| .. _building-c-or-cython-extensions: |
|
|
| ********************** |
| C or Cython Extensions |
| ********************** |
|
|
| Astropy supports using C extensions for wrapping C libraries and Cython for |
| speeding up computationally-intensive calculations. Both Cython and C extension |
| building can be customized using the ``get_extensions`` function of the |
| ``setup_package.py`` file. If defined, this function must return a list of |
| `distutils.core.Extension` objects. The creation process is left to the |
| subpackage designer, and can be customized however is relevant for the |
| extensions in the subpackage. |
|
|
| While C extensions must always be defined through the ``get_extensions`` |
| mechanism, Cython files (ending in ``.pyx``) are automatically located and |
| loaded in separate extensions if they are not in ``get_extensions``. For |
| Cython extensions located in this way, headers for numpy C functions are |
| included in the build, but no other external headers are included. ``.pyx`` |
| files present in the extensions returned by ``get_extensions`` are not |
| included in the list of extensions automatically generated extensions. Note |
| that this allows disabling a Cython file by providing an extension that |
| includes the Cython file, but giving it the special ``name`` 'cython_skip'. Any |
| extension with this package name will not be built by ``setup.py``. |
|
|
| .. note:: |
| |
| If an :class:`~distutils.core.Extension` object is provided for Cython |
| source files using the ``get_extensions`` mechanism, it is very |
| important that the ``.pyx`` files be given as the ``source``, rather than the |
| ``.c`` files generated by Cython. |
|
|
| Using Numpy C headers |
| ===================== |
|
|
| If your C or Cython extensions uses `numpy` at the C level, you probably |
| need access to the numpy C headers. A common idiom you can find in the numpy |
| docs or other examples involves getting the include directory by calling |
| ``numpy.get_include()``. However, using this in ``setup_package.py`` will *not* |
| work, because ``setup_package.py`` needs to be able to import even when none of |
| the dependencies are present. To work around this need, simply include the |
| string ``'numpy'`` in the list that is passed to the ``include_dirs`` argument |
| of `distutils.core.Extension`. The astropy setup helpers will then use |
| ``numpy.get_include()`` downstream once it is certain that the dependencies |
| have actually been processed. For example:: |
| |
| from distutils.extension import Extension |
|
|
| def get_extensions(): |
| return Extension(name='myextension', sources=['myext.pyx'], |
| include_dirs=['numpy']) |
|
|
|
|
|
|
| Installing C header files |
| ========================= |
|
|
| If your C extension needs to be linked from other third-party C code, |
| you probably want to install its header files along side the Python module. |
|
|
| 1) Create an ``include`` directory inside of your package for |
| all of the header files. |
|
|
| 2) Use the ``get_package_data`` hook in ``setup_package.py`` to |
| install those header files. For example, the `astropy.wcs` |
| package has this:: |
|
|
| def get_package_data(): |
| return {'astropy.wcs': ['include/*.h']} |
|
|
| Preventing importing at build time |
| ================================== |
|
|
| In rare cases, some packages may need to be imported at build time. |
| Unfortunately, anything that requires a C or Cython extension will fail to |
| import until the build phase has completed. In this cases, the |
| ``_ASTROPY_SETUP_`` variable can be used to determine if the package is being |
| imported as part of the build and choose to not import problematic modules. |
| ``_ASTROPY_SETUP_`` is inserted into the builtins, and is `True` when inside |
| of astropy's ``setup.py`` script, and `False` otherwise. |
|
|
| For example, suppose there is a subpackage ``foo`` that needs to |
| import a module called ``version.py`` at build time in order to set |
| some version information, and also has a C extension, ``process``, |
| that will not be available in the source tree. In this case, |
| ``astropy/foo/__init__.py`` would probably want to check the value of |
| ``_ASTROPY_SETUP_`` before importing the C extension:: |
|
|
| try: |
| from . import process |
| except ImportError: |
| if not _ASTROPY_SETUP_: |
| raise |
|
|
| from . import version |
|
|
| Speed up your builds with ccache |
| ================================ |
|
|
| `ccache <https://en.wikipedia.org/wiki/Ccache>`_ is a tool that caches |
| compiled sources so that they don't have to be recompiled (so long as they are |
| unchanged) even if the outputs have been deleted. This means that if you |
| switch branches or clean your source checkout you can save a lot of time by |
| avoiding the majority of re-compiles from scratch. |
|
|
| Because installation and configuration of ccache varies from platform to |
| platform, please consult the ccache documentation and/or Google to set up |
| ccache on your system--this is strongly encouraged for anyone doing significant |
| development of Astropy or scientific programming in general. |
|
|
