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/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