FoamPython
FoamPython is a minimal code that mimics OpenFOAM's basic structure and functionality. The similarity lies in the way that the base classes (fvMesh, fvMatrix, fields) are implemented.
As it is written in Python and as it consists of significantly less lines of code than OpenFOAM, experimenting with some base principles of Finite Volume Method CFD should be simpler and easier than doing the same in OpenFOAM. Also, as it is written in Python, as opposed to C++ in the case of OpenFOAM, it is more accessible to novice programmers and CFD users.
Training session in the 17th OpenFOAM Workshop, Cambridge, 2022