Python中将打印输出导向日志文件

a. 利用sys.stdout将print行导向到你定义的日志文件中，例如：

import sys

# make a copy of original stdout route
stdout_backup = sys.stdout
# define the log file that receives your log info
log_file = open("message.log", "w")
# redirect print output to log file
sys.stdout = log_file

print "Now all print info will be written to message.log"
# any command line that you will execute
...

log_file.close()
# restore the output to initial pattern
sys.stdout = stdout_backup

print "Now this will be presented on screen"

b. 利用logging模块（规范化日志输出，推荐！！）
由于logging模块的功能比较多，下面就放一些文档里介绍的简单的例子，更详细具体的用法请戳这里

而以下是根据事件的严重性程度而应采取的logging函数的建议：

默认的等级是WARNING，也就是说logging函数在没有特别配置的前提下只追踪比WARNING程度更严重的异常。

下面就用一些例子来具体说明如何用logging函数记录日志信息：

# this is a simple example
import logging
# define the log file, file mode and logging level
logging.basicConfig(filename=‘example.log‘, filemode="w", level=logging.DEBUG)
logging.debug(‘This message should go to the log file‘)
logging.info(‘So should this‘)
logging.warning(‘And this, too‘)

查看example.log文件可以看到以下信息：

DEBUG:root:This message should go to the log file
INFO:root:So should this
WARNING:root:And this, too

从多个文件记录日志

# myapp.py
import logging
import mylib

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

if __name__ == ‘__main__‘:
    main()

# mylib.py
import logging

def do_something():
    logging.info(‘Doing something‘)

输出的信息为

INFO:root:Started
INFO:root:Doing something
INFO:root:Finished

改变默认输出信息的格式

import logging
# output format: output time - logging level - log messages
logging.basicConfig(format=‘%(asctime)s - %(levelname)s - %(message)s‘)
logging.warning(‘This message will appear in python console.‘)

在python console中直接打印以下输出：

2016-8-2 2:59:11, 510 - WARNING - This message will appear in python console

logging高级用法
可以通过构建logger或者读取logging config文件对logging函数进行任意配置。

• 构建logger法

import logging

# create logger
logger = logging.getLogger(‘simple_example‘)
logger.setLevel(logging.DEBUG)

# create console handler and set level to debug
ch = logging.StreamHandler()
ch.setLevel(logging.DEBUG)

# create formatter
formatter = logging.Formatter(‘%(asctime)s - %(name)s - %(levelname)s - %(message)s‘)

# add formatter to ch
ch.setFormatter(formatter)

# add ch to logger
logger.addHandler(ch)

# ‘application‘ code
logger.debug(‘debug message‘)
logger.info(‘info message‘)
logger.warn(‘warn message‘)
logger.error(‘error message‘)
logger.critical(‘critical message‘)

以上程序结果会输出到python console中：

$ python simple_logging_module.py
2005-03-19 15:10:26,618 - simple_example - DEBUG - debug message
2005-03-19 15:10:26,620 - simple_example - INFO - info message
2005-03-19 15:10:26,695 - simple_example - WARNING - warn message
2005-03-19 15:10:26,697 - simple_example - ERROR - error message
2005-03-19 15:10:26,773 - simple_example - CRITICAL - critical message

• 读取配置文件法

import logging
import logging.config

logging.config.fileConfig(‘logging.conf‘)

# create logger
logger = logging.getLogger(‘simpleExample‘)

# ‘application‘ code
logger.debug(‘debug message‘)
logger.info(‘info message‘)
logger.warn(‘warn message‘)
logger.error(‘error message‘)
logger.critical(‘critical message‘)

其中logging.conf文件格式为：(其实就是将前一种方法的各项配置参数写到logging.conf文件中)

[loggers]
keys=root,simpleExample

[handlers]
keys=consoleHandler

[formatters]
keys=simpleFormatter

[logger_root]
level=DEBUG
handlers=consoleHandler

[logger_simpleExample]
level=DEBUG
handlers=consoleHandler
qualname=simpleExample
propagate=0

[handler_consoleHandler]
class=StreamHandler
level=DEBUG
formatter=simpleFormatter
args=(sys.stdout,)

[formatter_simpleFormatter]
format=%(asctime)s - %(name)s - %(levelname)s - %(message)s
datefmt= ‘%m/%d/%Y %I:%M:%S %p‘

与前面一样，上述文件输出结果为：

$ python simple_logging_config.py
2005-03-19 15:38:55,977 - simpleExample - DEBUG - debug message
2005-03-19 15:38:55,979 - simpleExample - INFO - info message
2005-03-19 15:38:56,054 - simpleExample - WARNING - warn message
2005-03-19 15:38:56,055 - simpleExample - ERROR - error message
2005-03-19 15:38:56,130 - simpleExample - CRITICAL - critical message

参考：stackoverflow: redirect prints to log file