EA: preliminary EA reshape ops

pandas/core/arrays/base.py

@@ -830,6 +830,20 @@ 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.
+        """
+        raise AbstractMethodError(self)
+
     # ------------------------------------------------------------------------
     # Printing
     # ------------------------------------------------------------------------
@@ -897,6 +911,23 @@ def _formatting_values(self) -> np.ndarray:
     # Reshaping
     # ------------------------------------------------------------------------
 
+    @property
+    def T(self) -> ABCExtensionArray:
+        """
+        Return a transposed view on self.  For 1-D arrays this is a no-op.
+        """
+        if self.ndim != 1:
+            raise NotImplementedError
+        return self
+
+    def ravel(self, order=None) -> ABCExtensionArray:
+        """
+        Return a flattened view on self.  For 1-D arrays this is a no-op.
+        """
+        if self.ndim != 1:
+            raise NotImplementedError
+        return self
+
     @classmethod
     def _concat_same_type(
             cls,

pandas/core/arrays/categorical.py

@@ -1675,19 +1675,7 @@ def _values_for_rank(self):
             )
         return values
 
-    def ravel(self, order='C'):
-        """
-        Return a flattened (numpy) array.
-
-        For internal compatibility with numpy arrays.
-
-        Returns
-        -------
-        numpy.array
-        """
-        return np.array(self)
-
-    def view(self):
+    def view(self, dtype=None):
         """
         Return a view of myself.
 
@@ -1698,7 +1686,11 @@ def view(self):
         view : Categorical
            Returns `self`!
         """
-        return self
+        if dtype is not None:
+            return NotImplementedError(dtype)
+        return self._constructor(values=self._codes,
+                                 dtype=self.dtype,
+                                 fastpath=True)
 
     def to_dense(self):
         """

pandas/core/arrays/datetimelike.py

@@ -554,9 +554,11 @@ 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)
 
     # ------------------------------------------------------------------

pandas/core/arrays/integer.py

@@ -380,6 +380,11 @@ def copy(self):
         mask = mask.copy()
         return type(self)(data, mask, copy=False)
 
+    def view(self, dtype=None):
+        if dtype is not None:
+            raise NotImplementedError
+        return type(self)(self._data, self._mask, copy=False)
+
     def __setitem__(self, key, value):
         _is_scalar = is_scalar(value)
         if _is_scalar:

pandas/core/arrays/interval.py

@@ -694,6 +694,13 @@ def copy(self):
         # TODO: Could skip verify_integrity here.
         return type(self).from_arrays(left, right, closed=closed)
 
+    def view(self, dtype=None):
+        if dtype is not None:
+            raise NotImplementedError
+        return type(self)._simple_new(self._left, self._right, self.closed,
+                                      copy=False, dtype=None,
+                                      verify_integrity=False)
+
     def isna(self):
         return isna(self.left)
 

pandas/core/arrays/numpy_.py

@@ -299,6 +299,11 @@ def unique(self):
 
         return type(self)(unique(self._ndarray))
 
+    def view(self, dtype=None):
+        if dtype is not None:
+            raise NotImplementedError(dtype)
+        return type(self)(self._ndarray)
+
     # ------------------------------------------------------------------------
     # Reductions
 

pandas/core/arrays/sparse.py

@@ -1266,6 +1266,11 @@ def copy(self):
         values = self.sp_values.copy()
         return self._simple_new(values, self.sp_index, self.dtype)
 
+    def view(self, dtype=None):
+        if dtype is not None:
+            raise NotImplementedError
+        return self._simple_new(self.sp_values, self.sp_index, self.dtype)
+
     @classmethod
     def _concat_same_type(cls, to_concat):
         fill_values = [x.fill_value for x in to_concat]

pandas/tests/extension/arrow/bool.py

@@ -49,6 +49,11 @@ def __init__(self, values):
         self._data = values
         self._dtype = ArrowBoolDtype()
 
+    def view(self, dtype=None):
+        if dtype is not None:
+            raise NotImplementedError
+        return type(self)(self._data)
+
     def __repr__(self):
         return "ArrowBoolArray({})".format(repr(self._data))
 

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):

pandas/tests/extension/base/interface.py

@@ -74,3 +74,14 @@ 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
+
+        result[1] = result[0]
+        assert data[1] == data[0]

pandas/tests/extension/base/reshaping.py

@@ -269,3 +269,15 @@ def test_unstack(self, data, index, obj):
             result = result.astype(object)
 
             self.assert_frame_equal(result, expected)
+
+    def test_ravel(self, data):
+        # Test is invalid if data.ndim > 1
+        assert data.ndim == 1
+        result = data.ravel()
+        assert result is data
+
+    def test_transpose(self, data):
+        # Test is invalid if data.ndim > 1
+        assert data.ndim == 1
+        result = data.T
+        assert result is data

pandas/tests/extension/decimal/array.py

@@ -104,6 +104,11 @@ def take(self, indexer, allow_fill=False, fill_value=None):
     def copy(self):
         return type(self)(self._data.copy())
 
+    def view(self, dtype=None):
+        if dtype is not None:
+            raise NotImplementedError
+        return type(self)(self._data)
+
     def astype(self, dtype, copy=True):
         if isinstance(dtype, type(self.dtype)):
             return type(self)(self._data, context=dtype.context)

pandas/tests/extension/json/array.py

@@ -146,6 +146,11 @@ def take(self, indexer, allow_fill=False, fill_value=None):
     def copy(self):
         return type(self)(self.data[:])
 
+    def view(self, dtype=None):
+        if dtype is not None:
+            raise NotImplementedError
+        return type(self)(self.data)
+
     def astype(self, dtype, copy=True):
         # NumPy has issues when all the dicts are the same length.
         # np.array([UserDict(...), UserDict(...)]) fails,

pandas/tests/extension/test_interval.py

@@ -35,12 +35,17 @@ def dtype():
     return IntervalDtype()
 
 
-@pytest.fixture
 def data():
     """Length-100 PeriodArray for semantics test."""
     return IntervalArray(make_data())
 
 
+@pytest.fixture(name="data")
+def data_fixture():
+    """Length-100 PeriodArray for semantics test."""
+    return IntervalArray(make_data())
+
+
 @pytest.fixture
 def data_missing():
     """Length 2 array with [NA, Valid]"""
@@ -95,7 +100,11 @@ 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):

pandas/tests/extension/test_sparse.py

@@ -102,6 +102,10 @@ 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