13.12 给内库增加日志功能

问题

You would like to add a logging capability to a library, but don’t want it to interfere withprograms that don’t use logging.

解决方案

For libraries that want to perform logging, you should create a dedicated logger object,and initially configure it as follows:

somelib.py

import logginglog = logging.getLogger(name)log.addHandler(logging.NullHandler())

Example function (for testing)def func():

log.critical(‘A Critical Error!')log.debug(‘A debug message')

With this configuration, no logging will occur by default. For example:

>>> import somelib
>>> somelib.func()
>>>

However, if the logging system gets configured, log messages will start to appear. Forexample:

>>> import logging
>>> logging.basicConfig()
>>> somelib.func()
CRITICAL:somelib:A Critical Error!
>>>

讨论

Libraries present a special problem for logging, since information about the environ‐ment in which they are used isn’t known. As a general rule, you should never writelibrary code that tries to configure the logging system on its own or which makes as‐sumptions about an already existing logging configuration. Thus, you need to take greatcare to provide isolation.The call to getLogger(name) creates a logger module that has the same name asthe calling module. Since all modules are unique, this creates a dedicated logger that islikely to be separate from other loggers.

The log.addHandler(logging.NullHandler()) operation attaches a null handler tothe just created logger object. A null handler ignores all logging messages by default.Thus, if the library is used and logging is never configured, no messages or warningswill appear.One subtle feature of this recipe is that the logging of individual libraries can be inde‐pendently configured, regardless of other logging settings. For example, consider thefollowing code:

>>> import logging
>>> logging.basicConfig(level=logging.ERROR)
>>> import somelib
>>> somelib.func()
CRITICAL:somelib:A Critical Error!

>>> # Change the logging level for 'somelib' only
>>> logging.getLogger('somelib').level=logging.DEBUG
>>> somelib.func()
CRITICAL:somelib:A Critical Error!
DEBUG:somelib:A debug message
>>>

Here, the root logger has been configured to only output messages at the ERROR level orhigher. However, the level of the logger for somelib has been separately configured tooutput debugging messages. That setting takes precedence over the global setting.The ability to change the logging settings for a single module like this can be a usefuldebugging tool, since you don’t have to change any of the global logging settings—simplychange the level for the one module where you want more output.The “Logging HOWTO” has more information about configuring the logging moduleand other useful tips.