my_dict

test.py

#!/usr/bin/env python3

from ruamel.yaml import YAML
from pathlib import Path
import re

my_dict = {}

path = Path('test.yml')
yaml = YAML(typ='safe')
data = yaml.load(path)

for insts in data:
  for host in data[insts]:
    first_group = host['groups'][0]
    result = re.split(r'-', first_group, 1)
    if result[0] not in my_dict:
      my_dict.update({result[0]: {first_group: [host.get('name')]}})
    else:
      if first_group not in my_dict[result[0]]:
        my_dict[result[0]].update({first_group: [host.get('name')]})
      else:
        my_dict[result[0]][first_group].append(host.get('name'))

print(f'{my_dict}')

test.yml

---
inst_one:
  - name: n1
    addr: 127.0.0.1
    groups:
      - v1-int
      - runner
  - name: n2
    addr: 127.0.0.1
    groups:
      - v2-int
      - rabbit
      - redis
inst_two:
  - name: n3
    addr: 127.0.0.1
    groups:
      - v1-prod
      - runner
  - name: n4
    addr: 127.0.0.1
    groups:
      - v2-int
      - rabbit
      - redis
  - name: n5
    addr: 127.0.0.1
    groups:
      - v2-demo
      - rabbit
  - name: n6
    addr: 127.0.0.1
    groups:
      - v2-demo
      - rabbit