[GitHub] [arrow] wjones127 commented on a diff in pull request #37797: GH-34031: C Data Interface PyCapsule Protocol

["wjones127 (via GitHub)" <gi...@apache.org>]

wjones127 commented on code in PR #37797:
URL: https://github.com/apache/arrow/pull/37797#discussion_r1334911576


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docs/source/format/CDataInterface/PyCapsuleInterface.rst:
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+.. Licensed to the Apache Software Foundation (ASF) under one
+.. or more contributor license agreements.  See the NOTICE file
+.. distributed with this work for additional information
+.. regarding copyright ownership.  The ASF licenses this file
+.. to you under the Apache License, Version 2.0 (the
+.. "License"); you may not use this file except in compliance
+.. with the License.  You may obtain a copy of the License at
+
+..   http://www.apache.org/licenses/LICENSE-2.0
+
+.. Unless required by applicable law or agreed to in writing,
+.. software distributed under the License is distributed on an
+.. "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+.. KIND, either express or implied.  See the License for the
+.. specific language governing permissions and limitations
+.. under the License.
+
+
+=============================
+The Arrow PyCapsule Interface
+=============================
+
+Rationale
+=========
+
+The :ref:`C data interface <c-data-interface>` and
+:ref:`C stream interface <c-stream-interface>` allow moving Arrow data between
+different implementations of Arrow. However, these interfaces don't specify how
+Python libraries should expose these structs to other libraries. Prior to this,
+many libraries simply provided export to PyArrow data structures, using the
+``_import_from_c`` and ``_export_from_c`` methods. However, this always required
+PyArrow to be installed.
+
+This interface allows any library to export Arrow data structures to other
+libraries that understand the same protocol.
+
+Goals
+-----
+
+* Standardize the PyCapsule objects that represent ``ArrowSchema``, ``ArrowArray``,
+  and ``ArrowArrayStream``.
+* Define standard methods to get these PyCapsules, so any Python objects that
+  support export via Arrow can be recognized by other libraries.
+
+
+Non-goals
+---------
+
+* Standardizing what public APIs should be used for import. This is left up to
+  individual libraries.
+
+
+Comparison to DataFrame Interchange Protocol
+--------------------------------------------
+
+:doc:`DataFrame Interchange Protocol <../dataframe.html>` is another protocol
+in Python that allows for the sharing of data between libraries. This protocol
+is complementary to the DataFrame Interchange Protocol. Many of the objects that
+implement this protocol will also implement the DataFrame Interchange Protocol.
+
+This protocol is specific to Arrow-based data structures, while the DataFrame
+Interchange Protocol allows non-Arrow data frames and arrays to be shared as well.
+Because of this, these PyCapsules can support Arrow-specific features such as
+nested columns.
+
+This protocol is also much more minimal than the DataFrame Interchange Protocol.
+It just handles data export, rather than defining accessors for details like
+number of rows or columns.
+
+In summary, if you are implementing this protocol, you should also consider
+implementing the DataFrame Interchange Protocol.
+
+
+PyCapsule Standard
+==================
+
+`PyCapsules`_ allow for a ``name`` to be associated with the capsule, allowing 
+consumers to verify that the capsule contains the expected data. To make sure
+Arrow structs are recognized, the following names should be used:
+
+.. list-table::
+   :widths: 25 25
+   :header-rows: 1
+
+   * - C Interface Type
+     - PyCapsule Name
+   * - ArrowSchema
+     - ``arrowschema``
+   * - ArrowArray
+     - ``arrowarray``
+   * - ArrowArrayStream
+     - ``arrowarraystream``
+
+
+Lifetime Semantics
+------------------
+
+The exported PyCapsules should have a destructor that calls the
+:ref:`release callback <c-data-interface-released>`
+of the Arrow struct, if it is not already null. This prevents a memory leak in
+case the capsule was never passed to another consumer.
+
+If the consumer has been passed to a consumer, the consumer should have moved
+the data and marked the release callback as null, so there isn’t a risk of
+releasing data the consumer is using. Read more in the C Data Interface
+specification.
+
+Because of the lifetime semantics of the underlying Arrow structs, the
+PyCapsules can only be used once.
+
+
+Export Protocol
+===============
+
+The interface is three separate protocols:
+
+* ``ArrowSchemaExportable``, which defines the ``__arrow_c_schema__`` method.
+* ``ArrowArrayExportable``, which defines the ``__arrow_c_array__``. It extends
+  ``ArrowSchemaExportable``, so it must also define ``__arrow_c_schema__``.
+* ``ArrowStreamExportable``, which defines the ``__arrow_c_stream__``. It extends
+  ``ArrowSchemaExportable``, so it must also define ``__arrow_c_schema__``.
+
+The protocols are defined below in terms of ``typing.Protocol``. These may be
+copied into a library for the purposes of static type checking, but this is not
+required to implement the protocol.
+
+.. TODO: should stream have a batch size parameter?
+.. TODO: should we have a allow_copy flag?

Review Comment:
   Do folks think we need these parameters?



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