code: def parse(directive):
      
      if not directive:
          # Handle null case
          return None
  
      if isinstance(directive, (list, tuple)):
          results = []
          for d in directive:
              results.extend(parse(d))
          return results
  
      if isinstance(directive, (dict, client.Placement)):
          # We've been handed something that we can simply hand back to
          # the api. (Forwards compatibility)
          return [directive]
  
      # Juju 2.0 can't handle lxc containers.
      directive = directive.replace('lxc', 'lxd')
  
      if ":" in directive:
          # Planner has given us a scope and directive in string form
          scope, directive = directive.split(":")
          return [client.Placement(scope=scope, directive=directive)]
  
      if directive.isdigit():
          # Planner has given us a machine id (we rely on juju core to
          # verify its validity.)
          return [client.Placement(scope=MACHINE_SCOPE, directive=directive)]
  
      if "/" in directive:
          # e.g. "0/lxd/0"
          # https://github.com/juju/juju/blob/master/instance/placement_test.go#L29
          return [
              client.Placement(scope=MACHINE_SCOPE, directive=directive),
          ]
  
      # Planner has probably given us a container type. Leave it up to
      # juju core to verify that it is valid.
      return [client.Placement(scope=directive)]
docs: Given a string in the format `scope:directive`, or simply `scope`
      or `directive`, return a Placement object suitable for passing
      back over the websocket API.