Python的日志是通过logging模块实现的

一、日志的基本使用

1. 日志输出到控制台

import logging

logging.warning('This is a warning message')  # will print a message to the console
logging.info('This is a nomal message')  # will not print anything

运行结果：

WARNING:root:This is a warning message

日志的默认级别是warning，所以上例中，只有warning的信息打印出来。

1. 日志输出到文件

import logging

logging.basicConfig(filename='example.log',level=logging.DEBUG)
logging.debug('This is a debug message')
logging.info('This is a nomal message')
logging.warning('This is a warning message')

运行代码后，会在根目录生成一个example.log，打开该日志文件，可以看到如下结果：

DEBUG:root:This is a debug message
INFO:root:This is a nomal message
WARNING:root:This is a warning message

注意：basicConfig()调用前不能进行任何日志的输出，否则basicConfig()的配置信息全部失效。

二、多模块使用日志

1. 日志输出到一个终端

# myapp.py
import logging
import mylib

def main():
    logging.basicConfig(filename='myapp.log', level=logging.INFO)
    logging.info('In myapp.py, Started')
    mylib.do_something()
    logging.info('In myapp.py, Finished')

if __name__ == '__main__':
    main()

# mylib.py
import logging

def do_something():
    logging.info('In mylib.py, Doing something')

运行myapp.py后，打开生成的日志文件myapp.log，可以看到如下结果：

INFO:root:In myapp.py, Started
INFO:root:In mylib.py, Doing something
INFO:root:In myapp.py, Finished

从运行结果可以看出，如上例实现的多模块日志，会根据方法调用的先后顺序打印日志信息。

2. 日志输出到一个终端

上面的例子都只能输出到一个终端，要么是控制台，要么是日志文件。
下面的例子将会展示如何将日志信既输出到控制台，又输出到日志文件中：

# myapp.py
import logging
import mylib

# create logger with 'myapp'
logger = logging.getLogger('myapp')
logger.setLevel(logging.DEBUG)
# create file handler which logs even debug messages
# file mode set write. myapp.log will be rewrite when every time myapp.py is runned.
fh = logging.FileHandler('myapp.log', mode='w')
fh.setLevel(logging.DEBUG)
# create console handler with a higher log level
ch = logging.StreamHandler()
ch.setLevel(logging.ERROR)
# create formatter and add it to the handlers
formatter = logging.Formatter('%(asctime)s - %(name)s - %(levelname)s - %(message)s')
fh.setFormatter(formatter)
ch.setFormatter(formatter)
# add the handlers to the logger
logger.addHandler(fh)
logger.addHandler(ch)

logger.info('In myapp.py, Started')
logger.error('This is a error message')
mylib.do_something()
logger.info('In myapp.py, Finished')

# mylib.py
import logging

# getLogger named myapp
mylib_logger = logging.getLogger('myapp')

def do_something():
    mylib_logger.info('In mylib.py, Doing something')

运行myapp.py，因为控制台的日志级别为ERROR，所以控制台只输出ERROR级别的信息：

2017-07-05 11:07:36,721 - myapp - ERROR - This is a error message

因为日志文件的日志级别为DEBUG，所以文件中包含DEBUG及DEBUG以上级别的信息：

2017-07-05 11:07:36,721 - myapp - INFO - In myapp.py, Started
2017-07-05 11:07:36,721 - myapp - ERROR - This is a error message
2017-07-05 11:07:36,721 - myapp - INFO - In mylib.py, Doing something
2017-07-05 11:07:36,721 - myapp - INFO - In myapp.py, Finished

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了解以上内容就可以实现基本的日志功能了，但是如果想要有效的使用Python的日志功能，还有很多东西需要学习。