From a9df51b017f02bd005972b2bdaf18a5e3c383541 Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Wed, 23 Jul 2025 00:46:10 +0200
Subject: [PATCH 1/7] CoW: add readonly flag to ExtensionArrays, return
 read-only EA/ndarray in .array/EA.to_numpy()

---
 pandas/core/arrays/_mixins.py           |  3 ++
 pandas/core/arrays/arrow/array.py       |  3 ++
 pandas/core/arrays/base.py              | 65 +++++++++++++++++++++++++
 pandas/core/arrays/datetimelike.py      |  7 ++-
 pandas/core/arrays/interval.py          |  3 ++
 pandas/core/arrays/masked.py            | 14 +++++-
 pandas/core/arrays/numpy_.py            | 23 +++++++--
 pandas/core/arrays/period.py            |  6 ++-
 pandas/core/arrays/sparse/array.py      |  8 ++-
 pandas/core/arrays/string_.py           |  3 ++
 pandas/core/dtypes/astype.py            | 11 +++++
 pandas/core/indexes/base.py             |  2 +
 pandas/core/internals/blocks.py         |  6 ++-
 pandas/core/series.py                   |  5 +-
 pandas/tests/extension/base/setitem.py  | 57 ++++++++++++++++++++++
 pandas/tests/extension/date/array.py    |  3 ++
 pandas/tests/extension/decimal/array.py |  3 ++
 pandas/tests/extension/json/array.py    |  3 ++
 18 files changed, 215 insertions(+), 10 deletions(-)

diff --git a/pandas/core/arrays/_mixins.py b/pandas/core/arrays/_mixins.py
index 26585e7bab8e3..2eb235f87d231 100644
--- a/pandas/core/arrays/_mixins.py
+++ b/pandas/core/arrays/_mixins.py
@@ -252,6 +252,9 @@ def shift(self, periods: int = 1, fill_value=None) -> Self:
         return self._from_backing_data(new_values)
 
     def __setitem__(self, key, value) -> None:
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         key = check_array_indexer(self, key)
         value = self._validate_setitem_value(value)
         self._ndarray[key] = value
diff --git a/pandas/core/arrays/arrow/array.py b/pandas/core/arrays/arrow/array.py
index 919453b29b7f9..61082f920a4ab 100644
--- a/pandas/core/arrays/arrow/array.py
+++ b/pandas/core/arrays/arrow/array.py
@@ -1967,6 +1967,9 @@ def __setitem__(self, key, value) -> None:
         -------
         None
         """
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         # GH50085: unwrap 1D indexers
         if isinstance(key, tuple) and len(key) == 1:
             key = key[0]
diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index d11e2271f9574..5d32e7f025416 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -40,6 +40,7 @@
     validate_insert_loc,
 )
 
+from pandas.core.dtypes.astype import astype_is_view
 from pandas.core.dtypes.cast import maybe_cast_pointwise_result
 from pandas.core.dtypes.common import (
     is_list_like,
@@ -269,6 +270,8 @@ class ExtensionArray:
     #  strictly less than 2000 to be below Index.__pandas_priority__.
     __pandas_priority__ = 1000
 
+    _readonly = False
+
     # ------------------------------------------------------------------------
     # Constructors
     # ------------------------------------------------------------------------
@@ -482,6 +485,11 @@ def __setitem__(self, key, value) -> None:
         Returns
         -------
         None
+
+        Raises
+        ------
+        ValueError
+            If the array is readonly and modification is attempted.
         """
         # Some notes to the ExtensionArray implementer who may have ended up
         # here. While this method is not required for the interface, if you
@@ -501,8 +509,59 @@ def __setitem__(self, key, value) -> None:
         #   __init__ method coerces that value, then so should __setitem__
         # Note, also, that Series/DataFrame.where internally use __setitem__
         # on a copy of the data.
+        # Check if the array is readonly
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         raise NotImplementedError(f"{type(self)} does not implement __setitem__.")
 
+    @property
+    def readonly(self) -> bool:
+        """
+        Whether the array is readonly.
+
+        If True, attempts to modify the array via __setitem__ will raise
+        a ValueError.
+
+        Returns
+        -------
+        bool
+            True if the array is readonly, False otherwise.
+
+        Examples
+        --------
+        >>> arr = pd.array([1, 2, 3])
+        >>> arr.readonly
+        False
+        >>> arr.readonly = True
+        >>> arr[0] = 5
+        Traceback (most recent call last):
+        ...
+        ValueError: Cannot modify readonly ExtensionArray
+        """
+        return getattr(self, "_readonly", False)
+
+    @readonly.setter
+    def readonly(self, value: bool) -> None:
+        """
+        Set the readonly state of the array.
+
+        Parameters
+        ----------
+        value : bool
+            True to make the array readonly, False to make it writable.
+
+        Examples
+        --------
+        >>> arr = pd.array([1, 2, 3])
+        >>> arr.readonly = True
+        >>> arr.readonly
+        True
+        """
+        if not isinstance(value, bool):
+            raise TypeError("readonly must be a boolean")
+        self._readonly = value
+
     def __len__(self) -> int:
         """
         Length of this array
@@ -595,8 +654,14 @@ def to_numpy(
         result = np.asarray(self, dtype=dtype)
         if copy or na_value is not lib.no_default:
             result = result.copy()
+        elif self._readonly and astype_is_view(self.dtype, result.dtype):
+            # If the ExtensionArray is readonly, make the numpy array readonly too
+            result = result.view()
+            result.flags.writeable = False
+
         if na_value is not lib.no_default:
             result[self.isna()] = na_value  # type: ignore[index]
+
         return result
 
     # ------------------------------------------------------------------------
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index d5e654c95577e..91f6646d07c21 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -368,7 +368,12 @@ def __array__(
 
         if copy is True:
             return np.array(self._ndarray, dtype=dtype)
-        return self._ndarray
+
+        result = self._ndarray
+        if self._readonly:
+            result = result.view()
+            result.flags.writeable = False
+        return result
 
     @overload
     def __getitem__(self, key: ScalarIndexer) -> DTScalarOrNaT: ...
diff --git a/pandas/core/arrays/interval.py b/pandas/core/arrays/interval.py
index 4bcbe2eedee47..9996eb93ea8f0 100644
--- a/pandas/core/arrays/interval.py
+++ b/pandas/core/arrays/interval.py
@@ -729,6 +729,9 @@ def __getitem__(self, key: PositionalIndexer) -> Self | IntervalOrNA:
         return self._simple_new(left, right, dtype=self.dtype)  # type: ignore[arg-type]
 
     def __setitem__(self, key, value) -> None:
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         value_left, value_right = self._validate_setitem_value(value)
         key = check_array_indexer(self, key)
 
diff --git a/pandas/core/arrays/masked.py b/pandas/core/arrays/masked.py
index fefd70fef35c9..59cbda6fd1e64 100644
--- a/pandas/core/arrays/masked.py
+++ b/pandas/core/arrays/masked.py
@@ -23,6 +23,7 @@
 from pandas.errors import AbstractMethodError
 from pandas.util._decorators import doc
 
+from pandas.core.dtypes.astype import astype_is_view
 from pandas.core.dtypes.base import ExtensionDtype
 from pandas.core.dtypes.common import (
     is_bool,
@@ -290,6 +291,9 @@ def _validate_setitem_value(self, value):
         raise TypeError(f"Invalid value '{value!s}' for dtype '{self.dtype}'")
 
     def __setitem__(self, key, value) -> None:
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         key = check_array_indexer(self, key)
 
         if is_scalar(value):
@@ -520,6 +524,9 @@ def to_numpy(
             with warnings.catch_warnings():
                 warnings.filterwarnings("ignore", category=RuntimeWarning)
                 data = self._data.astype(dtype, copy=copy)
+            if self._readonly and astype_is_view(self.dtype, dtype):
+                data = data.view()
+                data.flags.writeable = False
         return data
 
     @doc(ExtensionArray.tolist)
@@ -596,7 +603,12 @@ def __array__(
         if copy is False:
             if not self._hasna:
                 # special case, here we can simply return the underlying data
-                return np.array(self._data, dtype=dtype, copy=copy)
+                result = np.array(self._data, dtype=dtype, copy=copy)
+                # If the ExtensionArray is readonly, make the numpy array readonly too
+                if self._readonly:
+                    result = result.view()
+                    result.flags.writeable = False
+                return result
             raise ValueError(
                 "Unable to avoid copy while creating an array as requested."
             )
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index fd2c8c9d63362..ebede913e77fc 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -12,7 +12,10 @@
 from pandas._libs.tslibs import is_supported_dtype
 from pandas.compat.numpy import function as nv
 
-from pandas.core.dtypes.astype import astype_array
+from pandas.core.dtypes.astype import (
+    astype_array,
+    astype_is_view,
+)
 from pandas.core.dtypes.cast import construct_1d_object_array_from_listlike
 from pandas.core.dtypes.common import pandas_dtype
 from pandas.core.dtypes.dtypes import NumpyEADtype
@@ -160,8 +163,19 @@ def __array__(
     ) -> np.ndarray:
         if copy is not None:
             # Note: branch avoids `copy=None` for NumPy 1.x support
-            return np.array(self._ndarray, dtype=dtype, copy=copy)
-        return np.asarray(self._ndarray, dtype=dtype)
+            result = np.array(self._ndarray, dtype=dtype, copy=copy)
+        else:
+            result = np.asarray(self._ndarray, dtype=dtype)
+
+        if (
+            self._readonly
+            and not copy
+            and (dtype is None or astype_is_view(self.dtype, dtype))
+        ):
+            result = result.view()
+            result.flags.writeable = False
+
+        return result
 
     def __array_ufunc__(self, ufunc: np.ufunc, method: str, *inputs, **kwargs):
         # Lightly modified version of
@@ -512,6 +526,9 @@ def to_numpy(
             result[mask] = na_value
         else:
             result = self._ndarray
+            if not copy and self._readonly:
+                result = result.view()
+                result.flags.writeable = False
 
         result = np.asarray(result, dtype=dtype)
 
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index ae92e17332c76..d2b3448a59896 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -393,7 +393,11 @@ def __array__(
             # For NumPy 1.x compatibility we cannot use copy=None.  And
             # `copy=False` has the meaning of `copy=None` here:
             if not copy:
-                return np.asarray(self.asi8, dtype=dtype)
+                result = np.asarray(self.asi8, dtype=dtype)
+                if self._readonly:
+                    result = result.view()
+                    result.flags.writeable = False
+                return result
             else:
                 return np.array(self.asi8, dtype=dtype)
 
diff --git a/pandas/core/arrays/sparse/array.py b/pandas/core/arrays/sparse/array.py
index 137dbb6e4d139..fbf457e414c13 100644
--- a/pandas/core/arrays/sparse/array.py
+++ b/pandas/core/arrays/sparse/array.py
@@ -562,7 +562,11 @@ def __array__(
             if copy is True:
                 return np.array(self.sp_values)
             else:
-                return self.sp_values
+                result = self.sp_values
+                if self._readonly:
+                    result = result.view()
+                    result.flags.writeable = False
+                return result
 
         if copy is False:
             raise ValueError(
@@ -591,6 +595,8 @@ def __array__(
         return out
 
     def __setitem__(self, key, value) -> None:
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
         # I suppose we could allow setting of non-fill_value elements.
         # TODO(SparseArray.__setitem__): remove special cases in
         # ExtensionBlock.where
diff --git a/pandas/core/arrays/string_.py b/pandas/core/arrays/string_.py
index f52b709a59de9..abf77b57e2a49 100644
--- a/pandas/core/arrays/string_.py
+++ b/pandas/core/arrays/string_.py
@@ -769,6 +769,9 @@ def _maybe_convert_setitem_value(self, value):
         return value
 
     def __setitem__(self, key, value) -> None:
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         value = self._maybe_convert_setitem_value(value)
 
         key = check_array_indexer(self, key)
diff --git a/pandas/core/dtypes/astype.py b/pandas/core/dtypes/astype.py
index 086f7d2da6640..f9cdb50254730 100644
--- a/pandas/core/dtypes/astype.py
+++ b/pandas/core/dtypes/astype.py
@@ -24,8 +24,11 @@
     pandas_dtype,
 )
 from pandas.core.dtypes.dtypes import (
+    CategoricalDtype,
     ExtensionDtype,
+    IntervalDtype,
     NumpyEADtype,
+    PeriodDtype,
 )
 
 if TYPE_CHECKING:
@@ -283,6 +286,14 @@ def astype_is_view(dtype: DtypeObj, new_dtype: DtypeObj) -> bool:
         new_dtype = getattr(new_dtype, "numpy_dtype", new_dtype)
         return getattr(dtype, "unit", None) == getattr(new_dtype, "unit", None)
 
+    elif new_dtype == object and isinstance(dtype, (PeriodDtype, IntervalDtype)):
+        return False
+
+    elif isinstance(dtype, CategoricalDtype) and not isinstance(
+        new_dtype, CategoricalDtype
+    ):
+        return False
+
     numpy_dtype = getattr(dtype, "numpy_dtype", None)
     new_numpy_dtype = getattr(new_dtype, "numpy_dtype", None)
 
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index e8c5a03a6de50..4172e159b0d90 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -4972,6 +4972,8 @@ def array(self) -> ExtensionArray:
             from pandas.core.arrays.numpy_ import NumpyExtensionArray
 
             array = NumpyExtensionArray(array)
+        array = array.view()
+        array._readonly = True
         return array
 
     @property
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 6aa5062b8ed86..f778cf618d077 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -2359,7 +2359,9 @@ def external_values(values: ArrayLike) -> ArrayLike:
     if isinstance(values, np.ndarray):
         values = values.view()
         values.flags.writeable = False
-
-    # TODO(CoW) we should also mark our ExtensionArrays as read-only
+    else:
+        # ExtensionArrays
+        values = values.view()
+        values._readonly = True
 
     return values
diff --git a/pandas/core/series.py b/pandas/core/series.py
index ce5b2e5ed8de5..1bd46f779831b 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -820,7 +820,10 @@ def _references(self) -> BlockValuesRefs:
     @Appender(base.IndexOpsMixin.array.__doc__)  # type: ignore[prop-decorator]
     @property
     def array(self) -> ExtensionArray:
-        return self._mgr.array_values()
+        arr = self._mgr.array_values()
+        arr = arr.view()
+        arr._readonly = True
+        return arr
 
     def __len__(self) -> int:
         """
diff --git a/pandas/tests/extension/base/setitem.py b/pandas/tests/extension/base/setitem.py
index 185d6d750cace..3c81ff4d457ea 100644
--- a/pandas/tests/extension/base/setitem.py
+++ b/pandas/tests/extension/base/setitem.py
@@ -454,3 +454,60 @@ def test_setitem_2d_values(self, data):
         df.loc[[0, 1], :] = df.loc[[1, 0], :].values
         assert (df.loc[0, :] == original[1]).all()
         assert (df.loc[1, :] == original[0]).all()
+
+    def test_readonly_property(self, data):
+        assert data.readonly is False
+
+        data.readonly = True
+        assert data.readonly is True
+
+        data_orig = data.copy()
+        assert data_orig.readonly is False
+
+        with pytest.raises(ValueError, match="Cannot modify readonly array"):
+            data[0] = data[1]
+
+        with pytest.raises(ValueError, match="Cannot modify readonly array"):
+            data[0:3] = data[1]
+
+        with pytest.raises(ValueError, match="Cannot modify readonly array"):
+            data[np.array([True] * len(data))] = data[1]
+
+        tm.assert_extension_array_equal(data, data_orig)
+
+    def test_readonly_propagates_to_numpy_array(self, data):
+        data.readonly = True
+
+        # when we ask for a copy, the result should never be readonly
+        arr = np.array(data)
+        assert arr.flags.writeable
+
+        # when we don't ask for a copy -> if the conversion is zero-copy,
+        # the result should be readonly
+        arr1 = np.asarray(data)
+        arr2 = np.asarray(data)
+        if np.shares_memory(arr1, arr2):
+            assert not arr1.flags.writeable
+        else:
+            assert arr1.flags.writeable
+
+    def test_readonly_propagates_to_numpy_array_method(self, data):
+        data.readonly = True
+
+        # when we ask for a copy, the result should never be readonly
+        arr = data.to_numpy(copy=True)
+        assert arr.flags.writeable
+
+        # when we don't ask for a copy -> if the conversion is zero-copy,
+        # the result should be readonly
+        arr1 = data.to_numpy(copy=False)
+        arr2 = data.to_numpy(copy=False)
+        if np.shares_memory(arr1, arr2):
+            assert not arr1.flags.writeable
+        else:
+            assert arr1.flags.writeable
+
+        # non-NA fill value should always result in a copy
+        if data.isna().any():
+            arr = data.to_numpy(copy=False, na_value=data[0])
+            assert arr.flags.writeable
diff --git a/pandas/tests/extension/date/array.py b/pandas/tests/extension/date/array.py
index 0c51570189a7c..e6a4441bca97c 100644
--- a/pandas/tests/extension/date/array.py
+++ b/pandas/tests/extension/date/array.py
@@ -149,6 +149,9 @@ def __getitem__(self, item: PositionalIndexer):
             raise NotImplementedError("only ints are supported as indexes")
 
     def __setitem__(self, key: int | slice | np.ndarray, value: Any) -> None:
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         if not isinstance(key, int):
             raise NotImplementedError("only ints are supported as indexes")
 
diff --git a/pandas/tests/extension/decimal/array.py b/pandas/tests/extension/decimal/array.py
index 2ee6a73ec4054..de8e45679ea59 100644
--- a/pandas/tests/extension/decimal/array.py
+++ b/pandas/tests/extension/decimal/array.py
@@ -195,6 +195,9 @@ def astype(self, dtype, copy=True):
         return super().astype(dtype, copy=copy)
 
     def __setitem__(self, key, value) -> None:
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         if is_list_like(value):
             if is_scalar(key):
                 raise ValueError("setting an array element with a sequence.")
diff --git a/pandas/tests/extension/json/array.py b/pandas/tests/extension/json/array.py
index b110911bda400..7c01bdef88c0a 100644
--- a/pandas/tests/extension/json/array.py
+++ b/pandas/tests/extension/json/array.py
@@ -120,6 +120,9 @@ def __getitem__(self, item):
             return type(self)([self.data[i] for i in item])
 
     def __setitem__(self, key, value) -> None:
+        if self._readonly:
+            raise ValueError("Cannot modify readonly array")
+
         if isinstance(key, numbers.Integral):
             self.data[key] = value
         else:

From 9cd6e4f39fc109ed5326606cb11c6905d8b34716 Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Wed, 23 Jul 2025 01:16:39 +0200
Subject: [PATCH 2/7] cleanup

---
 pandas/core/arrays/base.py | 47 --------------------------------------
 1 file changed, 47 deletions(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 5d32e7f025416..5c488b6aa0906 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -515,53 +515,6 @@ def __setitem__(self, key, value) -> None:
 
         raise NotImplementedError(f"{type(self)} does not implement __setitem__.")
 
-    @property
-    def readonly(self) -> bool:
-        """
-        Whether the array is readonly.
-
-        If True, attempts to modify the array via __setitem__ will raise
-        a ValueError.
-
-        Returns
-        -------
-        bool
-            True if the array is readonly, False otherwise.
-
-        Examples
-        --------
-        >>> arr = pd.array([1, 2, 3])
-        >>> arr.readonly
-        False
-        >>> arr.readonly = True
-        >>> arr[0] = 5
-        Traceback (most recent call last):
-        ...
-        ValueError: Cannot modify readonly ExtensionArray
-        """
-        return getattr(self, "_readonly", False)
-
-    @readonly.setter
-    def readonly(self, value: bool) -> None:
-        """
-        Set the readonly state of the array.
-
-        Parameters
-        ----------
-        value : bool
-            True to make the array readonly, False to make it writable.
-
-        Examples
-        --------
-        >>> arr = pd.array([1, 2, 3])
-        >>> arr.readonly = True
-        >>> arr.readonly
-        True
-        """
-        if not isinstance(value, bool):
-            raise TypeError("readonly must be a boolean")
-        self._readonly = value
-
     def __len__(self) -> int:
         """
         Length of this array

From c6f37d1f0cfd50c70aa729da03c355eb2f17e24b Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Wed, 23 Jul 2025 01:36:38 +0200
Subject: [PATCH 3/7] fixup attribute name in tests

---
 pandas/tests/extension/base/setitem.py | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/pandas/tests/extension/base/setitem.py b/pandas/tests/extension/base/setitem.py
index 3c81ff4d457ea..853b314e76a8e 100644
--- a/pandas/tests/extension/base/setitem.py
+++ b/pandas/tests/extension/base/setitem.py
@@ -456,13 +456,13 @@ def test_setitem_2d_values(self, data):
         assert (df.loc[1, :] == original[0]).all()
 
     def test_readonly_property(self, data):
-        assert data.readonly is False
+        assert data._readonly is False
 
-        data.readonly = True
-        assert data.readonly is True
+        data._readonly = True
+        assert data._readonly is True
 
         data_orig = data.copy()
-        assert data_orig.readonly is False
+        assert data_orig._readonly is False
 
         with pytest.raises(ValueError, match="Cannot modify readonly array"):
             data[0] = data[1]
@@ -476,7 +476,7 @@ def test_readonly_property(self, data):
         tm.assert_extension_array_equal(data, data_orig)
 
     def test_readonly_propagates_to_numpy_array(self, data):
-        data.readonly = True
+        data._readonly = True
 
         # when we ask for a copy, the result should never be readonly
         arr = np.array(data)
@@ -492,7 +492,7 @@ def test_readonly_propagates_to_numpy_array(self, data):
             assert arr1.flags.writeable
 
     def test_readonly_propagates_to_numpy_array_method(self, data):
-        data.readonly = True
+        data._readonly = True
 
         # when we ask for a copy, the result should never be readonly
         arr = data.to_numpy(copy=True)

From 8058d9ad53ef933ec3fb650c6bd8caf597dc1da8 Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Wed, 23 Jul 2025 08:49:59 +0200
Subject: [PATCH 4/7] fix tests

---
 pandas/_libs/ops.pyx                         | 4 ++--
 pandas/tests/arrays/integer/test_indexing.py | 2 +-
 pandas/tests/arrays/numpy_/test_numpy.py     | 7 -------
 pandas/tests/arrays/test_datetimelike.py     | 4 ++--
 pandas/tests/extension/test_common.py        | 2 +-
 pandas/tests/extension/test_numpy.py         | 2 +-
 pandas/tests/internals/test_internals.py     | 2 +-
 pandas/tests/series/test_constructors.py     | 4 ++--
 8 files changed, 10 insertions(+), 17 deletions(-)

diff --git a/pandas/_libs/ops.pyx b/pandas/_libs/ops.pyx
index 567bfc02a2950..8b53e842a7988 100644
--- a/pandas/_libs/ops.pyx
+++ b/pandas/_libs/ops.pyx
@@ -177,7 +177,7 @@ def vec_compare(ndarray[object] left, ndarray[object] right, object op) -> ndarr
 
 @cython.wraparound(False)
 @cython.boundscheck(False)
-def scalar_binop(object[:] values, object val, object op) -> ndarray:
+def scalar_binop(ndarray[object] values, object val, object op) -> ndarray:
     """
     Apply the given binary operator `op` between each element of the array
     `values` and the scalar `val`.
@@ -214,7 +214,7 @@ def scalar_binop(object[:] values, object val, object op) -> ndarray:
 
 @cython.wraparound(False)
 @cython.boundscheck(False)
-def vec_binop(object[:] left, object[:] right, object op) -> ndarray:
+def vec_binop(ndarray[object] left, ndarray[object] right, object op) -> ndarray:
     """
     Apply the given binary operator `op` pointwise to the elements of
     arrays `left` and `right`.
diff --git a/pandas/tests/arrays/integer/test_indexing.py b/pandas/tests/arrays/integer/test_indexing.py
index 4b953d699108b..ce801db5cb58d 100644
--- a/pandas/tests/arrays/integer/test_indexing.py
+++ b/pandas/tests/arrays/integer/test_indexing.py
@@ -12,7 +12,7 @@ def test_array_setitem_nullable_boolean_mask():
 
 def test_array_setitem():
     # GH 31446
-    arr = pd.Series([1, 2], dtype="Int64").array
+    arr = pd.array([1, 2], dtype="Int64")
     arr[arr > 1] = 1
 
     expected = pd.array([1, 1], dtype="Int64")
diff --git a/pandas/tests/arrays/numpy_/test_numpy.py b/pandas/tests/arrays/numpy_/test_numpy.py
index 620a553d5a731..af3a793a3ef77 100644
--- a/pandas/tests/arrays/numpy_/test_numpy.py
+++ b/pandas/tests/arrays/numpy_/test_numpy.py
@@ -159,13 +159,6 @@ def test_to_numpy():
 # Setitem
 
 
-def test_setitem_series():
-    ser = pd.Series([1, 2, 3])
-    ser.array[0] = 10
-    expected = pd.Series([10, 2, 3])
-    tm.assert_series_equal(ser, expected)
-
-
 def test_setitem(any_numpy_array):
     nparr = any_numpy_array
     arr = NumpyExtensionArray(nparr, copy=True)
diff --git a/pandas/tests/arrays/test_datetimelike.py b/pandas/tests/arrays/test_datetimelike.py
index d1ef29b0bf8a0..a2eeee1447111 100644
--- a/pandas/tests/arrays/test_datetimelike.py
+++ b/pandas/tests/arrays/test_datetimelike.py
@@ -1248,8 +1248,8 @@ def test_invalid_nat_setitem_array(arr, non_casting_nats):
 @pytest.mark.parametrize(
     "arr",
     [
-        pd.date_range("2000", periods=4).array,
-        pd.timedelta_range("2000", periods=4).array,
+        pd.date_range("2000", periods=4).array.copy(),
+        pd.timedelta_range("2000", periods=4).array.copy(),
     ],
 )
 def test_to_numpy_extra(arr):
diff --git a/pandas/tests/extension/test_common.py b/pandas/tests/extension/test_common.py
index 5eda0f00f54ca..9f782770a02e4 100644
--- a/pandas/tests/extension/test_common.py
+++ b/pandas/tests/extension/test_common.py
@@ -101,5 +101,5 @@ def test_ellipsis_index():
     # String comparison because there's no native way to compare slices.
     # Before the fix for GH#42430, last_item_arg would get set to the 2D slice
     # (Ellipsis, slice(None, 1, None))
-    out = df["col1"].array.last_item_arg
+    out = df["col1"]._values.last_item_arg
     assert str(out) == "slice(None, 1, None)"
diff --git a/pandas/tests/extension/test_numpy.py b/pandas/tests/extension/test_numpy.py
index 79cfb736941d6..e6301d9a23604 100644
--- a/pandas/tests/extension/test_numpy.py
+++ b/pandas/tests/extension/test_numpy.py
@@ -77,7 +77,7 @@ def allow_in_pandas(monkeypatch):
 @pytest.fixture
 def data(allow_in_pandas, dtype):
     if dtype.numpy_dtype == "object":
-        return pd.Series([(i,) for i in range(100)]).array
+        return pd.Series([(i,) for i in range(100)]).array.copy()
     return NumpyExtensionArray(np.arange(1, 101, dtype=dtype._dtype))
 
 
diff --git a/pandas/tests/internals/test_internals.py b/pandas/tests/internals/test_internals.py
index 11e6b99204aee..2622310aaacd6 100644
--- a/pandas/tests/internals/test_internals.py
+++ b/pandas/tests/internals/test_internals.py
@@ -1347,7 +1347,7 @@ def check_series_setitem(self, elem, index: Index, inplace: bool):
             ser[: len(elem)] = elem
 
         if inplace:
-            assert ser.array is arr  # i.e. setting was done inplace
+            assert ser._values is arr  # i.e. setting was done inplace
         else:
             assert ser.dtype == object
 
diff --git a/pandas/tests/series/test_constructors.py b/pandas/tests/series/test_constructors.py
index f82451a2be84d..f8af23de89297 100644
--- a/pandas/tests/series/test_constructors.py
+++ b/pandas/tests/series/test_constructors.py
@@ -525,9 +525,9 @@ def test_categorical_sideeffects_free(self):
         # so this WILL change values
         cat = Categorical(["a", "b", "c", "a"])
         s = Series(cat, copy=False)
-        assert s.values is cat
+        assert s._values is cat
         s = s.cat.rename_categories([1, 2, 3])
-        assert s.values is not cat
+        assert s._values is not cat
         exp_s = np.array([1, 2, 3, 1], dtype=np.int64)
         tm.assert_numpy_array_equal(s.__array__(), exp_s)
 

From 91465ee00540b109892c38b40520cbe1ef112b44 Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Wed, 23 Jul 2025 10:05:00 +0200
Subject: [PATCH 5/7] more test fixes

---
 pandas/core/arrays/sparse/array.py       | 2 ++
 pandas/core/dtypes/astype.py             | 5 ++++-
 pandas/tests/base/test_conversion.py     | 2 +-
 pandas/tests/extension/base/setitem.py   | 7 ++++++-
 pandas/tests/extension/json/array.py     | 1 -
 pandas/tests/extension/json/test_json.py | 6 ++++++
 6 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/pandas/core/arrays/sparse/array.py b/pandas/core/arrays/sparse/array.py
index fbf457e414c13..2912444a5185e 100644
--- a/pandas/core/arrays/sparse/array.py
+++ b/pandas/core/arrays/sparse/array.py
@@ -975,6 +975,8 @@ def __getitem__(
             # _NestedSequence[Union[bool, int]]], ...]]"
             data_slice = self.to_dense()[key]  # type: ignore[index]
         elif isinstance(key, slice):
+            if key == slice(None):
+                return type(self)._simple_new(self.sp_values, self.sp_index, self.dtype)
             # 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
diff --git a/pandas/core/dtypes/astype.py b/pandas/core/dtypes/astype.py
index f9cdb50254730..75464cbb2b5f9 100644
--- a/pandas/core/dtypes/astype.py
+++ b/pandas/core/dtypes/astype.py
@@ -25,6 +25,7 @@
 )
 from pandas.core.dtypes.dtypes import (
     CategoricalDtype,
+    DatetimeTZDtype,
     ExtensionDtype,
     IntervalDtype,
     NumpyEADtype,
@@ -286,7 +287,9 @@ def astype_is_view(dtype: DtypeObj, new_dtype: DtypeObj) -> bool:
         new_dtype = getattr(new_dtype, "numpy_dtype", new_dtype)
         return getattr(dtype, "unit", None) == getattr(new_dtype, "unit", None)
 
-    elif new_dtype == object and isinstance(dtype, (PeriodDtype, IntervalDtype)):
+    elif new_dtype == object and isinstance(
+        dtype, (DatetimeTZDtype, PeriodDtype, IntervalDtype)
+    ):
         return False
 
     elif isinstance(dtype, CategoricalDtype) and not isinstance(
diff --git a/pandas/tests/base/test_conversion.py b/pandas/tests/base/test_conversion.py
index e3a821519c638..bddd28d29ba1c 100644
--- a/pandas/tests/base/test_conversion.py
+++ b/pandas/tests/base/test_conversion.py
@@ -285,7 +285,7 @@ def test_array(arr, attr, index_or_series):
         arr = getattr(arr, attr)
         result = getattr(result, attr)
 
-    assert result is arr
+    assert np.shares_memory(result, arr)
 
 
 def test_array_multiindex_raises():
diff --git a/pandas/tests/extension/base/setitem.py b/pandas/tests/extension/base/setitem.py
index 853b314e76a8e..e9389a4e57549 100644
--- a/pandas/tests/extension/base/setitem.py
+++ b/pandas/tests/extension/base/setitem.py
@@ -510,4 +510,9 @@ def test_readonly_propagates_to_numpy_array_method(self, data):
         # non-NA fill value should always result in a copy
         if data.isna().any():
             arr = data.to_numpy(copy=False, na_value=data[0])
-            assert arr.flags.writeable
+            if isinstance(data.dtype, pd.ArrowDtype) and data.dtype.kind == "f":
+                # for float dtype, after the fillna, the conversion from pyarrow to
+                # numpy is zero-copy, and pyarrow will mark the array as readonly
+                assert not arr.flags.writeable
+            else:
+                assert arr.flags.writeable
diff --git a/pandas/tests/extension/json/array.py b/pandas/tests/extension/json/array.py
index 7c01bdef88c0a..7e224b2416e10 100644
--- a/pandas/tests/extension/json/array.py
+++ b/pandas/tests/extension/json/array.py
@@ -155,7 +155,6 @@ def __array__(self, dtype=None, copy=None):
             raise ValueError(
                 "Unable to avoid copy while creating an array as requested."
             )
-
         if dtype is None:
             dtype = object
         if dtype == object:
diff --git a/pandas/tests/extension/json/test_json.py b/pandas/tests/extension/json/test_json.py
index 4bc9562f1895d..895d5df8810ad 100644
--- a/pandas/tests/extension/json/test_json.py
+++ b/pandas/tests/extension/json/test_json.py
@@ -418,6 +418,12 @@ def test_setitem_slice_array(self, data):
     def test_setitem_invalid(self, data, invalid_scalar):
         super().test_setitem_invalid(data, invalid_scalar)
 
+    @pytest.mark.xfail(
+        reason="result readonly flag is incorrect and does not support na_value"
+    )
+    def test_readonly_propagates_to_numpy_array_method(self, data):
+        super().test_readonly_propagates_to_numpy_array_method(data)
+
     @pytest.mark.xfail(reason="only integer scalar arrays can be converted")
     def test_setitem_2d_values(self, data):
         super().test_setitem_2d_values(data)

From 856dc0242d2f67c7caa52ec2a1d34729fb100a47 Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Wed, 23 Jul 2025 10:21:30 +0200
Subject: [PATCH 6/7] add tests for .array being readonly

---
 pandas/tests/copy_view/test_array.py | 37 +++++++++++++++++++---------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/pandas/tests/copy_view/test_array.py b/pandas/tests/copy_view/test_array.py
index 2b3ef9201d918..81b77bb4abdee 100644
--- a/pandas/tests/copy_view/test_array.py
+++ b/pandas/tests/copy_view/test_array.py
@@ -19,10 +19,11 @@
     "method",
     [
         lambda ser: ser.values,
+        lambda ser: np.asarray(ser.array),
         lambda ser: np.asarray(ser),
         lambda ser: np.array(ser, copy=False),
     ],
-    ids=["values", "asarray", "array"],
+    ids=["values", "array", "np.asarray", "np.array"],
 )
 def test_series_values(method):
     ser = Series([1, 2, 3], name="name")
@@ -105,24 +106,38 @@ def test_series_to_numpy():
     assert arr.flags.writeable is True
 
 
-def test_series_array_ea_dtypes():
+@pytest.mark.parametrize(
+    "method",
+    [
+        lambda ser: np.asarray(ser.array),
+        lambda ser: np.asarray(ser),
+        lambda ser: np.asarray(ser, dtype="int64"),
+        lambda ser: np.array(ser, copy=False),
+    ],
+    ids=["array", "np.asarray", "np.asarray-dtype", "np.array"],
+)
+def test_series_values_ea_dtypes(method):
     ser = Series([1, 2, 3], dtype="Int64")
-    arr = np.asarray(ser, dtype="int64")
-    assert np.shares_memory(arr, get_array(ser))
-    assert arr.flags.writeable is False
+    arr = method(ser)
 
-    arr = np.asarray(ser)
     assert np.shares_memory(arr, get_array(ser))
     assert arr.flags.writeable is False
 
 
-def test_dataframe_array_ea_dtypes():
+@pytest.mark.parametrize(
+    "method",
+    [
+        lambda df: df.values,
+        lambda df: np.asarray(df),
+        lambda df: np.asarray(df, dtype="int64"),
+        lambda df: np.array(df, copy=False),
+    ],
+    ids=["values", "np.asarray", "np.asarray-dtype", "np.array"],
+)
+def test_dataframe_array_ea_dtypes(method):
     df = DataFrame({"a": [1, 2, 3]}, dtype="Int64")
-    arr = np.asarray(df, dtype="int64")
-    assert np.shares_memory(arr, get_array(df, "a"))
-    assert arr.flags.writeable is False
+    arr = method(df)
 
-    arr = np.asarray(df)
     assert np.shares_memory(arr, get_array(df, "a"))
     assert arr.flags.writeable is False
 

From ee1ed6ed56f8ebfa1068fe9e5ae60694fa11001e Mon Sep 17 00:00:00 2001
From: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Date: Sun, 3 Aug 2025 11:48:12 +0200
Subject: [PATCH 7/7] typing

---
 pandas/core/arrays/_mixins.py | 6 ++++++
 pandas/core/arrays/_utils.py  | 2 +-
 pandas/core/arrays/base.py    | 6 ++++++
 pandas/core/arrays/numpy_.py  | 2 +-
 4 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/_mixins.py b/pandas/core/arrays/_mixins.py
index 2eb235f87d231..87eed4d8f32cc 100644
--- a/pandas/core/arrays/_mixins.py
+++ b/pandas/core/arrays/_mixins.py
@@ -115,6 +115,12 @@ def _validate_scalar(self, value):
 
     # ------------------------------------------------------------------------
 
+    @overload
+    def view(self) -> Self: ...
+
+    @overload
+    def view(self, dtype: Dtype | None = ...) -> ArrayLike: ...
+
     def view(self, dtype: Dtype | None = None) -> ArrayLike:
         # We handle datetime64, datetime64tz, timedelta64, and period
         #  dtypes here. Everything else we pass through to the underlying
diff --git a/pandas/core/arrays/_utils.py b/pandas/core/arrays/_utils.py
index 6b46396d5efdf..5a5ac6cb48039 100644
--- a/pandas/core/arrays/_utils.py
+++ b/pandas/core/arrays/_utils.py
@@ -22,7 +22,7 @@
 
 def to_numpy_dtype_inference(
     arr: ArrayLike, dtype: npt.DTypeLike | None, na_value, hasna: bool
-) -> tuple[npt.DTypeLike, Any]:
+) -> tuple[np.dtype | None, Any]:
     if dtype is None and is_numeric_dtype(arr.dtype):
         dtype_given = False
         if hasna:
diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 9a1bd9c453b21..4ac48716b9561 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -1853,6 +1853,12 @@ def copy(self) -> Self:
         """
         raise AbstractMethodError(self)
 
+    @overload
+    def view(self) -> Self: ...
+
+    @overload
+    def view(self, dtype: Dtype | None = ...) -> ArrayLike: ...
+
     def view(self, dtype: Dtype | None = None) -> ArrayLike:
         """
         Return a view on the array.
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index ebede913e77fc..37c3e574124ed 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -159,7 +159,7 @@ def dtype(self) -> NumpyEADtype:
     # NumPy Array Interface
 
     def __array__(
-        self, dtype: NpDtype | None = None, copy: bool | None = None
+        self, dtype: np.dtype | None = None, copy: bool | None = None
     ) -> np.ndarray:
         if copy is not None:
             # Note: branch avoids `copy=None` for NumPy 1.x support