NumPyBackedExtensionArray (pandas-dev#24227)

Author: TomAugspurger

doc/source/api.rst

@@ -3997,6 +3997,7 @@ objects.
    api.extensions.register_index_accessor
    api.extensions.ExtensionDtype
    api.extensions.ExtensionArray
+   arrays.PandasArray
 
 .. This is to prevent warnings in the doc build. We don't want to encourage
 .. these methods.

doc/source/basics.rst

@@ -71,8 +71,10 @@ the **array** property
    s.array
    s.index.array
 
-Depending on the data type (see :ref:`basics.dtypes`), :attr:`~Series.array`
-be either a NumPy array or an :ref:`ExtensionArray <extending.extension-type>`.
+:attr:`~Series.array` will always be an :class:`~pandas.api.extensions.ExtensionArray`.
+The exact details of what an ``ExtensionArray`` is and why pandas uses them is a bit
+beyond the scope of this introduction. See :ref:`basics.dtypes` for more.
+
 If you know you need a NumPy array, use :meth:`~Series.to_numpy`
 or :meth:`numpy.asarray`.
 
@@ -81,10 +83,30 @@ or :meth:`numpy.asarray`.
    s.to_numpy()
    np.asarray(s)
 
-For Series and Indexes backed by NumPy arrays (like we have here), this will
-be the same as :attr:`~Series.array`. When the Series or Index is backed by
-a :class:`~pandas.api.extension.ExtensionArray`, :meth:`~Series.to_numpy`
-may involve copying data and coercing values.
+When the Series or Index is backed by
+an :class:`~pandas.api.extension.ExtensionArray`, :meth:`~Series.to_numpy`
+may involve copying data and coercing values. See :ref:`basics.dtypes` for more.
+
+:meth:`~Series.to_numpy` gives some control over the ``dtype`` of the
+resulting :class:`ndarray`. For example, consider datetimes with timezones.
+NumPy doesn't have a dtype to represent timezone-aware datetimes, so there
+are two possibly useful representations:
+
+1. An object-dtype :class:`ndarray` with :class:`Timestamp` objects, each
+   with the correct ``tz``
+2. A ``datetime64[ns]`` -dtype :class:`ndarray`, where the values have
+   been converted to UTC and the timezone discarded
+
+Timezones may be preserved with ``dtype=object``
+
+.. ipython:: python
+
+   ser = pd.Series(pd.date_range('2000', periods=2, tz="CET"))
+   ser.to_numpy(dtype=object)
+
+Or thrown away with ``dtype='datetime64[ns]'``
+
+   ser.to_numpy(dtype="datetime64[ns]")
 
 :meth:`~Series.to_numpy` gives some control over the ``dtype`` of the
 resulting :class:`ndarray`. For example, consider datetimes with timezones.
@@ -109,7 +131,7 @@ Or thrown away with ``dtype='datetime64[ns]'``
 
 Getting the "raw data" inside a :class:`DataFrame` is possibly a bit more
 complex. When your ``DataFrame`` only has a single data type for all the
-columns, :attr:`DataFrame.to_numpy` will return the underlying data:
+columns, :meth:`DataFrame.to_numpy` will return the underlying data:
 
 .. ipython:: python
 
@@ -136,8 +158,9 @@ drawbacks:
 
 1. When your Series contains an :ref:`extension type <extending.extension-type>`, it's
    unclear whether :attr:`Series.values` returns a NumPy array or the extension array.
-   :attr:`Series.array` will always return the actual array backing the Series,
-   while :meth:`Series.to_numpy` will always return a NumPy array.
+   :attr:`Series.array` will always return an ``ExtensionArray``, and will never
+   copy data. :meth:`Series.to_numpy` will always return a NumPy array,
+   potentially at the cost of copying / coercing values.
 2. When your DataFrame contains a mixture of data types, :attr:`DataFrame.values` may
    involve copying data and coercing values to a common dtype, a relatively expensive
    operation. :meth:`DataFrame.to_numpy`, being a method, makes it clearer that the

doc/source/dsintro.rst

@@ -146,11 +146,15 @@ If you need the actual array backing a ``Series``, use :attr:`Series.array`.
 
    s.array
 
-Again, this is often a NumPy array, but may instead be a
-:class:`~pandas.api.extensions.ExtensionArray`. See :ref:`basics.dtypes` for more.
 Accessing the array can be useful when you need to do some operation without the
 index (to disable :ref:`automatic alignment <dsintro.alignment>`, for example).
 
+:attr:`Series.array` will always be an :class:`~pandas.api.extensions.ExtensionArray`.
+Briefly, an ExtensionArray is a thin wrapper around one or more *concrete* arrays like a
+:class:`numpy.ndarray`. Pandas knows how to take an ``ExtensionArray`` and
+store it in a ``Series`` or a column of a ``DataFrame``.
+See :ref:`basics.dtypes` for more.
+
 While Series is ndarray-like, if you need an *actual* ndarray, then use
 :meth:`Series.to_numpy`.
 

doc/source/whatsnew/v0.24.0.rst

@@ -65,8 +65,11 @@ If you need an actual NumPy array, use :meth:`Series.to_numpy` or :meth:`Index.t
    idx.to_numpy()
    pd.Series(idx).to_numpy()
 
-For Series and Indexes backed by normal NumPy arrays, this will be the same thing (and the same
-as ``.values``).
+For Series and Indexes backed by normal NumPy arrays, :attr:`Series.array` will return a
+new :class:`arrays.PandasArray`, which is a thin (no-copy) wrapper around a
+:class:`numpy.ndarray`. :class:`arrays.PandasArray` isn't especially useful on its own,
+but it does provide the same interface as any extension array defined in pandas or by
+a third-party library.
 
 .. ipython:: python
 
@@ -75,7 +78,7 @@ as ``.values``).
    ser.to_numpy()
 
 We haven't removed or deprecated :attr:`Series.values` or :attr:`DataFrame.values`, but we
-recommend and using ``.array`` or ``.to_numpy()`` instead.
+highly recommend and using ``.array`` or ``.to_numpy()`` instead.
 
 See :ref:`Dtypes <basics.dtypes>` and :ref:`Attributes and Underlying Data <basics.attrs>` for more.
 

pandas/__init__.py

@@ -49,6 +49,7 @@
 from pandas.io.api import *
 from pandas.util._tester import test
 import pandas.testing
+import pandas.arrays
 
 # use the closest tagged version if possible
 from ._version import get_versions

pandas/arrays/__init__.py

@@ -0,0 +1,11 @@
+"""
+All of pandas' ExtensionArrays.
+
+See :ref:`extending.extension-types` for more.
+"""
+from pandas.core.arrays import PandasArray
+
+
+__all__ = [
+    'PandasArray'
+]

pandas/core/arrays/__init__.py

@@ -9,3 +9,4 @@
 from .integer import (  # noqa
     IntegerArray, integer_array)
 from .sparse import SparseArray  # noqa
+from .numpy_ import PandasArray, PandasDtype  # noqa

pandas/core/arrays/categorical.py

@@ -2091,9 +2091,9 @@ def __setitem__(self, key, value):
             If (one or more) Value is not in categories or if a assigned
             `Categorical` does not have the same categories
         """
+        from pandas.core.internals.arrays import extract_array
 
-        if isinstance(value, (ABCIndexClass, ABCSeries)):
-            value = value.array
+        value = extract_array(value, extract_numpy=True)
 
         # require identical categories set
         if isinstance(value, Categorical):