学习 python logging（2）: 格式化日志输出

1. 简介

在实际项目中，我们可能需要根据实际需要去记录日志，包括记录的时间、进程号、日志的等级、出现的对应文件及行号等等，以帮助我们定位问题，分析过程。python已经为我们提供了基础的这些，包括:

name	format_str	meaning
asctime	%(asctime)s	Human-readable time when the LogRecord was created. By default this is of the form ‘2003-07-08 16:49:45,896’ (the numbers after the comma are millisecond
created	%(created)f	Time when the LogRecord was created (as returned by time.time()).
filename	%(filename)s	Filename portion of pathname.
funcName	%(funcName)s	Name of function containing the logging call.
levelname	%(levelname)s	Text logging level for the message (‘DEBUG’, ‘INFO’, ‘WARNING’, ‘ERROR’, ‘CRITICAL’).
levelno	%(levelno)s	Numeric logging level for the message (DEBUG, INFO, WARNING, ERROR, CRITICAL).
lineno	%(lineno)d	Source line number where the logging call was issued (if available).
message	%(message)s	The logged message, computed as msg % args. This is set when Formatter.format() is invoked.
module	%(module)s	Module (name portion of filename).
msecs	%(msecs)d	Millisecond portion of the time when the LogRecord was created.
name	%(name)s	Name of the logger used to log the call.
pathname	%(pathname)s	Full pathname of the source file where the logging call was issued (if available).
process	%(process)d	Process ID (if available).
processName	%(processName)s	Process name (if available).
relativeCreated	%(relativeCreated)d	Time in milliseconds when the LogRecord was created, relative to the time the logging module was loaded.
thread	%(thread)d	Thread ID (if available).
threadName	%(threadName)s	Thread name (if available).

在基本使用中，已经足够了, 简单的例子:

logging.basicConfig(
    format="%(asctime)s-%(filename)s-%(levelno)s-%(lineno)s, %(message)s"
)
logging.warning("test message")

日志输出为：
2019-01-03 21:36:53,772-logging_format_sample.py-30-11, test message

2. 三种格式化方式

在python中日志模块中，有三种格式化日志信息的方式: %-formatting, str.format(), Template, %s是默认的。

2.1 默认 `%-formatting`

以上例子已给出

2.2 使用 `str.format()`

需要将格式化的 style 参数设置成 {。

logging.basicConfig(
    style='{',
    format="{asctime}-{filename}-{levelno}-{lineno}, {message}"
)
logging.warning("test format message")

日志输出为
2019-01-03 21:49:59,689-logging_format_sample.py-30-11, test format message

2.3 使用`Template`

需要将格式化的 style 参数设置成 $。

logging.basicConfig(
    style='$',
    format="$asctime-$filename-$levelno-$lineno, $message"
)
logging.warning("test template message")

日志输出为
2019-01-03 21:53:56,873-logging_format_sample.py-30-11, test template message

以上，就是记录日志格式化消息的基本操作。

参考:

https://docs.python.org/3/howto/logging.html#logging-basic-tutorial

https://docs.python.org/3/library/logging.html

https://docs.python.org/3/howto/logging-cookbook.html#logging-cookbook