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| -rw-r--r-- | Doc/howto/logging-cookbook.rst | 135 |
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diff --git a/Doc/howto/logging-cookbook.rst b/Doc/howto/logging-cookbook.rst index 83b479c..cc3c26a 100644 --- a/Doc/howto/logging-cookbook.rst +++ b/Doc/howto/logging-cookbook.rst @@ -1694,3 +1694,138 @@ When the above script is run, it prints:: Note that the order of items might be different according to the version of Python used. +.. currentmodule:: logging.config + +Customising handlers with :func:`dictConfig` +-------------------------------------------- + +There are times when you want to customise logging handlers in particular ways, +and if you use :func:`dictConfig` you may be able to do this without +subclassing. As an example, consider that you may want to set the ownership of a +log file. On POSIX, this is easily done using :func:`shutil.chown`, but the file +handlers in the stdlib don't offer built-in support. You can customise handler +creation using a plain function such as:: + + def owned_file_handler(filename, mode='a', encoding=None, owner=None): + if owner: + if not os.path.exists(filename): + open(filename, 'a').close() + shutil.chown(filename, *owner) + return logging.FileHandler(filename, mode, encoding) + +You can then specify, in a logging configuration passed to :func:`dictConfig`, +that a logging handler be created by calling this function:: + + LOGGING = { + 'version': 1, + 'disable_existing_loggers': False, + 'formatters': { + 'default': { + 'format': '%(asctime)s %(levelname)s %(name)s %(message)s' + }, + }, + 'handlers': { + 'file':{ + # The values below are popped from this dictionary and + # used to create the handler, set the handler's level and + # its formatter. + '()': owned_file_handler, + 'level':'DEBUG', + 'formatter': 'default', + # The values below are passed to the handler creator callable + # as keyword arguments. + 'owner': ['pulse', 'pulse'], + 'filename': 'chowntest.log', + 'mode': 'w', + 'encoding': 'utf-8', + }, + }, + 'root': { + 'handlers': ['file'], + 'level': 'DEBUG', + }, + } + +In this example I am setting the ownership using the ``pulse`` user and group, +just for the purposes of illustration. Putting it together into a working +script, ``chowntest.py``:: + + import logging, logging.config, os, shutil + + def owned_file_handler(filename, mode='a', encoding=None, owner=None): + if owner: + if not os.path.exists(filename): + open(filename, 'a').close() + shutil.chown(filename, *owner) + return logging.FileHandler(filename, mode, encoding) + + LOGGING = { + 'version': 1, + 'disable_existing_loggers': False, + 'formatters': { + 'default': { + 'format': '%(asctime)s %(levelname)s %(name)s %(message)s' + }, + }, + 'handlers': { + 'file':{ + # The values below are popped from this dictionary and + # used to create the handler, set the handler's level and + # its formatter. + '()': owned_file_handler, + 'level':'DEBUG', + 'formatter': 'default', + # The values below are passed to the handler creator callable + # as keyword arguments. + 'owner': ['pulse', 'pulse'], + 'filename': 'chowntest.log', + 'mode': 'w', + 'encoding': 'utf-8', + }, + }, + 'root': { + 'handlers': ['file'], + 'level': 'DEBUG', + }, + } + + logging.config.dictConfig(LOGGING) + logger = logging.getLogger('mylogger') + logger.debug('A debug message') + +To run this, you will probably need to run as ``root``:: + + $ sudo python3.3 chowntest.py + $ cat chowntest.log + 2013-11-05 09:34:51,128 DEBUG mylogger A debug message + $ ls -l chowntest.log + -rw-r--r-- 1 pulse pulse 55 2013-11-05 09:34 chowntest.log + +Note that this example uses Python 3.3 because that's where :func:`shutil.chown` +makes an appearance. This approach should work with any Python version that +supports :func:`dictConfig` - namely, Python 2.7, 3.2 or later. With pre-3.3 +versions, you would need to implement the actual ownership change using e.g. +:func:`os.chown`. + +In practice, the handler-creating function may be in a utility module somewhere +in your project. Instead of the line in the configuration:: + + '()': owned_file_handler, + +you could use e.g.:: + + '()': 'ext://project.util.owned_file_handler', + +where ``project.util`` can be replaced with the actual name of the package +where the function resides. In the above working script, using +``'ext://__main__.owned_file_handler'`` should work. Here, the actual callable +is resolved by :func:`dictConfig` from the ``ext://`` specification. + +This example hopefully also points the way to how you could implement other +types of file change - e.g. setting specific POSIX permission bits - in the +same way, using :func:`os.chmod`. + +Of course, the approach could also be extended to types of handler other than a +:class:`~logging.FileHandler` - for example, one of the rotating file handlers, +or a different type of handler altogether. + |