def shellComplete(config, cmdName, words, shellCompFile):
    """
    Perform shell completion.

    A completion function (shell script) is generated for the requested
    shell and written to C{shellCompFile}, typically C{stdout}. The result
    is then eval'd by the shell to produce the desired completions.

    @type config: L{twisted.python.usage.Options}
    @param config: The L{twisted.python.usage.Options} instance to generate
        completions for.

    @type cmdName: C{str}
    @param cmdName: The name of the command we're generating completions for.
        In the case of zsh, this is used to print an appropriate
        "#compdef $CMD" line at the top of the output. This is
        not necessary for the functionality of the system, but it
        helps in debugging, since the output we produce is properly
        formed and may be saved in a file and used as a stand-alone
        completion function.

    @type words: C{list} of C{str}
    @param words: The raw command-line words passed to use by the shell
        stub function. argv[0] has already been stripped off.

    @type shellCompFile: C{file}
    @param shellCompFile: The file to write completion data to.
    """

    # If given a file with unicode semantics, such as sys.stdout on Python 3,
    # we must get at the the underlying buffer which has bytes semantics.
    if shellCompFile and ioType(shellCompFile) == unicode:
        shellCompFile = shellCompFile.buffer

    # shellName is provided for forward-compatibility. It is not used,
    # since we currently only support zsh.
    shellName, position = words[-1].split(":")
    position = int(position)
    # zsh gives the completion position ($CURRENT) as a 1-based index,
    # and argv[0] has already been stripped off, so we subtract 2 to
    # get the real 0-based index.
    position -= 2
    cWord = words[position]

    # since the user may hit TAB at any time, we may have been called with an
    # incomplete command-line that would generate getopt errors if parsed
    # verbatim. However, we must do *some* parsing in order to determine if
    # there is a specific subcommand that we need to provide completion for.
    # So, to make the command-line more sane we work backwards from the
    # current completion position and strip off all words until we find one
    # that "looks" like a subcommand. It may in fact be the argument to a
    # normal command-line option, but that won't matter for our purposes.
    while position >= 1:
        if words[position - 1].startswith("-"):
            position -= 1
        else:
            break
    words = words[:position]

    subCommands = getattr(config, 'subCommands', None)
    if subCommands:
        # OK, this command supports sub-commands, so lets see if we have been
        # given one.

        # If the command-line arguments are not valid then we won't be able to
        # sanely detect the sub-command, so just generate completions as if no
        # sub-command was found.
        args = None
        try:
            opts, args = getopt.getopt(words,
                                       config.shortOpt, config.longOpt)
        except getopt.error:
            pass

        if args:
            # yes, we have a subcommand. Try to find it.
            for (cmd, short, parser, doc) in config.subCommands:
                if args[0] == cmd or args[0] == short:
                    subOptions = parser()
                    subOptions.parent = config

                    gen = ZshSubcommandBuilder(subOptions, config, cmdName,
                                               shellCompFile)
                    gen.write()
                    return

        # sub-command not given, or did not match any knowns sub-command names
        genSubs = True
        if cWord.startswith("-"):
            # optimization: if the current word being completed starts
            # with a hyphen then it can't be a sub-command, so skip
            # the expensive generation of the sub-command list
            genSubs = False
        gen = ZshBuilder(config, cmdName, shellCompFile)
        gen.write(genSubs=genSubs)
    else:
        gen = ZshBuilder(config, cmdName, shellCompFile)
        gen.write()

    Perform shell completion.

    A completion function (shell script) is generated for the requested
    shell and written to C{shellCompFile}, typically C{stdout}. The result
    is then eval'd by the shell to produce the desired completions.

    @type config: L{twisted.python.usage.Options}
    @param config: The L{twisted.python.usage.Options} instance to generate
        completions for.

    @type cmdName: C{str}
    @param cmdName: The name of the command we're generating completions for.
        In the case of zsh, this is used to print an appropriate
        "#compdef $CMD" line at the top of the output. This is
        not necessary for the functionality of the system, but it
        helps in debugging, since the output we produce is properly
        formed and may be saved in a file and used as a stand-alone
        completion function.

    @type words: C{list} of C{str}
    @param words: The raw command-line words passed to use by the shell
        stub function. argv[0] has already been stripped off.

    @type shellCompFile: C{file}
    @param shellCompFile: The file to write completion data to.
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/_shellcomp.py