Question

When I run this fabfile.py…

from fabric.api import env, run, local, cd

def setenv(foo):
  env.hosts = ['myhost']

def mycmd(foo):
  setenv(foo)
  print(env.hosts)
  run('ls')

with this command fab mycmd:bar. I get this output…

['myhost']
No hosts found. Please specify (single) host string for connection:

What, what?! I don’t get it? I’ve set the env.hosts and it seems to be valid “inside” the mycmd function, but for some reason that run command doesn’t know about the hosts I’ve specified.

Color me confused. Any help would be appreciated!

Answer 1

@Chris, the reason you’re seeing this behavior is because the host list is constructed before the task function is called. So, even though you’re changing env.hosts inside the function, it is too late for it to have any effect.

Whereas the command fab setenv:foo mycmd:bar, would have resulted in something you would have expected:

$ fab setenv:foo mycmd:bar
[myhost] Executing task 'mycmd'
['myhost']
[myhost] run: ls

This is the same as the accepted answer, but because of the way setenv is defined, an argument is needed.

Another example:

from fabric.api import env, run, local, cd

env.hosts = ['other_host']

def setenv(foo):
    env.hosts = ['myhost']

def mycmd(foo):
    setenv(foo)
    print('env.hosts inside mycmd: %s' % env.hosts)
    run('ls')

The output of this is:

$ fab mycmd:bar
[other_host] Executing task 'mycmd'
env.hosts inside mycmd: ['myhost']
[other_host] run: ls

Fatal error: Name lookup failed for other_host

Underlying exception:
    (8, 'nodename nor servname provided, or not known')
Aborting.

As you can see, the host-list is already set to ['other_host', ] when fabric starts to execute mycmd.