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
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。
