前言
日志是非常重要的，最近有接觸到這個，所以系統的看一下Python這個模塊的用法。本文即為Logging模塊的用法簡介，主要參考文章為Python官方文檔，鏈接見參考列表。
另外，Python的HOWTOs文檔很詳細，連日志該怎么用都寫了，所以有英文閱讀能力的同學建議去閱讀一下。
Logging模塊構成
組成
主要分為四個部分：
- Loggers：提供應用程序直接使用的接口
- Handlers：將Loggers產生的日志傳到指定位置
- Filters：對輸出日志進行過濾
- Formatters：控制輸出格式
日志級別
| Level | Numeric value | When it’s used |
|---|---|---|
| DEBUG | 10 | Detailed information, typically of interest only when diagnosing problems. |
| INFO | 20 | Confirmation that things are working as expected. |
| WARNING | 30 | An indication that something unexpected happened, or indicative of some problem in the near future (e.g. ‘disk space low’). The software is still working as expected. |
| ERROR | 40 | Due to a more serious problem, the software has not been able to perform some function. |
| CRITICAL | 50 | A serious error, indicating that the program itself may be unable to continue running. |
| NOSET | 0 | getattr(logging, loglevel.upper()) |
默認級別是WARNING，可以使用打印到屏幕上的方式記錄，也可以記錄到文件中。
模塊使用示例
簡單例子
打印輸出
In [5]: import logging
In [6]: logging.warning("FBI warning")
WARNING:root:FBI warning
In [7]: logging.info("information")
# 沒有打印是因為默認級別是warning
輸出到文件中
In [4]: import logging
In [5]: logging.basicConfig(filename='example.log', level=logging.DEBUG)
In [6]: logging.debug("debug")
In [7]: logging.warning('warning')
In [8]: logging.info('info')
In [9]: ls
C++ STL/ a.py example.log parser/ test.sh*
In [10]: cat example.log
DEBUG:root:debug
WARNING:root:warning
INFO:root:info
In [14]: logging.warning('new warning')
# 注意這種是直接往后面添加，也就是add的，若是想覆蓋內容可以更改文件寫入方式
In [15]: cat example.log
DEBUG:root:debug
WARNING:root:warning
INFO:root:info
WARNING:root:new warning
# 覆蓋的方式寫入例子
(test) ➜ test ipython
WARNING: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
Python 2.7.11 (default, Jun 17 2016, 20:01:51)
Type "copyright", "credits" or "license" for more information.
IPython 4.2.0 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
In [1]: import logging
In [2]: logging.basicConfig(filename='example.log', filemode='w', level=logging.DEBUG)
In [3]: logging.warning('FBI warning')
In [4]: ls
C++ STL/ a.py example.log parser/ test.sh*
In [5]: cat example.log
WARNING:root:FBI warning
In [6]: quit
(test) ➜ test ipython
WARNING: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
Python 2.7.11 (default, Jun 17 2016, 20:01:51)
Type "copyright", "credits" or "license" for more information.
IPython 4.2.0 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
In [1]: import logging
In [2]: logging.basicConfig(filename='example.log', filemode='w', level=logging.DEBUG)
In [3]: logging.warning('warning')
In [4]: cat example.log
WARNING:root:warning
變量的使用
和print語句中使用變量一樣，如:
In [5]: logging.warning('%s before you %s', 'Look', 'leap!')
In [6]: cat example.log
WARNING:root:warning
WARNING:root:Look before you leap!
輸出格式
可以在basicConfig中設置，參數名稱可以參見鏈接，可以設置時間什么的，如：
In [2]: import logging
In [3]: logging.basicConfig(format='%(asctime)s:%(levelname)s:%(message)s', level=logging.DEBUG)
In [4]: logging.warning('And this, too')
2016-12-06 15:40:43,577:WARNING:And this, too
進階使用
當想項目中使用logging模塊的時候肯定不能在這樣一句句的寫了，一般可能會抽象出一個模塊來，這樣比較有效一些。logging模塊提供了四個類（Loggers，Formatters，Filtters,Handlers）來實現不同的功能，與此對應的我們如果想封裝一個函數，也就要針對這幾個功能來自定義一下。
想自定義的話思路也很簡單，首先實例化一個相應的對象，然后進行一些設置，可以簡單看一下下面的例子：
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')
# 輸出是：
$ 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
從配置文件導入配置
模塊內容：
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')
# 輸出
$ 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
配置文件內容：
[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
同時也可以使用YAML格式的配置文件，如：
version: 1
formatters:
simple:
format: '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
handlers:
console:
class: logging.StreamHandler
level: DEBUG
formatter: simple
stream: ext://sys.stdout
loggers:
simpleExample:
level: DEBUG
handlers: [console]
propagate: no
root:
level: DEBUG
handlers: [console]
PS
Logging模塊使用也會有很多坑，常見的是日志重復打印的問題，大家可以搜一下，這里提供一些鏈接供參考：
- http://www.jianshu.com/p/25f70905ae9d
- https://yinzo.github.io/14610807170718.html
- http://python.jobbole.com/86887/