Drivers and Application Handlers

Trigger

Use this skill when the user:

• Wants property A to follow property B with some math (driver)
• Asks about driver_add, FCurve.driver, driver.expression, driver_namespace
• Wants to run code on file save, file load, frame change, depsgraph update, or process exit
• Mentions bpy.app.handlers, save_pre, load_post, depsgraph_update_post, exit_pre
• Has a driver expression that fails security checks because it tries to call a Python function

This skill bundles two related "reactive" patterns. They show up together often (a driver that calls a function registered on a handler), so they share one skill.

Part 1: Drivers

What a driver is

A driver replaces a static animation curve with a real-time-evaluated expression. It is attached to one property (the target) and reads zero or more other properties (the variables), then evaluates a string expression to compute the target's value on each frame.