python logging 模块无法打印日志道文件

$ python ~/logfile.py
WARNING:root:This is warning message
只在屏幕输出上面这句，指定的日志文件是空的
将 logger=logging.getLogger('hello') 改为 logger=logging.getLogger() 才能输出到日志文件，不太明白为什么会这样子？


#!/usr/bin/python2
# coding=utf8

import os
import sys
import logging
import logging.handlers

## 日志级别大小关系为：NOTSET < DEBUG < INFO < WARNING < ERROR < CRITICAL
log_file='myapp.log'

logger=logging.getLogger('hello') ##########
logger.setLevel(logging.NOTSET)

formatter=logging.Formatter('%(asctime)s %(levelname)s %(message)s')

file_handler=logging.handlers.RotatingFileHandler(log_file,maxBytes=1000,backupCount=5)
file_handler.setFormatter(formatter)

logger.addHandler(file_handler)

logging.debug('This is debug message')
logging.info('This is info message')
logging.warning('This is warning message')