ENH: Array Interface and Categorical internals Refactor (#19268)

* REF: Define extension base classes

* Updated for comments

* removed take_nd
* Changed to_dense to return get_values
* Fixed docstrings, types
* Removed is_sparse

* Remove metaclasses from PeriodDtype and IntervalDtype

* Fixup form_blocks rebase

* Restore concat casting cat -> object

* Remove _slice, clarify semantics around __getitem__

* Document and use take.

* Clarify type, kind, init

* Remove base

* API: Remove unused __iter__ and get_values

* API: Implement repr and str

* Remove default value_counts for now

* Fixed merge conflicts

* Remove implementation of construct_from_string

* Example implementation of take

* Cleanup ExtensionBlock

* Pass through ndim

* Use series._values

* Removed repr, updated take doc

* Various cleanups

* Handle get_values, to_dense, is_view

* Docs

* Remove is_extension, is_bool

Remove inherited convert

* Sparse formatter

* Revert "Sparse formatter"

This reverts commit ab2f045.

* Unbox SparseSeries

* Added test for sparse consolidation

* Docs

* Moved to errors

* Handle classmethods, properties

* Use our AbstractMethodError

* Lint

* Cleanup

* Move ndim validation to a method.

* Try this

* Make ExtensionBlock._holder a property

Removed ExtensionBlock.__init__

* Make _holder a property for all

* Refactored validate_ndim

* fixup! Refactored validate_ndim

* lint

pandas/core/arrays/__init__.py

@@ -1 +1,2 @@
+from .base import ExtensionArray  # noqa
 from .categorical import Categorical  # noqa

pandas/core/arrays/base.py

@@ -0,0 +1,247 @@
+"""An interface for extending pandas with custom arrays."""
+from pandas.errors import AbstractMethodError
+
+_not_implemented_message = "{} does not implement {}."
+
+
+class ExtensionArray(object):
+    """Abstract base class for custom 1-D array types.
+
+    pandas will recognize instances of this class as proper arrays
+    with a custom type and will not attempt to coerce them to objects. They
+    may be stored directly inside a :class:`DataFrame` or :class:`Series`.
+
+    Notes
+    -----
+    The interface includes the following abstract methods that must be
+    implemented by subclasses:
+
+    * __getitem__
+    * __len__
+    * dtype
+    * nbytes
+    * isna
+    * take
+    * copy
+    * _formatting_values
+    * _concat_same_type
+
+    Some additional methods are required to satisfy pandas' internal, private
+    block API.
+
+    * _concat_same_type
+    * _can_hold_na
+
+    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.
+
+    ExtensionArrays are limited to 1 dimension.
+
+    They may be backed by none, one, or many NumPy ararys. For example,
+    ``pandas.Categorical`` is an extension array backed by two arrays,
+    one for codes and one for categories. An array of IPv6 address may
+    be backed by a NumPy structured array with two fields, one for the
+    lower 64 bits and one for the upper 64 bits. Or they may be backed
+    by some other storage type, like Python lists. Pandas makes no
+    assumptions on how the data are stored, just that it can be converted
+    to a NumPy array.
+
+    Extension arrays should be able to be constructed with instances of
+    the class, i.e. ``ExtensionArray(extension_array)`` should return
+    an instance, not error.
+
+    Additionally, certain methods and interfaces are required for proper
+    this array to be properly stored inside a ``DataFrame`` or ``Series``.
+    """
+    # ------------------------------------------------------------------------
+    # Must be a Sequence
+    # ------------------------------------------------------------------------
+    def __getitem__(self, item):
+        # type (Any) -> Any
+        """Select a subset of self.
+
+        Parameters
+        ----------
+        item : int, slice, or ndarray
+            * int: The position in 'self' to get.
+
+            * slice: A slice object, where 'start', 'stop', and 'step' are
+              integers or None
+
+            * ndarray: A 1-d boolean NumPy ndarray the same length as 'self'
+
+        Returns
+        -------
+        item : scalar or ExtensionArray
+
+        Notes
+        -----
+        For scalar ``item``, return a scalar value suitable for the array's
+        type. This should be an instance of ``self.dtype.type``.
+
+        For slice ``key``, return an instance of ``ExtensionArray``, even
+        if the slice is length 0 or 1.
+
+        For a boolean mask, return an instance of ``ExtensionArray``, filtered
+        to the values where ``item`` is True.
+        """
+        raise AbstractMethodError(self)
+
+    def __setitem__(self, key, value):
+        # type: (Any, Any) -> None
+        raise NotImplementedError(_not_implemented_message.format(
+            type(self), '__setitem__')
+        )
+
+    def __len__(self):
+        """Length of this array
+
+        Returns
+        -------
+        length : int
+        """
+        # type: () -> int
+        raise AbstractMethodError(self)
+
+    # ------------------------------------------------------------------------
+    # Required attributes
+    # ------------------------------------------------------------------------
+    @property
+    def dtype(self):
+        # type: () -> ExtensionDtype
+        """An instance of 'ExtensionDtype'."""
+        raise AbstractMethodError(self)
+
+    @property
+    def shape(self):
+        # type: () -> Tuple[int, ...]
+        return (len(self),)
+
+    @property
+    def ndim(self):
+        # type: () -> int
+        """Extension Arrays are only allowed to be 1-dimensional."""
+        return 1
+
+    @property
+    def nbytes(self):
+        # type: () -> int
+        """The number of bytes needed to store this object in memory.
+
+        If this is expensive to compute, return an approximate lower bound
+        on the number of bytes needed.
+        """
+        raise AbstractMethodError(self)
+
+    # ------------------------------------------------------------------------
+    # Additional Methods
+    # ------------------------------------------------------------------------
+    def isna(self):
+        # type: () -> np.ndarray
+        """Boolean NumPy array indicating if each value is missing.
+
+        This should return a 1-D array the same length as 'self'.
+        """
+        raise AbstractMethodError(self)
+
+    # ------------------------------------------------------------------------
+    # Indexing methods
+    # ------------------------------------------------------------------------
+    def take(self, indexer, allow_fill=True, fill_value=None):
+        # type: (Sequence[int], bool, Optional[Any]) -> ExtensionArray
+        """Take elements from an array.
+
+        Parameters
+        ----------
+        indexer : sequence of integers
+            indices to be taken. -1 is used to indicate values
+            that are missing.
+        allow_fill : bool, default True
+            If False, indexer is assumed to contain no -1 values so no filling
+            will be done. This short-circuits computation of a mask. Result is
+            undefined if allow_fill == False and -1 is present in indexer.
+        fill_value : any, default None
+            Fill value to replace -1 values with. By default, this uses
+            the missing value sentinel for this type, ``self._fill_value``.
+
+        Notes
+        -----
+        This should follow pandas' semantics where -1 indicates missing values.
+        Positions where indexer is ``-1`` should be filled with the missing
+        value for this type.
+
+        This is called by ``Series.__getitem__``, ``.loc``, ``iloc``, when the
+        indexer is a sequence of values.
+
+        Examples
+        --------
+        Suppose the extension array somehow backed by a NumPy structured array
+        and that the underlying structured array is stored as ``self.data``.
+        Then ``take`` may be written as
+
+        .. code-block:: python
+
+           def take(self, indexer, allow_fill=True, fill_value=None):
+               mask = indexer == -1
+               result = self.data.take(indexer)
+               result[mask] = self._fill_value
+               return type(self)(result)
+        """
+        raise AbstractMethodError(self)
+
+    def copy(self, deep=False):
+        # type: (bool) -> ExtensionArray
+        """Return a copy of the array.
+
+        Parameters
+        ----------
+        deep : bool, default False
+            Also copy the underlying data backing this array.
+
+        Returns
+        -------
+        ExtensionArray
+        """
+        raise AbstractMethodError(self)
+
+    # ------------------------------------------------------------------------
+    # Block-related methods
+    # ------------------------------------------------------------------------
+    @property
+    def _fill_value(self):
+        # type: () -> Any
+        """The missing value for this type, e.g. np.nan"""
+        return None
+
+    def _formatting_values(self):
+        # type: () -> np.ndarray
+        # At the moment, this has to be an array since we use result.dtype
+        """An array of values to be printed in, e.g. the Series repr"""
+        raise AbstractMethodError(self)
+
+    @classmethod
+    def _concat_same_type(cls, to_concat):
+        # type: (Sequence[ExtensionArray]) -> ExtensionArray
+        """Concatenate multiple array
+
+        Parameters
+        ----------
+        to_concat : sequence of this type
+
+        Returns
+        -------
+        ExtensionArray
+        """
+        raise AbstractMethodError(cls)
+
+    def _can_hold_na(self):
+        # type: () -> bool
+        """Whether your array can hold missing values. True by default.
+
+        Notes
+        -----
+        Setting this to false will optimize some operations like fillna.
+        """
+        return True

pandas/core/arrays/categorical.py

@@ -43,6 +43,8 @@
 from pandas.util._validators import validate_bool_kwarg
 from pandas.core.config import get_option
 
+from .base import ExtensionArray
+
 
 def _cat_compare_op(op):
     def f(self, other):
@@ -148,7 +150,7 @@ def _maybe_to_categorical(array):
 """
 
 
-class Categorical(PandasObject):
+class Categorical(ExtensionArray, PandasObject):
     """
     Represents a categorical variable in classic R / S-plus fashion
 
@@ -2130,6 +2132,20 @@ def repeat(self, repeats, *args, **kwargs):
         return self._constructor(values=codes, categories=self.categories,
                                  ordered=self.ordered, fastpath=True)
 
+    # Implement the ExtensionArray interface
+    @property
+    def _can_hold_na(self):
+        return True
+
+    @classmethod
+    def _concat_same_type(self, to_concat):
+        from pandas.core.dtypes.concat import _concat_categorical
+
+        return _concat_categorical(to_concat)
+
+    def _formatting_values(self):
+        return self
+
 # The Series.cat accessor
 
 

pandas/core/common.py

@@ -25,7 +25,8 @@
 
 # compat
 from pandas.errors import (  # noqa
-    PerformanceWarning, UnsupportedFunctionCall, UnsortedIndexError)
+    PerformanceWarning, UnsupportedFunctionCall, UnsortedIndexError,
+    AbstractMethodError)
 
 # back-compat of public API
 # deprecate these functions
@@ -88,19 +89,6 @@ class SettingWithCopyWarning(Warning):
     pass
 
 
-class AbstractMethodError(NotImplementedError):
-    """Raise this error instead of NotImplementedError for abstract methods
-    while keeping compatibility with Python 2 and Python 3.
-    """
-
-    def __init__(self, class_instance):
-        self.class_instance = class_instance
-
-    def __str__(self):
-        msg = "This method must be defined in the concrete class of {name}"
-        return (msg.format(name=self.class_instance.__class__.__name__))
-
-
 def flatten(l):
     """Flatten an arbitrarily nested sequence.