Set the log level for python Bokeh code.

def py_log_level(self, default='none'):
        ''' Set the log level for python Bokeh code.

        '''
        level = self._get_str("PY_LOG_LEVEL", default, "debug")
        LEVELS = {'trace': logging.TRACE,
                  'debug': logging.DEBUG,
                  'info' : logging.INFO,
                  'warn' : logging.WARNING,
                  'error': logging.ERROR,
                  'fatal': logging.CRITICAL,
                  'none' : None}
        return LEVELS[level]