Horizontal constraint in 3D using the code (slvs_py) about solvespace

[ruevs]

In the code I'm defining two 3d Points: (0, 5, 8) and (5, 3, 24).
Then I define horizontal constraint in workplane defined by point (0,0,0) and normal (0,0,1).
When I run the solver - (0, 5, 8) and (5, 3, 24).are constrained to (0,4,8), (5,4,24). When I'd expect them to be constrained to (0,4,8), (5,4,8) (paralle to plane horizontal axis, X axis).

Constraints can be in 3d or projected in a workplane (when one is active). To quote from the reference manual:

Most constraints are available in both 3d and projected versions. If a workplane is active, then the constraint applies on the projection of the geometry into that workplane. If no workplane is active, then the constraint applies to the actual geometry in free space.

So the horizontal constraint you are defining works exactly as intended. The line between the points projected in the XY workplane is forced horizontal in the workplane. Attached is a file with the two points to experiment with visually.
1404_TwoPoints.zip

So your guess:

Is it that LineHorizontal assures that line projectd on a workplane will be horizontal for the specified workplane and not that actual line will be horizontal to the workplane / line will be parallel to workplane horizontal X axis?

is correct.

And perhaps a bit similar question - how would I create a constraint to make sure two arbitrary 3d lines will be parallel or keep the same distance between them?

Make sure that you are in 3d and apply a parallel constraint.

[Andrej730]

Thank you! It worked. Completely missed that some constraints methods doesn't require workplane argument to be provided and therefore can work in 3d.