An add-on example for creating a persistent library of Python-powered Blender driver functions (find out more about Blender Python drivers here https://www.youtube.com/watch?v=yiMGxlRv8h4&lc=UgwlEGLUyWsiKhPPlml4AaABAg )

blender_driver_functions_persistent_library.py

# name this file __init__.py

import bpy

ADDON_NAME = "My Diver Library"

bl_info = {
    "name": "My Diver Library",
    "author": "Viktor Stepanov",
    "version": (0, 0, 1),
    "blender": (2, 83, 0),
    "description": "A collection of Python diver functions.",
    "category": "Development",
}


def custom_driver_1(v):
    return v * 2


def custom_driver_2(v):
    return v * 3


DRIVERS = {
    "my_driver_1": custom_driver_1,
    "my_driver_2": custom_driver_2,
}


def register_drivers():
    print(f"Delayed driver registration:")
    for driver_name, driver_function in DRIVERS.items():
        print(f"\tRegistering driver {driver_name}")
        bpy.app.driver_namespace[driver_name] = driver_function


def register():
    print(f'ENABLED "{ADDON_NAME}" addon')

    # HACK: if we register the drivers right away they will not appear in the bpy.app.driver_namespace
    # adding a 2 second delay to register them
    bpy.app.timers.register(register_drivers, first_interval=2.0)


def unregister():
    print(f'DISABLE "{ADDON_NAME}" addon')

    for driver_name, _ in DRIVERS.items():
        print(f"\tDeregistering driver {driver_name}")
        bpy.app.driver_namespace.pop(driver_name, None)


if __name__ == "__main__":
    register()