You can see if jinja filters work. Ansible has a bunch, but I’m not sure if it’s an Ansible thing or some of it is native to jinja. I know Ansible defines a lot of custom ones.

Assuming it works, for what you’re doing it could be simplified to…

script:
  sequence:
    target:
      entity_id: "{{ entity }}"
      data: "{{ variable | default(omit) }}"

This basically says for data, use {{ variable }}, unless it doesn’t exist, in which case omit data. Omit could also be replaced with some another variable or static string (quoted) to use that as a default if {{ variable }} doesn’t exist.

I tried quickly looking on the Home Assistant site to see if this would work and didn’t see anything, but if it’s anything like Ansible, the documentation leaves a lot of room for finding stuff through experience and leaning on other resources. I’d say try it and see what happens. That’s probably the fastest way to know if it will work.

Depending on what data does here, you might be able to just add an else condition to set it to a blank string. That works better in some cases than others. I’m not sure if there is an “official” way to do this, but {{- '' -}} seems to work for me. I usually add those extra minus signs on everything when using jinja to avoid issues with extra spaces (those trim white space). If you don’t want to go the empty string route, or it doesn’t play well, you might be able to try NONE as well, as mentioned here.

To give more context I’m working on a media control dashboard. The script or rather scripts I have to send commands to kodi is as follows

kodi_control:
  sequence:
  - service: kodi.call_method
    target:
      entity_id: '{{ kodi_entity }}'
    data:
      method: '{{ kodi_method }}'

kodi_control_playback:
  sequence:
  - service: kodi.call_method
    target:
      entity_id: '{{ kodi_entity }}'
    data:
      method: '{{ kodi_method }}'
      playerid: '{{ kodi_playerid }}'

kodi_control_subtitles:
  sequence:
  - service: kodi.call_method
    target:
      entity_id: '{{ kodi_entity }}'
    data:
      method: '{{ kodi_method }}'
      action: '{{ kodi_action }}'

kodi_control_seek:
  sequence:
  - service: kodi.call_method
    target:
      entity_id: '{{ kodi_entity }}'
    data:
      method: '{{ kodi_method }}'
      playerid: '{{ kodi_playerid }}'
      value: '{{ kodi_value }}'

kodi_control_playlist:
  sequence:
  - service: kodi.call_method
    target:
      entity_id: '{{ kodi_entity }}'
    data:
      method: '{{ kodi_method }}'
      window: '{{ kodi_window }}'
      parameters: '{{ [ kodi_parameters ] }}'

I would like to condense all of this down to a single script using “is defined” to omit the parts not needed for certain commands so something like

kodi_control:
  sequence:
  - service: kodi.call_method
    target:
      entity_id: '{{ kodi_entity }}'
    data: >-
      method: '{{ kodi_method }}'
      {% if kodi_playerid is defined %}
        playerid: '{{ kodi_playerid }}'
      {% endif %}
      {% if kodi_action is defined %}
        action: '{{ kodi_action }}'
      {% endif %}
      {% if kodi_value is defined %}
        value: '{{ kodi_value }}'
      {% endif %}
      {% if kodi_window is defined %}
        window: '{{ kodi_window }}'
      {% endif %}
      {% if kodi_parameters is defined %}
        parameters: '{{ [ kodi_parameters ] }}'
      {% endif %}

Problem with the above is I get “result is not a dictionary”