Mathics3 is a computer algebra system. It is a free, open-source alternative to Mathematica® or the Wolfram Language. However, Mathics3 builds around and on top of the Python ecosystem of libraries and tools. So in a sense, you can think of it as a WMA front-end to the Python ecosystem of tools.
Mathics3 is free both as in “free beer”, but more importantly, as in “freedom”. Mathics3 can be run locally. To facilitate installation of the vast amount of software needed to run this, there is a docker image available on dockerhub.
The programming language and built-in functions of Mathics3 try to match the Wolfram Language, which is continually evolving.
Mathics3 is in no way affiliated or supported by Wolfram. Mathics3 will probably never have the power to compete with Mathematica® in industrial applications; it is a free alternative though. It also invites community development at all levels.
See the installation instructions for the most recent instructions for installing from PyPI, or the source.
For implementation details, please refer to the Developers Guide.