Driver value does not update

I am trying to animate a rope coiled around a cylinder. When the rope uncoils, a piece of rope will be extended downward.

I am doing this with drivers, using the bevel start and end factors (the rope is a bezier curve) to control the length of rope that has been coiled/extended. However, although everything looks nice when I manipulate the values manually, when I add a driver to allow the start factor to drive the end factor, the end factor gets stuck at zero and the value of the variable holding the start value will not update, despite the fact that I can change the value itself.

If anyone has experienced this problem and/or nows a solution, that would be excellent. Thanks in advance.

Aha! I found the problem.

I failed to notice the warning in the info window asking me if I wanted to trust the driver. It seems that the new Python security features extend to driver expressions as well, which I did not realize. I just said to trust the driver and everything started working.

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