def _formatSystem(event):
    """
    Format the system specified in the event in the "log_system" key if set,
    otherwise the C{"log_namespace"} and C{"log_level"}, joined by a C{u"#"}.
    Each defaults to C{u"-"} is not set.  If formatting fails completely,
    "UNFORMATTABLE" is returned.

    @param event: The event containing the system specification.
    @type event: L{dict}

    @return: A formatted string representing the "log_system" key.
    @rtype: L{unicode}
    """
    system = event.get("log_system", None)
    if system is None:
        level = event.get("log_level", None)
        if level is None:
            levelName = u"-"
        else:
            levelName = level.name

        system = u"{namespace}#{level}".format(
            namespace=event.get("log_namespace", u"-"),
            level=levelName,
        )
    else:
        try:
            system = unicode(system)
        except Exception:
            system = u"UNFORMATTABLE"
    return system

    Format the system specified in the event in the "log_system" key if set,
    otherwise the C{"log_namespace"} and C{"log_level"}, joined by a C{u"#"}.
    Each defaults to C{u"-"} is not set.  If formatting fails completely,
    "UNFORMATTABLE" is returned.

    @param event: The event containing the system specification.
    @type event: L{dict}

    @return: A formatted string representing the "log_system" key.
    @rtype: L{unicode}
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/_format.py