pitrou commented on code in PR #37797: URL: https://github.com/apache/arrow/pull/37797#discussion_r1338892247
########## docs/source/format/CDataInterface/PyCapsuleInterface.rst: ########## @@ -0,0 +1,238 @@ +.. 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? + + + +.. code-block:: python + + from typing import Tuple, Protocol + from typing_extensions import Self + + class ArrowSchemaExportable(Protocol): + def __arrow_c_schema__(self) -> object: + """ + Get a PyCapsule containing a C ArrowSchema representation of the object. + + The capsule will have a name of "arrowschema". + """ + ... + + class ArrowArrayExportable(ArrowSchemaExportable, Protocol): + def __arrow_c_array__( + self, + requested_schema: object | None = None + ) -> Tuple[object, object]: + """ + Export array as a pair of PyCapsules for the ArrowSchema and ArrowArray. + + The capsule will have a name of "arrowarray". + + If requested_schema is passed, the callee should attempt to provide the + data in the requested schema. However, this is best-effort, and the + callee may return a PyCapsule containing an ArrowArray with a different + schema. This parameter is useful for cases where the underlying data + could be represented in multiple ways, and the caller has a preference + for how it is represented. For example, some systems have a single + integer type, but Arrow has multiple integer types with different + sizes and sign. + + Parameters + ---------- + requested_schema : PyCapsule | None + A PyCapsule containing a C ArrowSchema representation of a requested + schema. Conversion to this schema is best-effort. + + Returns + ------- + Tuple[PyCapsule, PyCapsule] + A pair of PyCapsules containing a C ArrowSchema and ArrowArray, + respectively. + """ + ... + + + class ArrowStreamExportable(ArrowSchemaExportable, Protocol): Review Comment: > And for ArrowArrayExportable, we also want to expose the schema directly, I think, so this keeps it consistent This means you could eventually pass an array or record batch to `pa.schema`, which seems weird. -- This is an automated message from the Apache Git Service. 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