Operating system: Pop!_OS (Debian) 18.10
Graphics card: GTX Titan X
A driver calls on a python function. The python function is internal and registered.
When the project is opened, the driver can't find the function, throwing up "ERROR: Invalid Python expression".
Pressing "Update Dependencies" fixes the problem
Exact steps for others to reproduce the error
- Open the project.
- Enter pose mode for the armature
- Try moving it along the X axis (Moving the armature along the x axis should make the cube move along the Y-axis, but it doesn't)
- Exit pose mode
- Select the cube
- In the Drivers panel, you'll see in the Driver Property panel an error "ERROR: Invalid Python expression"
- Clicking 'Update Dependencies' allows the driver to start working correctly.
This happens each time the project is opened.
I'd be happy to just click this button each time I load, but if you have a project with many items being driven by functions, it's not practical at all.