Groupby usage and creating script dict variable using template

I had to do so much google-fu to figure this script out that I decided to share my learnings.

For context, this script allows me to make a Sonos in my house (I have 9) join the largest group already playing. When I walk into a room, I can push a button and have the Sonos automatically join the largest Sonos group (usually what I want to listen to).

The challenge I had was identifying the largest group of Sonos in the house. I struggled with jinja2, specifically, how to use groupby as a filter. The next thing I struggled with was how to create a script variable that creates a dictionary from a template (using dict.from_keys).

Format disclaimer: I use both the visual and YAML editor modes. The visual mode mangles the template code. I apologize for how messy it looks.

Here it is:

alias: Sonos - Join Largest Group playing something
    name: sonos
    description: entity ID of the Sonos media player to have join the group
    advanced: false
    required: true
    default: media_player.downstairs_portable
  largest_group: >
    {% set ns = namespace(max_count=0, members=[], master='') -%} {%- for
    group_item in expand(integration_entities("sonos") | select("match",
    "media_player")) | groupby('attributes.group_members', default=[]) |
    map(attribute='grouper')  -%}
      {%- if group_item | length > ns.max_count -%}
        {%- if is_state(group_item | first, 'playing') -%}
          {%- set ns.max_count = group_item | length -%}
          {%- set ns.members = group_item -%}
          {%- set ns.master = group_item | first -%}
        {%- endif -%}
      {%- endif -%}
    {%- endfor -%}  {{ dict.from_keys([('members', ns.members), ('master',
    ns.master)]) }}
  - service: system_log.write
    enabled: false
      message: >-
        group members: {{largest_group.members}} and master:
  - service: media_player.join
      group_members: "{{sonos}}"
      entity_id: "{{largest_group.master}}"
mode: single