Blender drivers work differently on frame stepping than frame selecting I created a driver to move an edge as a constant size hypotenuse of an animated right angle triangle.

import bpy
import math as m

def rigthAngleTransform(rot, length):
    locY = length-(m.cos(rot)*length)-length
    return locY

bpy.app.driver_namespace["rigthAngleTransform"] = rigthAngleTransform


The representation of these values:


This screenshot is not at the start frame! At the start frame the pivot is from "length" distance from the X axis and rot is 0d

Where rot is animated from 0d to 90d (the script uses radians!), while the pivot Y location coordinate is decreasing according to the following function:

locY = length-(cos(rot)*length)-length


The fun part it works perfectly as it should by math if I choose a frame with the mouse cursor. however if I play the animation or change the frames with the arrow keys it will deviate like this:



The end point at the x axis should not deviate from it

A good clue if I change the direction of the animation playback the direction of the deviation will be the opposite (hypotenuse will go under the x axis)

If my perception is correct, during animation the driver has a 1 frame latency. 

Update:

My perception was correct I modified the script to use the z rotation value of the next frame, by adding 90/45d to the z rotation. Now it works perfectly during animation, if the interpolation is linear, and it will generate a deviation if I select a frame by clicking.

updated script:

import bpy
import math as m

def rigthAngleTransform(rot, length):
    locY = length-(m.cos(rot+m.radians(90/45))*length)-length
    return locY

bpy.app.driver_namespace["rigthAngleTransform"] = rigthAngleTransform
Well I found a very easy solution to the problem.

I use an empty proxy object to drive the rotation and location of my edge.



So I copy this z rotation with a simple driver, and my original script gets the z rotation input from this proxy. Now it works perfectly regardless of the interpolation.

It is important to note: That the one frame latency occurred, because in the question the driver and driven properties belonged to the same object. In a case when these porps reside on different object it should not occur.

I experimented further this issue, and the problem also shows itself if two objects mutually drive themselves.