Python按照重要程度把日志分为5个级别，如下：
可以通过level参数，设置不同的日志级别。当设置为高的日志级别时，低于此级别的日志不再打印。
五种日志级别按从低到高排序：
DEBUG < INFO < WARNING < ERROR < CRITICAL
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level设置为DEBUG级别，所有的日志都会打印
import logging
logging.basicConfig(level=logging.DEBUG, format=' %(asctime)s - %(levelname)s -%(message)s')
logging.debug('Some debugging details.')
logging.info('The logging module is working')
logging.warning('An error message is about to be logged.')
logging.error('An error has occurred.')
logging.critical('The program is unable to recover!')
2019-11-17 15:24:30,065 - DEBUG -Some debugging details.
2019-11-17 15:24:30,074 - INFO -The logging module is working
2019-11-17 15:24:30,086 - WARNING -An error message is about to be logged.
2019-11-17 15:24:30,105 - ERROR -An error has occurred.
2019-11-17 15:24:30,107 - CRITICAL -The program is unable to recover!
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level设置为ERROR级别时，只显示ERROR和CRITICAL日志
import logging
logging.basicConfig(level=logging.ERROR, format=' %(asctime)s - %(levelname)s -%(message)s')
logging.debug('Some debugging details.')
logging.info('The logging module is working')
logging.warning('An error message is about to be logged.')
logging.error('An error has occurred.')
logging.critical('The program is unable to recover!')
2019-11-17 15:30:46,767 - ERROR -An error has occurred.
2019-11-17 15:30:46,768 - CRITICAL -The program is unable to recover!