There are many cases where we would like to change some property by referring to another property but where the relationship cannot be captured by driving an IPO channel by another one. Often, this is because the relation is not a simple linear dependency, for example, a piston driven by a circular motion. Another case would be if the relation is non continuous, for example, a light that is turned on when a switch is in a certain position.

In these cases the relationship may be defined by a Python expression or by the so-called pydriver. A pydriver takes the IPO channel of another object as input and produces output that will drive a channel on the current object. Because these Python expressions can access the complete Blender API, these relations can be made very complex indeed.