fabfile.py

from fabric.api import run, task, roles, env, execute


env.roledefs = {'web': ['host1.example.com', 
                        'host2.example.com'],
                'db': ['host3.example.com']}


@task
@roles('db')
def echo():
  run('echo db')
  

@task
@roles('web')
def test():
  run('echo web')
  execute(echo)

out

$ fab test
[host1.example.com] Executing task 'test'
[host1.example.com] run: echo web
[host1.example.com] out: web
[host1.example.com] out:

[host3.example.com] Executing task 'echo'
[host3.example.com] run: echo db
[host3.example.com] out: db
[host3.example.com] out:

[host2.example.com] Executing task 'test'
[host2.example.com] run: echo web
[host2.example.com] out: web
[host2.example.com] out:

[host3.example.com] Executing task 'echo'
[host3.example.com] run: echo db
[host3.example.com] out: db
[host3.example.com] out:


Done.
Disconnecting from host1.example.com... done.
Disconnecting from host2.example.com... done.
Disconnecting from host3.example.com... done.