[Fab-user] Providing a fallback for roles at runtime

[Paul Walsh]

I have a setup like this:

env.roledefs = {'default': ['1.2.3.4'], 'db': ['5.6.7.8']},
env.roles: ['default', 'db']
# other custom keys on env

@roles('db')
@task
def drop():
    utilities.alert(u'Dropping the database.')
    run('dropdb {name}'.format(name=env.db_name))


This is fine: the task only executes for the 'db' role, as expected.

But, this is part of a larger set of tasks that I have. And, what I actually 
want to do is say:

"execute this task on db if db is in roles, otherwise, execute on default."

So, to achieve that, I tried:

import utilities

@roles(utilities.get_role('db'))
@task
def drop():
    utilities.alert(u'Dropping the database.')
    run('dropdb {name}'.format(name=env.db_name))


# utilities.py
def get_role(target_role):
    if target_role in env.roles:
        return target_role
    else:
        return 'default'


However, the task still attempts to run on roles['default']. I'm diving in but 
I'm also wondering if there is another way I could, in the fabfile itself, 
dynamically set the role for a task like this.