'Set C{fileLogObserverFactory} to the default appropriate for the
chosen C{logFile}.'

def selectDefaultLogObserver(self):

    if ('fileLogObserverFactory' not in self):
        logFile = self['logFile']
        if (hasattr(logFile, 'isatty') and logFile.isatty()):
            self['fileLogObserverFactory'] = textFileLogObserver
            self['logFormat'] = 'text'
        else:
            self['fileLogObserverFactory'] = jsonFileLogObserver
            self['logFormat'] = 'json'
