+raisadz@users.noreply.github.com>
Date: Sun, 26 Jan 2025 10:54:56 +0000
Subject: [PATCH] feat: raise if expression changes length in `.over()` (#1867)

---
 narwhals/expr.py                   |  4 ++++
 tests/expr_and_series/over_test.py | 13 +++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/narwhals/expr.py b/narwhals/expr.py
index 5fbb4336d..ea912807a 100644
--- a/narwhals/expr.py
+++ b/narwhals/expr.py
@@ -13,6 +13,7 @@
 from narwhals._expression_parsing import operation_changes_length
 from narwhals._expression_parsing import operation_is_order_dependent
 from narwhals.dtypes import _validate_dtype
+from narwhals.exceptions import LengthChangingExprError
 from narwhals.expr_cat import ExprCatNamespace
 from narwhals.expr_dt import ExprDateTimeNamespace
 from narwhals.expr_list import ExprListNamespace
@@ -2702,6 +2703,9 @@ def over(self: Self, *keys: str | Iterable[str]) -> Self:
             │ 3   ┆ 2   ┆ 3   │
             └─────┴─────┴─────┘
         """
+        if self._changes_length:
+            msg = "`.over()` can not be used for expressions which change length."
+            raise LengthChangingExprError(msg)
         return self.__class__(
             lambda plx: self._to_compliant_expr(plx).over(flatten(keys)),
             self._is_order_dependent,
diff --git a/tests/expr_and_series/over_test.py b/tests/expr_and_series/over_test.py
index 2ccbe070d..ee8a14806 100644
--- a/tests/expr_and_series/over_test.py
+++ b/tests/expr_and_series/over_test.py
@@ -1,9 +1,12 @@
 from __future__ import annotations
 
+import re
+
 import pandas as pd
 import pytest
 
 import narwhals.stable.v1 as nw
+from narwhals.exceptions import LengthChangingExprError
 from tests.utils import PANDAS_VERSION
 from tests.utils import Constructor
 from tests.utils import ConstructorEager
@@ -207,3 +210,13 @@ def test_over_cum_reverse() -> None:
         match=r"Cumulative operation with `reverse=True` is not supported",
     ):
         nw.from_native(df).select(nw.col("b").cum_max(reverse=True).over("a"))
+
+
+def test_over_raise_len_change(constructor: Constructor) -> None:
+    df = nw.from_native(constructor(data))
+
+    with pytest.raises(
+        LengthChangingExprError,
+        match=re.escape("`.over()` can not be used for expressions which change length."),
+    ):
+        nw.from_native(df).select(nw.col("b").drop_nulls().over("a"))