fix(python): Add height check to frame-level row indexing when key is…

… int (#20778)

py-polars/polars/_utils/getitem.py

@@ -291,6 +291,10 @@ def _select_rows(
 ) -> DataFrame | Series:
     """Select one or more rows from the DataFrame."""
     if isinstance(key, int):
+        num_rows = len(df)
+        if (key >= num_rows) or (key < -num_rows):
+            msg = f"index {key} is out of bounds for DataFrame of height {len(df)}"
+            raise IndexError(msg)
         return df.slice(key, 1)
 
     if isinstance(key, slice):

py-polars/tests/unit/dataframe/test_df.py

@@ -2995,3 +2995,22 @@ def test_get_column_after_drop_20119() -> None:
     df.drop_in_place("a")
     c = df.get_column("c")
     assert_series_equal(c, pl.Series("c", ["C"]))
+
+
+def test_select_oob_row_20775() -> None:
+    df = pl.DataFrame({"a": [1, 2, 3]})
+    with pytest.raises(
+        IndexError,
+        match="index 99 is out of bounds for DataFrame of height 3",
+    ):
+        df[99]
+
+
+@pytest.mark.parametrize("idx", [3, 99, -4, -99])
+def test_select_oob_element_20775_too_large(idx: int) -> None:
+    df = pl.DataFrame({"a": [1, 2, 3]})
+    with pytest.raises(
+        IndexError,
+        match=f"index {idx} is out of bounds for sequence of length 3",
+    ):
+        df[idx, "a"]