you should only put variable part in curly braces

I define a include_var task playbook, it use local facts to determine which file to load

include_vars: "{{ ansible_local[preference][role].yml }}"

it failed with an error message like

The task includes an option with an undefined variable.

I should put the file extension .yml outside of the curly braces

include_vars: "{{ ansible_local['preferences']['role'] }}.yml"