Support for using Giac within SageMath

Introduction

Giac is a full-featured, open source mathematics library written by Bernard Parisse:

• https://www-fourier.ujf-grenoble.fr/~parisse/giac.html

For many years, SageMath has included an interface to Giac based on the GiacPy python interface by Frederic Han:

• https://pypi.org/project/giacpy/

This interface has now been split off into a separate package. The main reason you would want to install it is to add Giac as a backend for SageMath's integrate command. For example, without this package, Sage cannot integrate the following example:

>>> from sage.all import *
>>> from sagemath_giac.giac import libgiac
>>> x = SR.symbol("x", domain="real")
>>> libgiac.integrate(2*min_symbolic(x,2*x),x).sage()
-1/2*x^2*sgn(x) + 3/2*x^2

After installing sagemath-giac, Sage will automatically gain the ability to do these integrals. Of course, you also gain access to the rest of the Giac library, with easy conversions to and from Sage objects.

Previously the contents of sagemath-giac were available (within SageMath) in the sage.libs.giac module. In this package, we have been forced to rename it to sagemath_giac due to some (hopefully temporary) limitations of Cython in regard to namespace packages:

• cython/cython#5237
• cython/cython#5335
• cython/cython#6287

If we had kept the old name, for example, you would not be able to install sagemath-giac as a normal user for use with the system installation of Sage. If sagemath-giac is installed, however, SageMath will export its contents under the old name to avoid breaking backwards compatibility. If/when Cython support for namespace packages materializes, the contents of sagemath-giac can be moved back under sage.libs.giac.

Building

It is possible to build the package directly through meson:

$ meson setup --prefix=$HOME/.local build
$ meson compile -C build

The prefix is not used until install-time, but eventually it tells meson where to install the built files. The location $HOME/.local is most likely your personal python "user site" directory, where python looks for packages installed with pip install --user.

Of course, it is also possible to build a wheel the usual way, using:

$ python -m pip wheel --no-build-isolation --verbose .

or:

$ python -m build --no-isolation --skip-dependency-check .

Skipping the dependency check is necessary if you installed SageMath using meson, because meson install installs only the library and not the python packaging metadata. It is recommended to skip the dependency check in any case, opting instead to ensure that SageMath and all of its dependencies are installed (systemwide or in the active venv) beforehand. Otherwise there is a risk that the build system will try to download and isolate a new set of build dependencies, which involves building all of SageMath again.

Testing

A few doctests within the module ensure that everything is working. If you have built the project using meson and a build directory named build, you can run the tests with:

$ PYTHONPATH=src:build/src python -m doctest \
    README.rst src/sagemath_giac/*.py* \
    2>/dev/null

We need to add both src and build/src (or whatever directory you passed to meson as your build directory) to PYTHONPATH so that python can find both our python modules and the compiled C extension module. We have redirected stderr to /dev/null because, otherwise, a mountain of debug output is printed to the console. A small amount is still printed to stdout, but that is most likely a bug in libgiac.

If you have pytest installed, it can also be used:

$ PYTHONPATH=src:build/src pytest

Finally, meson is capable of running pytest on your behalf:

$ PYTHONPATH=src:build/src meson test -C build

Installation

After building/testing, you can install the package either using meson:

$ meson install -C build

or from the wheel that you generated earlier:

$ pip install $(find ./ -type f -name 'sagemath_giac-*.whl')