def METHOD_NAME(usernames_cli_args, pacu_main):
    session = pacu_main.get_active_session()
    print = pacu_main.print
    fetch_data = pacu_main.fetch_data
    usernames = []
    if usernames_cli_args is not None:
        if "," in usernames_cli_args:
            usernames = usernames_cli_args.split(",")
        else:
            usernames = [usernames_cli_args]
    else:
        if (
            fetch_data(
                ["IAM", "Users"], module_info["prerequisite_modules"][0], "--users"
            )
            is False
        ):
            print("FAILURE")
            print("  SUB-MODULE EXECUTION FAILED")
        for user in session.IAM["Users"]:
            usernames.append(user["UserName"])
    return usernames