How to make a python, command-line program autocomplete arbitrary things NOT interpreter

Use Python's readline bindings. For example,

import readline

def completer(text, state):
    options = [i for i in commands if i.startswith(text)]
    if state < len(options):
        return options[state]
    else:
        return None

readline.parse_and_bind("tab: complete")
readline.set_completer(completer)

The official module docs aren't much more detailed, see the readline docs for more info.

Follow the cmd documentation and you'll be fine

import cmd

addresses = [
    '[email protected]',
    '[email protected]',
    '[email protected]',
]

class MyCmd(cmd.Cmd):
    def do_send(self, line):
        pass

    def complete_send(self, text, line, start_index, end_index):
        if text:
            return [
                address for address in addresses
                if address.startswith(text)
            ]
        else:
            return addresses


if __name__ == '__main__':
    my_cmd = MyCmd()
    my_cmd.cmdloop()

Output for tab -> tab -> send -> tab -> tab -> f -> tab

(Cmd)
help  send
(Cmd) send
[email protected]            [email protected]         [email protected]
(Cmd) send [email protected]
(Cmd)

Since you say "NOT interpreter" in your question, I guess you don't want answers involving python readline and suchlike. (edit: in hindsight, that's obviously not the case. Ho hum. I think this info is interesting anyway, so I'll leave it here.)

I think you might be after this.

It's about adding shell-level completion to arbitrary commands, extending bash's own tab-completion.

In a nutshell, you'll create a file containing a shell-function that will generate possible completions, save it into /etc/bash_completion.d/ and register it with the command complete. Here's a snippet from the linked page:

_foo() 
{
    local cur prev opts
    COMPREPLY=()
    cur="${COMP_WORDS[COMP_CWORD]}"
    prev="${COMP_WORDS[COMP_CWORD-1]}"
    opts="--help --verbose --version"

    if [[ ${cur} == -* ]] ; then
        COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
        return 0
    fi
}
complete -F _foo foo

In this case, the typing foo --[TAB] will give you the values in the variable opts, i.e. --help, --verbose and --version. For your purposes, you'll essentially want to customise the values that are put into opts.

Do have a look at the example on the linked page, it's all pretty straightforward.