completer.py

class Completer:
    def __init__(self, commands):
        self.commands = commands
        self.completed = None

        for name, c in self.commands.items():
            c._comp_positionals = len([a for a in c.parser._actions
                                       if not a.option_strings])

    def complete_cmd(self, command_name, argv):
        argv_wos = [a for a in argv if a]

        command = self.commands[command_name]
        if len(argv_wos) < command._comp_positionals:
            # Cannot complete positionals
            return []

        arg = argv[-1] if argv else ''  # the one we want to complete
        prev = argv[-2] if len(argv) > 1 else ''  # the one before

        actions = command.parser._option_string_actions
        if prev in actions and isinstance(actions[prev], argparse._StoreAction):
            # Expecting a value
            return []

        options = actions.keys()
        base = ' '.join(argv[:-1]) + ' '
        return [base + k for k in options if k.startswith(arg)]

    def complete(self, text, state=0):
        if self.completed != text:
            sl = text.split(' ')
            cmd, argv = sl[0], sl[1:]

            if cmd in self.commands:
                self.matches = [cmd + ' ' + c for c in self.complete_cmd(cmd, argv)]
            else:
                self.matches = [c for c in self.commands.keys()
                                if c.startswith(cmd)]
            self.completed = text

        if not self.matches:
            return None
        return self.matches[state]