Can't add special variables like "$fn" from python code #104

This is definitely a hole in the documentation! The most common case is, as you say, $fn. For that, you can specify a segments argument for any function (sphere(), cylinder(), etc.) that would take $fn. You can also check out the pattern in the solidpython_template example where you speficy SEGMENTS at the top of your SolidPython file, and add that as a header to the rendered SCAD code. This lets you control the segments for the entire model in one place. I've found this the easiest way to work for quick renders (SEGMENTS = 12) which I can increase for really smooth curves once I'm ready to render out (SEGMENTS = 120).

I'll open an issue specifically to document OpenSCAD special variables and how to deal with them.

You're correct that there's not a programmatic way to set $fn globally. Maybe there should be?

Here's the pattern I use (and it's in the solidpython_template example ) to specify $fn globally:

SEGMENTS = 12
a = assembly() # some SolidPython object
scad_render_to_file(a, file_header='$fn = %s;' % SEGMENTS)