VirtualEnvWrappers with Fabric

fabfile.py

def once(s):
    "command_prefixes is a list of prefixes"
    if s not in env.command_prefixes:
        return s
    return 'true'

@contextlib.contextmanager
def mycd(dir):
    with prefix(once('cd %s' % dir)):
        yield

@contextlib.contextmanager
def vwrap():
    with prefix(once('source /usr/local/bin/virtualenvwrapper.sh')):
        yield

@contextlib.contextmanager
def workon(ve):
    with vwrap():
        with prefix(once('workon %s' %ve)):
            yield