docs/source/python/extending.rst

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.. to you under the Apache License, Version 2.0 (the
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..   http://www.apache.org/licenses/LICENSE-2.0

.. Unless required by applicable law or agreed to in writing,
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.. KIND, either express or implied.  See the License for the
.. specific language governing permissions and limitations
.. under the License.

.. currentmodule:: pyarrow
.. _extending:

Using pyarrow from C++ and Cython Code
======================================

pyarrow features both a Cython and C++ API.

C++ API
-------

.. default-domain:: cpp

The Arrow C++ header files are bundled with a pyarrow installation.
To get the absolute path to this directory (like ``numpy.get_include()``), use:

.. code-block:: python

   import pyarrow as pa
   pa.get_include()

Assuming the path above is on your compiler's include path, the pyarrow API
can be included using the following directive:

.. code-block:: cpp

   #include <arrow/python/pyarrow.h>

This will not include other parts of the Arrow API, which you will need
to include yourself (for example ``arrow/api.h``).

When building C extensions that use the Arrow C++ libraries, you must add
appropriate linker flags. We have provided functions ``pyarrow.get_libraries``
and ``pyarrow.get_library_dirs`` which return a list of library names and
likely library install locations (if you installed pyarrow with pip or
conda). These must be included when declaring your C extensions with distutils
(see below).

Initializing the API
~~~~~~~~~~~~~~~~~~~~

.. function:: int import_pyarrow()

   Initialize inner pointers of the pyarrow API.  On success, 0 is
   returned.  Otherwise, -1 is returned and a Python exception is set.

   It is mandatory to call this function before calling any other function
   in the pyarrow C++ API.  Failing to do so will likely lead to crashes.

Wrapping and Unwrapping
~~~~~~~~~~~~~~~~~~~~~~~

pyarrow provides the following functions to go back and forth between
Python wrappers (as exposed by the pyarrow Python API) and the underlying
C++ objects.

.. function:: bool is_array(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`Array` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.Array` instance.

.. function:: bool is_buffer(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`Buffer` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.Buffer` instance.

.. function:: bool is_data_type(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`DataType` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.DataType` instance.

.. function:: bool is_field(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`Field` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.Field` instance.

.. function:: bool is_record_batch(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`RecordBatch` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.RecordBatch` instance.

.. function:: bool is_schema(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`Schema` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.Schema` instance.

.. function:: bool is_table(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`Table` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.Table` instance.

.. function:: bool is_tensor(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`Tensor` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.Tensor` instance.

.. function:: bool is_sparse_coo_tensor(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`SparseCOOTensor` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.SparseCOOTensor` instance.

.. function:: bool is_sparse_csr_matrix(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`SparseCSRMatrix` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.SparseCSRMatrix` instance.

.. function:: bool is_sparse_csc_matrix(PyObject* obj)

   Return whether *obj* wraps an Arrow C++ :class:`SparseCSCMatrix` pointer;
   in other words, whether *obj* is a :py:class:`pyarrow.SparseCSCMatrix` instance.

The following functions expect a pyarrow object, unwrap the underlying
Arrow C++ API pointer, and put it in the *out* parameter.  The returned
:class:`Status` object must be inspected first to know whether any error
occurred.  If successful, *out* is guaranteed to be non-NULL.

.. function:: Status unwrap_array(PyObject* obj, std::shared_ptr<Array>* out)

   Unwrap the Arrow C++ :class:`Array` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_buffer(PyObject* obj, std::shared_ptr<Buffer>* out)

   Unwrap the Arrow C++ :class:`Buffer` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_data_type(PyObject* obj, std::shared_ptr<DataType>* out)

   Unwrap the Arrow C++ :class:`DataType` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_field(PyObject* obj, std::shared_ptr<Field>* out)

   Unwrap the Arrow C++ :class:`Field` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_record_batch(PyObject* obj, std::shared_ptr<RecordBatch>* out)

   Unwrap the Arrow C++ :class:`RecordBatch` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_schema(PyObject* obj, std::shared_ptr<Schema>* out)

   Unwrap the Arrow C++ :class:`Schema` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_table(PyObject* obj, std::shared_ptr<Table>* out)

   Unwrap the Arrow C++ :class:`Table` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_tensor(PyObject* obj, std::shared_ptr<Tensor>* out)

   Unwrap the Arrow C++ :class:`Tensor` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_sparse_coo_tensor(PyObject* obj, std::shared_ptr<SparseCOOTensor>* out)

   Unwrap the Arrow C++ :class:`SparseCOOTensor` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_sparse_csr_matrix(PyObject* obj, std::shared_ptr<SparseCSRMatrix>* out)

   Unwrap the Arrow C++ :class:`SparseCSRMatrix` pointer from *obj* and put it in *out*.

.. function:: Status unwrap_sparse_csc_matrix(PyObject* obj, std::shared_ptr<SparseCSCMatrix>* out)

   Unwrap the Arrow C++ :class:`SparseCSCMatrix` pointer from *obj* and put it in *out*.

The following functions take an Arrow C++ API pointer and wrap it in a
pyarray object of the corresponding type.  A new reference is returned.
On error, NULL is returned and a Python exception is set.

.. function:: PyObject* wrap_array(const std::shared_ptr<Array>& array)

   Wrap the Arrow C++ *array* in a :py:class:`pyarrow.Array` instance.

.. function:: PyObject* wrap_buffer(const std::shared_ptr<Buffer>& buffer)

   Wrap the Arrow C++ *buffer* in a :py:class:`pyarrow.Buffer` instance.

.. function:: PyObject* wrap_data_type(const std::shared_ptr<DataType>& data_type)

   Wrap the Arrow C++ *data_type* in a :py:class:`pyarrow.DataType` instance.

.. function:: PyObject* wrap_field(const std::shared_ptr<Field>& field)

   Wrap the Arrow C++ *field* in a :py:class:`pyarrow.Field` instance.

.. function:: PyObject* wrap_record_batch(const std::shared_ptr<RecordBatch>& batch)

   Wrap the Arrow C++ record *batch* in a :py:class:`pyarrow.RecordBatch` instance.

.. function:: PyObject* wrap_schema(const std::shared_ptr<Schema>& schema)

   Wrap the Arrow C++ *schema* in a :py:class:`pyarrow.Schema` instance.

.. function:: PyObject* wrap_table(const std::shared_ptr<Table>& table)

   Wrap the Arrow C++ *table* in a :py:class:`pyarrow.Table` instance.

.. function:: PyObject* wrap_tensor(const std::shared_ptr<Tensor>& tensor)

   Wrap the Arrow C++ *tensor* in a :py:class:`pyarrow.Tensor` instance.

.. function:: PyObject* wrap_sparse_coo_tensor(const std::shared_ptr<SparseCOOTensor>& sparse_tensor)

   Wrap the Arrow C++ *COO sparse tensor* in a :py:class:`pyarrow.SparseCOOTensor` instance.

.. function:: PyObject* wrap_sparse_csr_matrix(const std::shared_ptr<SparseCSRMatrix>& sparse_tensor)

   Wrap the Arrow C++ *CSR sparse tensor* in a :py:class:`pyarrow.SparseCSRMatrix` instance.

.. function:: PyObject* wrap_sparse_csc_matrix(const std::shared_ptr<SparseCSCMatrix>& sparse_tensor)

   Wrap the Arrow C++ *CSC sparse tensor* in a :py:class:`pyarrow.SparseCSCMatrix` instance.


Cython API
----------

.. default-domain:: py

The Cython API more or less mirrors the C++ API, but the calling convention
can be different as required by Cython.  In Cython, you don't need to
initialize the API as that will be handled automatically by the ``cimport``
directive.

.. note::
   Classes from the Arrow C++ API are renamed when exposed in Cython, to
   avoid named clashes with the corresponding Python classes.  For example,
   C++ Arrow arrays have the ``CArray`` type and ``Array`` is the
   corresponding Python wrapper class.

Wrapping and Unwrapping
~~~~~~~~~~~~~~~~~~~~~~~

The following functions expect a pyarrow object, unwrap the underlying
Arrow C++ API pointer, and return it.  NULL is returned (without setting
an exception) if the input is not of the right type.

.. function:: pyarrow_unwrap_array(obj) -> shared_ptr[CArray]

   Unwrap the Arrow C++ :cpp:class:`Array` pointer from *obj*.

.. function:: pyarrow_unwrap_batch(obj) -> shared_ptr[CRecordBatch]

   Unwrap the Arrow C++ :cpp:class:`RecordBatch` pointer from *obj*.

.. function:: pyarrow_unwrap_buffer(obj) -> shared_ptr[CBuffer]

   Unwrap the Arrow C++ :cpp:class:`Buffer` pointer from *obj*.

.. function:: pyarrow_unwrap_data_type(obj) -> shared_ptr[CDataType]

   Unwrap the Arrow C++ :cpp:class:`CDataType` pointer from *obj*.

.. function:: pyarrow_unwrap_field(obj) -> shared_ptr[CField]

   Unwrap the Arrow C++ :cpp:class:`Field` pointer from *obj*.

.. function:: pyarrow_unwrap_schema(obj) -> shared_ptr[CSchema]

   Unwrap the Arrow C++ :cpp:class:`Schema` pointer from *obj*.

.. function:: pyarrow_unwrap_table(obj) -> shared_ptr[CTable]

   Unwrap the Arrow C++ :cpp:class:`Table` pointer from *obj*.

.. function:: pyarrow_unwrap_tensor(obj) -> shared_ptr[CTensor]

   Unwrap the Arrow C++ :cpp:class:`Tensor` pointer from *obj*.

.. function:: pyarrow_unwrap_sparse_coo_tensor(obj) -> shared_ptr[CSparseCOOTensor]

   Unwrap the Arrow C++ :cpp:class:`SparseCOOTensor` pointer from *obj*.

.. function:: pyarrow_unwrap_sparse_csr_matrix(obj) -> shared_ptr[CSparseCSRMatrix]

   Unwrap the Arrow C++ :cpp:class:`SparseCSRMatrix` pointer from *obj*.

.. function:: pyarrow_unwrap_sparse_csc_matrix(obj) -> shared_ptr[CSparseCSCMatrix]

   Unwrap the Arrow C++ :cpp:class:`SparseCSCMatrix` pointer from *obj*.

The following functions take a Arrow C++ API pointer and wrap it in a
pyarray object of the corresponding type.  An exception is raised on error.

.. function:: pyarrow_wrap_array(sp_array: const shared_ptr[CArray]& array) -> object

   Wrap the Arrow C++ *array* in a Python :class:`pyarrow.Array` instance.

.. function:: pyarrow_wrap_batch(sp_array: const shared_ptr[CRecordBatch]& batch) -> object

   Wrap the Arrow C++ record *batch* in a Python :class:`pyarrow.RecordBatch` instance.

.. function:: pyarrow_wrap_buffer(sp_array: const shared_ptr[CBuffer]& buffer) -> object

   Wrap the Arrow C++ *buffer* in a Python :class:`pyarrow.Buffer` instance.

.. function:: pyarrow_wrap_data_type(sp_array: const shared_ptr[CDataType]& data_type) -> object

   Wrap the Arrow C++ *data_type* in a Python :class:`pyarrow.DataType` instance.

.. function:: pyarrow_wrap_field(sp_array: const shared_ptr[CField]& field) -> object

   Wrap the Arrow C++ *field* in a Python :class:`pyarrow.Field` instance.

.. function:: pyarrow_wrap_resizable_buffer(sp_array: const shared_ptr[CResizableBuffer]& buffer) -> object

   Wrap the Arrow C++ resizable *buffer* in a Python :class:`pyarrow.ResizableBuffer` instance.

.. function:: pyarrow_wrap_schema(sp_array: const shared_ptr[CSchema]& schema) -> object

   Wrap the Arrow C++ *schema* in a Python :class:`pyarrow.Schema` instance.

.. function:: pyarrow_wrap_table(sp_array: const shared_ptr[CTable]& table) -> object

   Wrap the Arrow C++ *table* in a Python :class:`pyarrow.Table` instance.

.. function:: pyarrow_wrap_tensor(sp_array: const shared_ptr[CTensor]& tensor) -> object

   Wrap the Arrow C++ *tensor* in a Python :class:`pyarrow.Tensor` instance.

.. function:: pyarrow_wrap_sparse_coo_tensor(sp_array: const shared_ptr[CSparseCOOTensor]& sparse_tensor) -> object

   Wrap the Arrow C++ *COO sparse tensor* in a Python :class:`pyarrow.SparseCOOTensor` instance.

.. function:: pyarrow_wrap_sparse_csr_matrix(sp_array: const shared_ptr[CSparseCSRMatrix]& sparse_tensor) -> object

   Wrap the Arrow C++ *CSR sparse tensor* in a Python :class:`pyarrow.SparseCSRMatrix` instance.

.. function:: pyarrow_wrap_sparse_csc_matrix(sp_array: const shared_ptr[CSparseCSCMatrix]& sparse_tensor) -> object

   Wrap the Arrow C++ *CSC sparse tensor* in a Python :class:`pyarrow.SparseCSCMatrix` instance.

Example
~~~~~~~

The following Cython module shows how to unwrap a Python object and call
the underlying C++ object's API.

.. code-block:: python

   # distutils: language=c++

   from pyarrow.lib cimport *


   def get_array_length(obj):
       # Just an example function accessing both the pyarrow Cython API
       # and the Arrow C++ API
       cdef shared_ptr[CArray] arr = pyarrow_unwrap_array(obj)
       if arr.get() == NULL:
           raise TypeError("not an array")
       return arr.get().length()

To build this module, you will need a slightly customized ``setup.py`` file
(this is assuming the file above is named ``example.pyx``):

.. code-block:: python

    from distutils.core import setup
    from Cython.Build import cythonize

    import os
    import numpy as np
    import pyarrow as pa


    ext_modules = cythonize("example.pyx")

    for ext in ext_modules:
        # The Numpy C headers are currently required
        ext.include_dirs.append(np.get_include())
        ext.include_dirs.append(pa.get_include())
        ext.libraries.extend(pa.get_libraries())
        ext.library_dirs.extend(pa.get_library_dirs())

        if os.name == 'posix':
            ext.extra_compile_args.append('-std=c++11')

        # Try uncommenting the following line on Linux
        # if you get weird linker errors or runtime crashes
        # ext.define_macros.append(("_GLIBCXX_USE_CXX11_ABI", "0"))


    setup(ext_modules=ext_modules)


Compile the extension:

.. code-block:: bash

    python setup.py build_ext --inplace

Building Extensions against PyPI Wheels
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The Python wheels have the Arrow C++ libraries bundled in the top level
``pyarrow/`` install directory. On Linux and macOS, these libraries have an ABI
tag like ``libarrow.so.17`` which means that linking with ``-larrow`` using the
linker path provided by ``pyarrow.get_library_dirs()`` will not work right out
of the box. To fix this, you must run ``pyarrow.create_library_symlinks()``
once as a user with write access to the directory where pyarrow is
installed. This function will attempt to create symlinks like
``pyarrow/libarrow.so``. For example:

.. code-block:: bash

   pip install pyarrow
   python -c "import pyarrow; pyarrow.create_library_symlinks()"