How to set up driver via python script between Custom Properties of objects?

Question

I want to set up drivers: from Custom Properties of a Camera to Custom Properties of selected object.

Based on a script from 6 year old post: How to put together a driver with python, I manage to have this code,

import bpy
def add_driver(
        source, target, prop, dataPath,
        index = -1, negative = False, func = ''
    ):
    ''' Add driver to source prop (at index), driven by target dataPath '''

    if index != -1:
        d = source.driver_add( prop, index ).driver
    else:
        d = source.driver_add( prop ).driver

    v = d.variables.new()
    v.name                 = prop
    v.targets[0].id        = target
    v.targets[0].data_path = dataPath

    d.expression = func + "(" + v.name + ")" if func else v.name
    d.expression = d.expression if not negative else "-1 * " + d.expression

#-------------------------------------------- 
ob = bpy.context.active_object
camera = bpy.context.scene.objects['Camera']

prop_name1 = ob.name + "_Opacity"
prop_name2 = ob.name + "_Highlight"


add_driver( ob, camera, prop_name1, prop_name1, -1)
add_driver( ob, camera, prop_name2, prop_name2, -1)

but executing it always give me error:

"Python: TypeError: bpy_struct.driver_add(): property "FirstCube_Opacity" not found

...even if I'm sure that all the objects have proper named Custom Properties. The function of setting up driver works great with other parameters in blender like 'scale'. What Am I doing wrong?

###UPDATE CODE####

Script1 - Creating Custom Properties

import bpy

ob = bpy.context.active_object

prop_name1 = ob.name + "_Opacity"
ob[prop_name1] = 0.0
ob.id_properties_ensure()  # Make sure the manager is updated
property_manager = ob.id_properties_ui(prop_name1)
property_manager.update(min=0.0, max=1.0, soft_min=0.0, soft_max=1.0)

camera = bpy.context.scene.objects['Camera']

prop_name2 = ob.name + "_Opacity"
camera[prop_name2] = 0.0
camera.id_properties_ensure()  # Make sure the manager is updated
property_manager = camera.id_properties_ui(prop_name2)
property_manager.update(min=0.0, max=1.0, soft_min=0.0, soft_max=1.0)

Script2 - Setting up driver, "Object_Opacity" CAMERA's Custom Property should drive "Object_Opacity" Object's Custom Property

import bpy
def add_driver(
        source, target, prop, dataPath,
        index = -1, negative = False, func = ''
    ):
    ''' Add driver to source prop (at index), driven by target dataPath '''

    if index != -1:
        d = source.driver_add( prop, index ).driver
    else:
        d = source.driver_add( prop ).driver

    v = d.variables.new()
    v.name                 = prop
    v.targets[0].id        = target
    v.targets[0].data_path = dataPath

    d.expression = func + "(" + v.name + ")" if func else v.name
    d.expression = d.expression if not negative else "-1 * " + d.expression
ob = bpy.context.active_object
camera = bpy.context.scene.objects['Camera']

prop_name1 = ob.name + "_Opacity"


add_driver( ob, camera, f'["{prop_name1}"]', f'["{prop_name1}"]', -1)

Answer 1

Replace add_driver(ob, camera, prop_name1, prop_name1, -1)

With add_driver(ob, camera, "prop", f'["{prop_name1}"]', -1)

Custom property are accessed with square brackets and double quotes, when regular properties are accessed directly.

In python variable names can't contain brackets nor quotes so I suggest you use "prop" string in this case which will create a variable named prop.