From c368fb864a4f78ad3cfcf8f3965eaa7ce3bb2493 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Sun, 28 Jul 2019 19:43:46 -0700
Subject: [PATCH 1/9] implement+test EA.view

---
 pandas/core/arrays/base.py                | 20 +++++++++++++++++++-
 pandas/core/arrays/categorical.py         | 16 +++++-----------
 pandas/core/arrays/datetimelike.py        |  4 +++-
 pandas/core/arrays/interval.py            |  8 ++------
 pandas/core/arrays/numpy_.py              |  4 ++--
 pandas/core/arrays/sparse.py              |  8 ++++----
 pandas/tests/extension/arrow/test_bool.py |  4 ++++
 pandas/tests/extension/base/interface.py  | 12 ++++++++++++
 pandas/tests/extension/decimal/array.py   |  4 ++--
 pandas/tests/extension/json/array.py      |  7 +++++--
 pandas/tests/extension/test_interval.py   |  5 ++++-
 pandas/tests/extension/test_sparse.py     |  4 ++++
 12 files changed, 66 insertions(+), 30 deletions(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index ee796f9896b52..11809106e7bf0 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -351,7 +351,7 @@ def ndim(self) -> int:
         """
         Extension Arrays are only allowed to be 1-dimensional.
         """
-        return 1
+        return len(self.shape)
 
     @property
     def nbytes(self) -> int:
@@ -867,6 +867,24 @@ def copy(self) -> ABCExtensionArray:
         """
         raise AbstractMethodError(self)
 
+    def view(self, dtype=None) -> ABCExtensionArray:
+        """
+        Return a view on the array.
+
+        Returns
+        -------
+        ExtensionArray
+
+        Notes
+        -----
+        - This must return a *new* object, not self.
+        - The only case that *must* be implemented is with dtype=None,
+          giving a view with the same dtype as self.
+        """
+        if dtype is not None:
+            raise NotImplementedError(dtype)
+        return self[:]
+
     # ------------------------------------------------------------------------
     # Printing
     # ------------------------------------------------------------------------
diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index c22f7e0429433..a06ae572b6664 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -517,19 +517,12 @@ def astype(self, dtype: Dtype, copy: bool = True) -> ArrayLike:
             return self._set_dtype(dtype)
         return np.array(self, dtype=dtype, copy=copy)
 
-    @cache_readonly
-    def ndim(self) -> int:
-        """
-        Number of dimensions of the Categorical
-        """
-        return self._codes.ndim
-
     @cache_readonly
     def size(self) -> int:
         """
         return the len of myself
         """
-        return len(self)
+        return self._codes.size
 
     @cache_readonly
     def itemsize(self) -> int:
@@ -1764,7 +1757,7 @@ def ravel(self, order="C"):
         )
         return np.array(self)
 
-    def view(self):
+    def view(self, dtype=None):
         """
         Return a view of myself.
 
@@ -1773,9 +1766,10 @@ def view(self):
         Returns
         -------
         view : Categorical
-           Returns `self`!
         """
-        return self
+        if dtype is not None:
+            raise NotImplementedError(dtype)
+        return self._constructor(values=self._codes, dtype=self.dtype, fastpath=True)
 
     def to_dense(self):
         """
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 932d96a37c04c..c6d57d22f9bc5 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -566,9 +566,11 @@ def view(self, dtype=None):
 
         Returns
         -------
-        ndarray
+        ndarray or ExtensionArray
             With the specified `dtype`.
         """
+        if dtype is None:
+            return type(self)(self._data, dtype=self.dtype, freq=self.freq)
         return self._data.view(dtype=dtype)
 
     # ------------------------------------------------------------------
diff --git a/pandas/core/arrays/interval.py b/pandas/core/arrays/interval.py
index 2a0d2c8770063..43cc8ca51401f 100644
--- a/pandas/core/arrays/interval.py
+++ b/pandas/core/arrays/interval.py
@@ -738,18 +738,14 @@ def isna(self):
         return isna(self.left)
 
     @property
-    def nbytes(self):
+    def nbytes(self) -> int:
         return self.left.nbytes + self.right.nbytes
 
     @property
-    def size(self):
+    def size(self) -> int:
         # Avoid materializing self.values
         return self.left.size
 
-    @property
-    def shape(self):
-        return self.left.shape
-
     def take(self, indices, allow_fill=False, fill_value=None, axis=None, **kwargs):
         """
         Take elements from the IntervalArray.
diff --git a/pandas/core/arrays/numpy_.py b/pandas/core/arrays/numpy_.py
index 39529177b9e35..e8397341a1a1d 100644
--- a/pandas/core/arrays/numpy_.py
+++ b/pandas/core/arrays/numpy_.py
@@ -241,11 +241,11 @@ def __setitem__(self, key, value):
         else:
             self._ndarray[key] = value
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self._ndarray)
 
     @property
-    def nbytes(self):
+    def nbytes(self) -> int:
         return self._ndarray.nbytes
 
     def isna(self):
diff --git a/pandas/core/arrays/sparse.py b/pandas/core/arrays/sparse.py
index 048f6c6f5c680..7a3cb51a05fb4 100644
--- a/pandas/core/arrays/sparse.py
+++ b/pandas/core/arrays/sparse.py
@@ -839,7 +839,7 @@ def fill_value(self, value):
         self._dtype = SparseDtype(self.dtype.subtype, value)
 
     @property
-    def kind(self):
+    def kind(self) -> str:
         """
         The kind of sparse index for this array. One of {'integer', 'block'}.
         """
@@ -854,7 +854,7 @@ def _valid_sp_values(self):
         mask = notna(sp_vals)
         return sp_vals[mask]
 
-    def __len__(self):
+    def __len__(self) -> int:
         return self.sp_index.length
 
     @property
@@ -868,7 +868,7 @@ def _fill_value_matches(self, fill_value):
             return self.fill_value == fill_value
 
     @property
-    def nbytes(self):
+    def nbytes(self) -> int:
         return self.sp_values.nbytes + self.sp_index.nbytes
 
     @property
@@ -886,7 +886,7 @@ def density(self):
         return r
 
     @property
-    def npoints(self):
+    def npoints(self) -> int:
         """
         The number of non- ``fill_value`` points.
 
diff --git a/pandas/tests/extension/arrow/test_bool.py b/pandas/tests/extension/arrow/test_bool.py
index 205edf5da5b74..d959afc5b871f 100644
--- a/pandas/tests/extension/arrow/test_bool.py
+++ b/pandas/tests/extension/arrow/test_bool.py
@@ -41,6 +41,10 @@ def test_copy(self, data):
         # __setitem__ does not work, so we only have a smoke-test
         data.copy()
 
+    def test_view(self, data):
+        # __setitem__ does not work, so we only have a smoke-test
+        data.view()
+
 
 class TestConstructors(BaseArrowTests, base.BaseConstructorsTests):
     def test_from_dtype(self, data):
diff --git a/pandas/tests/extension/base/interface.py b/pandas/tests/extension/base/interface.py
index dee8021f5375f..dfbf0b1adf5ef 100644
--- a/pandas/tests/extension/base/interface.py
+++ b/pandas/tests/extension/base/interface.py
@@ -75,3 +75,15 @@ def test_copy(self, data):
 
         data[1] = data[0]
         assert result[1] != result[0]
+
+    def test_view(self, data):
+        # view with no dtype should return a shallow copy, *not* the same
+        #  object
+        assert data[1] != data[0]
+
+        result = data.view()
+        assert result is not data
+        assert type(result) == type(data)
+
+        result[1] = result[0]
+        assert data[1] == data[0]
diff --git a/pandas/tests/extension/decimal/array.py b/pandas/tests/extension/decimal/array.py
index c28ff956a33a4..a1988744d76a1 100644
--- a/pandas/tests/extension/decimal/array.py
+++ b/pandas/tests/extension/decimal/array.py
@@ -137,11 +137,11 @@ def __setitem__(self, key, value):
             value = decimal.Decimal(value)
         self._data[key] = value
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self._data)
 
     @property
-    def nbytes(self):
+    def nbytes(self) -> int:
         n = len(self)
         if n:
             return n * sys.getsizeof(self[0])
diff --git a/pandas/tests/extension/json/array.py b/pandas/tests/extension/json/array.py
index 21c4ac8f055a2..b64ddbd6ac84d 100644
--- a/pandas/tests/extension/json/array.py
+++ b/pandas/tests/extension/json/array.py
@@ -80,6 +80,9 @@ def __getitem__(self, item):
         elif isinstance(item, abc.Iterable):
             # fancy indexing
             return type(self)([self.data[i] for i in item])
+        elif isinstance(item, slice) and item == slice(None):
+            # Make sure we get a view
+            return type(self)(self.data)
         else:
             # slice
             return type(self)(self.data[item])
@@ -103,11 +106,11 @@ def __setitem__(self, key, value):
                     assert isinstance(v, self.dtype.type)
                     self.data[k] = v
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self.data)
 
     @property
-    def nbytes(self):
+    def nbytes(self) -> int:
         return sys.getsizeof(self.data)
 
     def isna(self):
diff --git a/pandas/tests/extension/test_interval.py b/pandas/tests/extension/test_interval.py
index 1aab71286b4a6..4fdcf930d224f 100644
--- a/pandas/tests/extension/test_interval.py
+++ b/pandas/tests/extension/test_interval.py
@@ -95,7 +95,10 @@ class TestGrouping(BaseInterval, base.BaseGroupbyTests):
 
 
 class TestInterface(BaseInterval, base.BaseInterfaceTests):
-    pass
+    def test_view(self, data):
+        # __setitem__ incorrectly makes a copy (GH#27147), so we only
+        #  have a smoke-test
+        data.view()
 
 
 class TestReduce(base.BaseNoReduceTests):
diff --git a/pandas/tests/extension/test_sparse.py b/pandas/tests/extension/test_sparse.py
index 84d59902d2aa7..6ebe71e173ec2 100644
--- a/pandas/tests/extension/test_sparse.py
+++ b/pandas/tests/extension/test_sparse.py
@@ -103,6 +103,10 @@ def test_copy(self, data):
         # __setitem__ does not work, so we only have a smoke-test
         data.copy()
 
+    def test_view(self, data):
+        # __setitem__ does not work, so we only have a smoke-test
+        data.view()
+
 
 class TestConstructors(BaseSparseTests, base.BaseConstructorsTests):
     pass

From 6636c893fa623572ed06b7d405899aa5ef045506 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Tue, 30 Jul 2019 09:57:10 -0700
Subject: [PATCH 2/9] test, comment

---
 pandas/core/arrays/base.py               | 3 ++-
 pandas/tests/extension/base/interface.py | 7 +++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 55b62f9f948ab..71f128bbcb0a5 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -64,6 +64,7 @@ class ExtensionArray:
     shift
     take
     unique
+    view
     _concat_same_type
     _formatter
     _formatting_values
@@ -147,7 +148,7 @@ class ExtensionArray:
     If implementing NumPy's ``__array_ufunc__`` interface, pandas expects
     that
 
-    1. You defer by raising ``NotImplemented`` when any Series are present
+    1. You defer by returning ``NotImplemented`` when any Series are present
        in `inputs`. Pandas will extract the arrays and call the ufunc again.
     2. You define a ``_HANDLED_TYPES`` tuple as an attribute on the class.
        Pandas inspect this to determine whether the ufunc is valid for the
diff --git a/pandas/tests/extension/base/interface.py b/pandas/tests/extension/base/interface.py
index dfbf0b1adf5ef..9af7aed000573 100644
--- a/pandas/tests/extension/base/interface.py
+++ b/pandas/tests/extension/base/interface.py
@@ -87,3 +87,10 @@ def test_view(self, data):
 
         result[1] = result[0]
         assert data[1] == data[0]
+
+        # check that `dtype` kwarg is accepted; it is OK if it
+        #  raises NotImplementedError
+        try:
+            data.view(data.dtype)
+        except NotImplementedError:
+            pass

From f99afb509613c8ea1168da9b4481e14d9e22c862 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Tue, 30 Jul 2019 15:18:03 -0700
Subject: [PATCH 3/9] fix broken case

---
 pandas/core/arrays/datetimelike.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 313c1f219f4ca..44c9aa5a99115 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -569,8 +569,8 @@ def view(self, dtype=None):
         ndarray or ExtensionArray
             With the specified `dtype`.
         """
-        if dtype is None:
-            return type(self)(self._data, dtype=self.dtype, freq=self.freq)
+        if dtype is None or dtype is self.dtype:
+            return type(self)(self._data, dtype=self.dtype)
         return self._data.view(dtype=dtype)
 
     # ------------------------------------------------------------------

From aa290bf4f843081ff0ee3326d686d53a157f7a8d Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Thu, 1 Aug 2019 09:22:11 -0700
Subject: [PATCH 4/9] docstring edits

---
 pandas/core/arrays/base.py         |  5 +++++
 pandas/core/arrays/categorical.py  |  9 ---------
 pandas/core/arrays/datetimelike.py | 12 ------------
 3 files changed, 5 insertions(+), 21 deletions(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 71f128bbcb0a5..480de2ac46e77 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -867,6 +867,11 @@ def view(self, dtype=None) -> ABCExtensionArray:
         """
         Return a view on the array.
 
+        Parameters
+        ----------
+        dtype : str, np.dtype, or ExtensionDtype, optional
+            Default None
+
         Returns
         -------
         ExtensionArray
diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index b053aa35a4a09..e56f623962fa3 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -1758,15 +1758,6 @@ def ravel(self, order="C"):
         return np.array(self)
 
     def view(self, dtype=None):
-        """
-        Return a view of myself.
-
-        For internal compatibility with numpy arrays.
-
-        Returns
-        -------
-        view : Categorical
-        """
         if dtype is not None:
             raise NotImplementedError(dtype)
         return self._constructor(values=self._codes, dtype=self.dtype, fastpath=True)
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 44c9aa5a99115..cf836408a3aed 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -557,18 +557,6 @@ def astype(self, dtype, copy=True):
             return np.asarray(self, dtype=dtype)
 
     def view(self, dtype=None):
-        """
-        New view on this array with the same data.
-
-        Parameters
-        ----------
-        dtype : numpy dtype, optional
-
-        Returns
-        -------
-        ndarray or ExtensionArray
-            With the specified `dtype`.
-        """
         if dtype is None or dtype is self.dtype:
             return type(self)(self._data, dtype=self.dtype)
         return self._data.view(dtype=dtype)

From 6053a509872257dcb566bbb583268024a69f2a9c Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Thu, 1 Aug 2019 11:24:48 -0700
Subject: [PATCH 5/9] add view to doc

---
 doc/source/reference/extensions.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/reference/extensions.rst b/doc/source/reference/extensions.rst
index 407aab4bb1f1b..04974f05164f8 100644
--- a/doc/source/reference/extensions.rst
+++ b/doc/source/reference/extensions.rst
@@ -45,6 +45,7 @@ objects.
       api.extensions.ExtensionArray.argsort
       api.extensions.ExtensionArray.astype
       api.extensions.ExtensionArray.copy
+      api.extensions.ExtensionArray.view
       api.extensions.ExtensionArray.dropna
       api.extensions.ExtensionArray.factorize
       api.extensions.ExtensionArray.fillna

From d9158ef8cd06bcb246c162c2df7c9c7e6f938a44 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Fri, 2 Aug 2019 07:36:18 -0700
Subject: [PATCH 6/9] simplify test

---
 pandas/tests/extension/base/interface.py | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/pandas/tests/extension/base/interface.py b/pandas/tests/extension/base/interface.py
index 9af7aed000573..a29f6deeffae6 100644
--- a/pandas/tests/extension/base/interface.py
+++ b/pandas/tests/extension/base/interface.py
@@ -88,9 +88,5 @@ def test_view(self, data):
         result[1] = result[0]
         assert data[1] == data[0]
 
-        # check that `dtype` kwarg is accepted; it is OK if it
-        #  raises NotImplementedError
-        try:
-            data.view(data.dtype)
-        except NotImplementedError:
-            pass
+        # check specifically that the `dtype` kwarg is accepted
+        data.view(dtype=None)

From 2b5e5fa6911c2631eb7305694e3dd1c85f591207 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Fri, 2 Aug 2019 09:46:20 -0700
Subject: [PATCH 7/9] docstring/comment edits

---
 pandas/core/arrays/base.py | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 480de2ac46e77..31949de018a3d 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -875,13 +875,11 @@ def view(self, dtype=None) -> ABCExtensionArray:
         Returns
         -------
         ExtensionArray
-
-        Notes
-        -----
-        - This must return a *new* object, not self.
-        - The only case that *must* be implemented is with dtype=None,
-          giving a view with the same dtype as self.
         """
+        # NB:
+        # - This must return a *new* object referencing the same data, not self.
+        # - The only case that *must* be implemented is with dtype=None,
+        #   giving a view with the same dtype as self.
         if dtype is not None:
             raise NotImplementedError(dtype)
         return self[:]

From 24b235c761f85062091c2a8393d690d3b015e5c3 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Fri, 2 Aug 2019 13:46:25 -0700
Subject: [PATCH 8/9] requested edits

---
 pandas/core/arrays/base.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 31949de018a3d..74e459622574d 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -355,7 +355,7 @@ def ndim(self) -> int:
         """
         Extension Arrays are only allowed to be 1-dimensional.
         """
-        return len(self.shape)
+        return 1
 
     @property
     def nbytes(self) -> int:
@@ -863,7 +863,7 @@ def copy(self) -> ABCExtensionArray:
         """
         raise AbstractMethodError(self)
 
-    def view(self, dtype=None) -> ABCExtensionArray:
+    def view(self, dtype=None) -> Union[ABCExtensionArray, np.ndarray]:
         """
         Return a view on the array.
 
@@ -874,7 +874,7 @@ def view(self, dtype=None) -> ABCExtensionArray:
 
         Returns
         -------
-        ExtensionArray
+        ExtensionArray or np.ndarray
         """
         # NB:
         # - This must return a *new* object referencing the same data, not self.

From c44aada5f9c45da39457b2059608852127e59e86 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Sat, 3 Aug 2019 13:08:44 -0700
Subject: [PATCH 9/9] revert return type

---
 pandas/core/arrays/base.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/arrays/base.py b/pandas/core/arrays/base.py
index 74e459622574d..41d84b0ae4853 100644
--- a/pandas/core/arrays/base.py
+++ b/pandas/core/arrays/base.py
@@ -874,7 +874,7 @@ def view(self, dtype=None) -> Union[ABCExtensionArray, np.ndarray]:
 
         Returns
         -------
-        ExtensionArray or np.ndarray
+        ExtensionArray
         """
         # NB:
         # - This must return a *new* object referencing the same data, not self.