This is a demonstration of using numpy and cffi in order to build a grand Python
package on top of existing C libaries. For the purpose of this demonstration
we condider the case of grand.coordinates.
Coordinates objects are created by subclassing numpy.ndarray. The TURTLE C library is used for transforms between different coordinates systems.
In order to be efficient the CPU time spent at interfaces between Python and C
must be minimal. In numpy this is achieved by vectorizing operations. Instead
of repeating Python calls on each element of a collection a single Python call
is made on a numpy.ndarray. The looping over the array elements is done at the
C level, not at the Python one. However, the TURTLE library functions are not
natively vectorized. Yet, this is simple to achieve by providing
straightforward C wrappers. Examples of those are located in
vectorization.h and
vectorization.c.
The compilation requires to clone the present repository as well as
TURTLE ones. The latter is expected to be
cloned under share/turtle in the local copy of the present repository.
The demonstration grand package can be compiled from the repository by using
the provided Makefile. This generates libgrand.abi3.so. Note that
the latter is a standard dynamic library that can be linked to C applications as
well.
The compilation is done with cffi. This is taken care by the
grand_build.py script. A local installation of
grand.libgrand is done with setuptools, see e.g. setup.py.
Note that this procedure should allow to generate binary wheels of the grand
package as well for distributing over PyPI, though this was not yet tested.
An example of usage is provided as
examples/coordinates.py. Note that you might need to
export the repository path to your PYTHONPATH in order to locate the grand
package.