fix: Fix DataFrame.__getitem__ when slicing with tuple and null sli…

…ce (#1123)

* fix: Fix `DataFrame.__getitem__` when slicing with tuple and null slice

* old polars compat

narwhals/_arrow/dataframe.py

@@ -176,6 +176,10 @@ def __getitem__(
         elif isinstance(item, tuple) and len(item) == 2:
             if isinstance(item[1], slice):
                 columns = self.columns
+                if item[1] == slice(None):
+                    if isinstance(item[0], Sequence) and len(item[0]) == 0:
+                        return self._from_native_frame(self._native_frame.slice(0, 0))
+                    return self._from_native_frame(self._native_frame.take(item[0]))
                 if isinstance(item[1].start, str) or isinstance(item[1].stop, str):
                     start, stop, step = convert_str_slice_to_int_slice(item[1], columns)
                     return self._from_native_frame(

narwhals/_pandas_like/dataframe.py

@@ -186,6 +186,8 @@ def __getitem__(
 
         elif isinstance(item, tuple) and len(item) == 2 and isinstance(item[1], slice):
             columns = self._native_frame.columns
+            if item[1] == slice(None):
+                return self._from_native_frame(self._native_frame.iloc[item[0], :])
             if isinstance(item[1].start, str) or isinstance(item[1].stop, str):
                 start, stop, step = convert_str_slice_to_int_slice(item[1], columns)
                 return self._from_native_frame(

narwhals/_polars/dataframe.py

@@ -2,6 +2,7 @@
 
 from typing import TYPE_CHECKING
 from typing import Any
+from typing import Sequence
 
 from narwhals._polars.namespace import PolarsNamespace
 from narwhals._polars.utils import convert_str_slice_to_int_slice
@@ -117,6 +118,12 @@ def __getitem__(self, item: Any) -> Any:
 
             columns = self.columns
             if isinstance(item, tuple) and len(item) == 2 and isinstance(item[1], slice):
+                if item[1] == slice(None):
+                    if isinstance(item[0], Sequence) and not len(item[0]):
+                        return self._from_native_frame(self._native_frame[0:0])
+                    return self._from_native_frame(
+                        self._native_frame.__getitem__(item[0])
+                    )
                 if isinstance(item[1].start, str) or isinstance(item[1].stop, str):
                     start, stop, step = convert_str_slice_to_int_slice(item[1], columns)
                     return self._from_native_frame(

narwhals/dataframe.py

@@ -843,8 +843,6 @@ def __getitem__(
         ):
             if item[1] == slice(None) and item[0] == slice(None):
                 return self
-            if item[1] == slice(None):
-                return self._from_compliant_dataframe(self._compliant_frame[item[0]])
             return self._from_compliant_dataframe(self._compliant_frame[item])
         if isinstance(item, str) or (isinstance(item, tuple) and len(item) == 2):
             return self._series(

tests/frame/getitem_test.py

@@ -177,6 +177,15 @@ def test_slice_slice_columns(constructor_eager: Any) -> None:  # noqa: PLR0915
     result = df["a":"b"]  # type: ignore[misc]
     expected = {"a": [1, 2, 3], "b": [4, 5, 6]}
     compare_dicts(result, expected)
+    result = df[(0, 1), :]
+    expected = {"a": [1, 2], "b": [4, 5], "c": [7, 8], "d": [1, 4]}
+    compare_dicts(result, expected)
+    result = df[[0, 1], :]
+    expected = {"a": [1, 2], "b": [4, 5], "c": [7, 8], "d": [1, 4]}
+    compare_dicts(result, expected)
+    result = df[[0, 1], df.columns]
+    expected = {"a": [1, 2], "b": [4, 5], "c": [7, 8], "d": [1, 4]}
+    compare_dicts(result, expected)
 
 
 def test_slice_invalid(constructor_eager: Any) -> None: