username:
password:

- name: Get username and password
  shell: python some_script.py
  register: result

output should be
username: bob
password: getmein

- name: Retrieve username
  shell: echo {{ item | regex_replace('^.*:(.*)$', 'username:\1')  }} >> playbooks/defaults/main.yml
  with_items: result.stdout_lines[1]