Thanks for your contribution!

I agree that it would be a nice feature to make device_id available to use in functions.
However, it would override device_id if it's used in parameters of spec.

I think it would be better if we change two things:

1. Create reserved word which contains any data, such as user_input or exposed_entities, needed for users. The reserved word can be something like _params, _args, or word starting with _, so that it reduces possibility of conflict. Then your function should look like this.

   - service: python_script.play_music
     data:
       device_id: '{{_params.user_input.device_id}}'
       ...

2. Make it available not only to script function but also to any functions.
   (Probably move your code to here and here would work)