Some problem when using Dynamic Inventory Sources and python API

[Young Yang]

Hi,

I'm developing a script using ansible python API (http://docs.ansible.com/ansible/developing_api.html)

Here  is my ansible version

```

ansible 2.2.0 (devel af150ea43a) last updated 2016/06/18 02:00:10 (GMT +800)

  lib/ansible/modules/core: (detached HEAD f0f5272f90) last updated 2016/06/19 09:59:32 (GMT +800)

  lib/ansible/modules/extras: (detached HEAD 6cb6829384) last updated 2016/06/19 09:59:53 (GMT +800)

  config file =

  configured module search path = Default w/o overrides

```

My inventory is dynamic, so I follow the  guide here : http://docs.ansible.com/ansible/developing_inventory.html

I found my script must support the `--list` option and `--host <hostname>` option.  

What stumbles me  is that when I use `--host <hostname>`,  the `<hostname>` must appear in the output of the --list option.

But I can't get all the hosts that I have access in advance.   I just render my options with `<hostname>` when I call the  `--host <hostname>` option.

Actually, all I need is to  render my  connection options based on the `<hostname>` option and I can't know the list of <hostname> in advance. 

How can I achieve this goal? 

Thanks in advance :)