This posts contains summary of two weeks work.

I was working on introducing transformation equations in CoordSysCartesian. We decide, up to now, to connect every other curvilinear coordinate system to Cartesian, so equations maps the rectangular one to the desired one. We allow user to set new coordinate system in three possible way:

a = CoordSys3D('a') a.connect_to_cartesian('spherical') a.connect_to_cartesian((a.x, a.y, a.z)) x, y, z = Symbols('x y z') a.connect_to_cartesian((x, y, z), (x, y, z))

So we can set coordinate system by str from pre-defined dict. The second way is to do it by equations, where variables are base scalars. The last possibility is to create tuple which contains two tuples. In first, there should be variables which we want to use to define transformation equations, and in the second one transformation equation itself.

During this time I’ve created several PR, which corresponds directly to the transformation of coordinate system #12803, #128822, #12872, but I also updated documentation #12871 and refactor some code. So now we have CoordSys3D instead of CoordSysCartesian #12829. Updated documentation went into 1.1 release of SymPy. It was necessary to avoid mismatch between the code which was previously updated or deprecated and documentation