Nevermind, I think I've got it:

moment_of_inertia = 2/5 * power(my.mass * radius, 2);

At least it't the best so far.

I found a useful chart of common moments of inertia: http://hyperphysics.phy-astr.gsu.edu/HBASE/mi.html

Thanks for explaining and pointing me in the right direction.