let getitem propagate readonly property

Author: jorisvandenbossche

pandas/core/arrays/_mixins.py

@@ -284,7 +284,10 @@ def __getitem__(
             result = self._ndarray[key]
             if self.ndim == 1:
                 return self._box_func(result)
-            return self._from_backing_data(result)
+            result = self._from_backing_data(result)
+            if self._getitem_returns_view(key):
+                result._readonly = self._readonly
+            return result
 
         # error: Incompatible types in assignment (expression has type "ExtensionArray",
         # variable has type "Union[int, slice, ndarray]")
@@ -295,6 +298,8 @@ def __getitem__(
             return self._box_func(result)
 
         result = self._from_backing_data(result)
+        if self._getitem_returns_view(key):
+            result._readonly = self._readonly
         return result
 
     def _pad_or_backfill(

pandas/core/arrays/arrow/array.py

@@ -730,7 +730,10 @@ def __getitem__(self, item: PositionalIndexer):
 
         value = self._pa_array[item]
         if isinstance(value, pa.ChunkedArray):
-            return self._from_pyarrow_array(value)
+            result = self._from_pyarrow_array(value)
+            if self._getitem_returns_view(item):
+                result._readonly = self._readonly
+            return result
         else:
             pa_type = self._pa_array.type
             scalar = value.as_py()

pandas/core/arrays/base.py

@@ -433,6 +433,22 @@ 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

@@ -724,7 +724,10 @@ def __getitem__(self, key: PositionalIndexer) -> Self | IntervalOrNA:
         # "Union[Period, Timestamp, Timedelta, NaTType, DatetimeArray, TimedeltaArray,
         # ndarray[Any, Any]]"; expected "Union[Union[DatetimeArray, TimedeltaArray],
         # ndarray[Any, Any]]"
-        return self._simple_new(left, right, dtype=self.dtype)  # type: ignore[arg-type]
+        result = self._simple_new(left, right, dtype=self.dtype)
+        if self._getitem_returns_view(key):
+            result._readonly = self._readonly
+        return result  # type: ignore[arg-type]
 
     def __setitem__(self, key, value) -> None:
         if self._readonly:

pandas/core/arrays/masked.py

@@ -200,7 +200,10 @@ def __getitem__(self, item: PositionalIndexer) -> Self | Any:
                 return self.dtype.na_value
             return self._data[item]
 
-        return self._simple_new(self._data[item], newmask)
+        result = self._simple_new(self._data[item], newmask)
+        if self._getitem_returns_view(item):
+            result._readonly = self._readonly
+        return result
 
     def _pad_or_backfill(
         self,

pandas/core/arrays/sparse/array.py

@@ -1008,7 +1008,11 @@ def __getitem__(
         elif isinstance(key, slice):
             if key == slice(None):
                 # to ensure arr[:] (used by view()) does not make a copy
-                return type(self)._simple_new(self.sp_values, self.sp_index, self.dtype)
+                result = type(self)._simple_new(
+                    self.sp_values, self.sp_index, self.dtype
+                )
+                result._readonly = self._readonly
+                return result
             # Avoid densifying when handling contiguous slices
             if key.step is None or key.step == 1:
                 start = 0 if key.start is None else key.start

pandas/tests/extension/base/getitem.py

@@ -467,3 +467,10 @@ def test_item(self, data):
 
         with pytest.raises(ValueError, match=msg):
             s.item()
+
+    def test_getitem_propagates_readonly_property(self, data):
+        # ensure read-only propagates if getitem returns view
+        data._readonly = True
+
+        result = data[:]
+        assert result._readonly

pandas/tests/extension/decimal/array.py

@@ -177,7 +177,10 @@ def __getitem__(self, item):
         else:
             # array, slice.
             item = pd.api.indexers.check_array_indexer(self, item)
-            return type(self)(self._data[item])
+            result = type(self)(self._data[item])
+            if self._getitem_returns_view(item):
+                result._readonly = self._readonly
+            return result
 
     def take(self, indexer, allow_fill=False, fill_value=None):
         from pandas.api.extensions import take

pandas/tests/extension/json/array.py

@@ -105,10 +105,15 @@ def __getitem__(self, item):
             return self.data[item]
         elif isinstance(item, slice) and item == slice(None):
             # Make sure we get a view
-            return type(self)(self.data)
+            result = type(self)(self.data)
+            result._readonly = self._readonly
+            return result
         elif isinstance(item, slice):
             # slice
-            return type(self)(self.data[item])
+            result = type(self)(self.data[item])
+            if self._getitem_returns_view(item):
+                result._readonly = self._readonly
+            return result
         elif not is_list_like(item):
             # e.g. "foo" or 2.5
             # exception message copied from numpy