move getitem_returns_view from method to helper function

Author: jorisvandenbossche

pandas/core/arrays/_mixins.py

@@ -54,7 +54,10 @@
 from pandas.core.array_algos.transforms import shift
 from pandas.core.arrays.base import ExtensionArray
 from pandas.core.construction import extract_array
-from pandas.core.indexers import check_array_indexer
+from pandas.core.indexers import (
+    check_array_indexer,
+    getitem_returns_view,
+)
 from pandas.core.sorting import nargminmax
 
 if TYPE_CHECKING:
@@ -287,7 +290,7 @@ def __getitem__(
             if self.ndim == 1:
                 return self._box_func(result)
             result = self._from_backing_data(result)
-            if self._getitem_returns_view(key):
+            if getitem_returns_view(self, key):
                 result._readonly = self._readonly
             return result
 
@@ -300,7 +303,7 @@ def __getitem__(
             return self._box_func(result)
 
         result = self._from_backing_data(result)
-        if self._getitem_returns_view(key):
+        if getitem_returns_view(self, key):
             result._readonly = self._readonly
         return result
 

pandas/core/arrays/arrow/array.py

@@ -78,6 +78,7 @@
 from pandas.core.construction import extract_array
 from pandas.core.indexers import (
     check_array_indexer,
+    getitem_returns_view,
     unpack_tuple_and_ellipses,
     validate_indices,
 )
@@ -791,7 +792,7 @@ def __getitem__(self, item: PositionalIndexer):
         value = self._pa_array[item]
         if isinstance(value, pa.ChunkedArray):
             result = self._from_pyarrow_array(value)
-            if self._getitem_returns_view(item):
+            if getitem_returns_view(self, item):
                 result._readonly = self._readonly
             return result
         else:

pandas/core/arrays/base.py

@@ -433,22 +433,6 @@ def __getitem__(self, item: PositionalIndexer) -> Self | Any:
         """
         raise AbstractMethodError(self)
 
-    def _getitem_returns_view(self, key) -> bool:
-        if not isinstance(key, tuple):
-            key = (key,)
-
-        # filter out Ellipsis and np.newaxis
-        key = tuple(k for k in key if k is not Ellipsis and k is not np.newaxis)
-        if not key:
-            return True
-        # single integer gives view if selecting subset of 2D array
-        if self.ndim == 2 and lib.is_integer(key[0]):
-            return True
-        # slices always give views
-        if all(isinstance(k, slice) for k in key):
-            return True
-        return False
-
     def __setitem__(self, key, value) -> None:
         """
         Set one or more values inplace.

pandas/core/arrays/interval.py

@@ -87,7 +87,10 @@
     ensure_wrapped_if_datetimelike,
     extract_array,
 )
-from pandas.core.indexers import check_array_indexer
+from pandas.core.indexers import (
+    check_array_indexer,
+    getitem_returns_view,
+)
 from pandas.core.ops import (
     invalid_comparison,
     unpack_zerodim_and_defer,
@@ -843,7 +846,7 @@ def __getitem__(self, key: PositionalIndexer) -> Self | IntervalOrNA:
         # ndarray[Any, Any]]"; expected "Union[Union[DatetimeArray, TimedeltaArray],
         # ndarray[Any, Any]]"
         result = self._simple_new(left, right, dtype=self.dtype)  # type: ignore[arg-type]
-        if self._getitem_returns_view(key):
+        if getitem_returns_view(self, key):
             result._readonly = self._readonly
         return result
 

pandas/core/arrays/masked.py

@@ -76,7 +76,10 @@
     ensure_wrapped_if_datetimelike,
     extract_array,
 )
-from pandas.core.indexers import check_array_indexer
+from pandas.core.indexers import (
+    check_array_indexer,
+    getitem_returns_view,
+)
 from pandas.core.ops import invalid_comparison
 from pandas.core.util.hashing import hash_array
 
@@ -214,7 +217,7 @@ def __getitem__(self, item: PositionalIndexer) -> Self | Any:
             return self._data[item]
 
         result = self._simple_new(self._data[item], newmask)
-        if self._getitem_returns_view(item):
+        if getitem_returns_view(self, item):
             result._readonly = self._readonly
         return result
 

pandas/core/indexers/__init__.py

@@ -3,6 +3,7 @@
     check_key_length,
     check_setitem_lengths,
     disallow_ndim_indexing,
+    getitem_returns_view,
     is_empty_indexer,
     is_list_like_indexer,
     is_scalar_indexer,
@@ -19,6 +20,7 @@
     "check_key_length",
     "check_setitem_lengths",
     "disallow_ndim_indexing",
+    "getitem_returns_view",
     "is_empty_indexer",
     "is_list_like_indexer",
     "is_scalar_indexer",

pandas/core/indexers/utils.py

@@ -414,6 +414,27 @@ def unpack_tuple_and_ellipses(item: tuple):
     return item
 
 
+def getitem_returns_view(arr, key) -> bool:
+    """
+    Check if an ``arr.__getitem__`` call with given ``key`` would return a view
+    or not.
+    """
+    if not isinstance(key, tuple):
+        key = (key,)
+
+    # filter out Ellipsis and np.newaxis
+    key = tuple(k for k in key if k is not Ellipsis and k is not np.newaxis)
+    if not key:
+        return True
+    # single integer gives view if selecting subset of 2D array
+    if arr.ndim == 2 and lib.is_integer(key[0]):
+        return True
+    # slices always give views
+    if all(isinstance(k, slice) for k in key):
+        return True
+    return False
+
+
 # -----------------------------------------------------------
 # Public indexer validation
 

pandas/tests/extension/decimal/array.py

@@ -30,7 +30,10 @@
     ExtensionArray,
     ExtensionScalarOpsMixin,
 )
-from pandas.core.indexers import check_array_indexer
+from pandas.core.indexers import (
+    check_array_indexer,
+    getitem_returns_view,
+)
 
 if TYPE_CHECKING:
     from pandas._typing import type_t
@@ -178,7 +181,7 @@ def __getitem__(self, item):
             # array, slice.
             item = pd.api.indexers.check_array_indexer(self, item)
             result = type(self)(self._data[item])
-            if self._getitem_returns_view(item):
+            if getitem_returns_view(self, item):
                 result._readonly = self._readonly
             return result
 

pandas/tests/extension/json/array.py

@@ -41,7 +41,10 @@
     ExtensionArray,
     ExtensionDtype,
 )
-from pandas.core.indexers import unpack_tuple_and_ellipses
+from pandas.core.indexers import (
+    getitem_returns_view,
+    unpack_tuple_and_ellipses,
+)
 
 if TYPE_CHECKING:
     from collections.abc import Mapping
@@ -111,7 +114,7 @@ def __getitem__(self, item):
         elif isinstance(item, slice):
             # slice
             result = type(self)(self.data[item])
-            if self._getitem_returns_view(item):
+            if getitem_returns_view(self, item):
                 result._readonly = self._readonly
             return result
         elif not is_list_like(item):