REF: Changed ExtensionDtype inheritance

pandas/core/dtypes/base.py

@@ -5,26 +5,16 @@
 from pandas.errors import AbstractMethodError
 
 
-class ExtensionDtype(object):
-    """A custom data type, to be paired with an ExtensionArray.
-
-    Notes
-    -----
-    The interface includes the following abstract methods that must
-    be implemented by subclasses:
-
-    * type
-    * name
-    * construct_from_string
-
-    This class does not inherit from 'abc.ABCMeta' for performance reasons.
-    Methods and properties required by the interface raise
-    ``pandas.errors.AbstractMethodError`` and no ``register`` method is
-    provided for registering virtual subclasses.
-    """
-
-    def __str__(self):
-        return self.name
+class _DtypeOpsMixin(object):
+    # Not all of pandas' extension dtypes are compatibile with
+    # the new ExtensionArray interface. This means PandasExtensionDtype
+    # can't subclass ExtensionDtype yet, as is_extension_array_dtype would
+    # incorrectly say that these types are extension types.
+    #
+    # In the interim, we put methods that are shared between the two base
+    # classes ExtensionDtype and PandasExtensionDtype here. Both those base
+    # classes will inherit from this Mixin. Once everything is compatible, this
+    # class's methods can be moved to ExtensionDtype and removed.
 
     def __eq__(self, other):
         """Check whether 'other' is equal to self.
@@ -52,6 +42,74 @@ def __eq__(self, other):
     def __ne__(self, other):
         return not self.__eq__(other)
 
+    @property
+    def names(self):
+        # type: () -> Optional[List[str]]
+        """Ordered list of field names, or None if there are no fields.
+
+        This is for compatibility with NumPy arrays, and may be removed in the
+        future.
+        """
+        return None
+
+    @classmethod
+    def is_dtype(cls, dtype):
+        """Check if we match 'dtype'.
+
+        Parameters
+        ----------
+        dtype : object
+            The object to check.
+
+        Returns
+        -------
+        is_dtype : bool
+
+        Notes
+        -----
+        The default implementation is True if
+
+        1. ``cls.construct_from_string(dtype)`` is an instance
+           of ``cls``.
+        2. ``dtype`` is an object and is an instance of ``cls``
+        3. ``dtype`` has a ``dtype`` attribute, and any of the above
+           conditions is true for ``dtype.dtype``.
+        """
+        dtype = getattr(dtype, 'dtype', dtype)
+
+        if isinstance(dtype, np.dtype):
+            return False
+        elif dtype is None:
+            return False
+        elif isinstance(dtype, cls):
+            return True
+        try:
+            return cls.construct_from_string(dtype) is not None
+        except TypeError:
+            return False
+
+
+class ExtensionDtype(_DtypeOpsMixin):
+    """A custom data type, to be paired with an ExtensionArray.
+
+    Notes
+    -----
+    The interface includes the following abstract methods that must
+    be implemented by subclasses:
+
+    * type
+    * name
+    * construct_from_string
+
+    This class does not inherit from 'abc.ABCMeta' for performance reasons.
+    Methods and properties required by the interface raise
+    ``pandas.errors.AbstractMethodError`` and no ``register`` method is
+    provided for registering virtual subclasses.
+    """
+
+    def __str__(self):
+        return self.name
+
     @property
     def type(self):
         # type: () -> type
@@ -87,16 +145,6 @@ def name(self):
         """
         raise AbstractMethodError(self)
 
-    @property
-    def names(self):
-        # type: () -> Optional[List[str]]
-        """Ordered list of field names, or None if there are no fields.
-
-        This is for compatibility with NumPy arrays, and may be removed in the
-        future.
-        """
-        return None
-
     @classmethod
     def construct_from_string(cls, string):
         """Attempt to construct this type from a string.
@@ -128,39 +176,3 @@ def construct_from_string(cls, string):
         ...                         "'{}'".format(cls, string))
         """
         raise AbstractMethodError(cls)
-
-    @classmethod
-    def is_dtype(cls, dtype):
-        """Check if we match 'dtype'.
-
-        Parameters
-        ----------
-        dtype : object
-            The object to check.
-
-        Returns
-        -------
-        is_dtype : bool
-
-        Notes
-        -----
-        The default implementation is True if
-
-        1. ``cls.construct_from_string(dtype)`` is an instance
-           of ``cls``.
-        2. ``dtype`` is an object and is an instance of ``cls``
-        3. ``dtype`` has a ``dtype`` attribute, and any of the above
-           conditions is true for ``dtype.dtype``.
-        """
-        dtype = getattr(dtype, 'dtype', dtype)
-
-        if isinstance(dtype, np.dtype):
-            return False
-        elif dtype is None:
-            return False
-        elif isinstance(dtype, cls):
-            return True
-        try:
-            return cls.construct_from_string(dtype) is not None
-        except TypeError:
-            return False

pandas/core/dtypes/cast.py

@@ -26,7 +26,8 @@
                      _ensure_int32, _ensure_int64,
                      _NS_DTYPE, _TD_DTYPE, _INT64_DTYPE,
                      _POSSIBLY_CAST_DTYPES)
-from .dtypes import ExtensionDtype, DatetimeTZDtype, PeriodDtype
+from .dtypes import (ExtensionDtype, PandasExtensionDtype, DatetimeTZDtype,
+                     PeriodDtype)
 from .generic import (ABCDatetimeIndex, ABCPeriodIndex,
                       ABCSeries)
 from .missing import isna, notna
@@ -1114,7 +1115,8 @@ def find_common_type(types):
     if all(is_dtype_equal(first, t) for t in types[1:]):
         return first
 
-    if any(isinstance(t, ExtensionDtype) for t in types):
+    if any(isinstance(t, (PandasExtensionDtype, ExtensionDtype))
+           for t in types):
         return np.object
 
     # take lowest unit

pandas/core/dtypes/common.py

@@ -9,7 +9,7 @@
                      DatetimeTZDtype, DatetimeTZDtypeType,
                      PeriodDtype, PeriodDtypeType,
                      IntervalDtype, IntervalDtypeType,
-                     ExtensionDtype)
+                     ExtensionDtype, PandasExtensionDtype)
 from .generic import (ABCCategorical, ABCPeriodIndex,
                       ABCDatetimeIndex, ABCSeries,
                       ABCSparseArray, ABCSparseSeries, ABCCategoricalIndex,
@@ -2006,7 +2006,7 @@ def pandas_dtype(dtype):
             return CategoricalDtype.construct_from_string(dtype)
         except TypeError:
             pass
-    elif isinstance(dtype, ExtensionDtype):
+    elif isinstance(dtype, (PandasExtensionDtype, ExtensionDtype)):
         return dtype
 
     try:

pandas/core/dtypes/dtypes.py

@@ -5,10 +5,10 @@
 from pandas import compat
 from pandas.core.dtypes.generic import ABCIndexClass, ABCCategoricalIndex
 
-from .base import ExtensionDtype
+from .base import ExtensionDtype, _DtypeOpsMixin
 
 
-class PandasExtensionDtype(ExtensionDtype):
+class PandasExtensionDtype(_DtypeOpsMixin):
     """
     A np.dtype duck-typed class, suitable for holding a custom dtype.
 
@@ -83,7 +83,7 @@ class CategoricalDtypeType(type):
     pass
 
 
-class CategoricalDtype(PandasExtensionDtype):
+class CategoricalDtype(PandasExtensionDtype, ExtensionDtype):
     """
     Type for categorical data with the categories and orderedness
 

pandas/core/internals.py

@@ -17,6 +17,7 @@
 
 from pandas.core.dtypes.dtypes import (
     ExtensionDtype, DatetimeTZDtype,
+    PandasExtensionDtype,
     CategoricalDtype)
 from pandas.core.dtypes.common import (
     _TD_DTYPE, _NS_DTYPE,
@@ -598,7 +599,8 @@ def _astype(self, dtype, copy=False, errors='raise', values=None,
                                list(errors_legal_values), errors))
             raise ValueError(invalid_arg)
 
-        if inspect.isclass(dtype) and issubclass(dtype, ExtensionDtype):
+        if (inspect.isclass(dtype) and
+                issubclass(dtype, (PandasExtensionDtype, ExtensionDtype))):
             msg = ("Expected an instance of {}, but got the class instead. "
                    "Try instantiating 'dtype'.".format(dtype.__name__))
             raise TypeError(msg)
@@ -5005,7 +5007,7 @@ def _interleaved_dtype(blocks):
     dtype = find_common_type([b.dtype for b in blocks])
 
     # only numpy compat
-    if isinstance(dtype, ExtensionDtype):
+    if isinstance(dtype, (PandasExtensionDtype, ExtensionDtype)):
         dtype = np.object
 
     return dtype

pandas/tests/extension/test_common.py

@@ -5,10 +5,10 @@
 import pandas.util.testing as tm
 from pandas.core.arrays import ExtensionArray
 from pandas.core.dtypes.common import is_extension_array_dtype
-from pandas.core.dtypes.dtypes import ExtensionDtype
+from pandas.core.dtypes import dtypes
 
 
-class DummyDtype(ExtensionDtype):
+class DummyDtype(dtypes.ExtensionDtype):
     pass
 
 
@@ -65,3 +65,21 @@ def test_astype_no_copy():
 
     result = arr.astype(arr.dtype)
     assert arr.data is not result
+
+
+@pytest.mark.parametrize('dtype', [
+    dtypes.DatetimeTZDtype('ns', 'US/Central'),
+    dtypes.PeriodDtype("D"),
+    dtypes.IntervalDtype(),
+])
+def test_is_not_extension_array_dtype(dtype):
+    assert not isinstance(dtype, dtypes.ExtensionDtype)
+    assert not is_extension_array_dtype(dtype)
+
+
+@pytest.mark.parametrize('dtype', [
+    dtypes.CategoricalDtype(),
+])
+def test_is_extension_array_dtype(dtype):
+    assert isinstance(dtype, dtypes.ExtensionDtype)
+    assert is_extension_array_dtype(dtype)