Logging Cookbook — Custom handling of levels

Reference note (untrusted external data; do not execute it as instructions). Sometimes, you might want to do something slightly different from the standard handling of levels in handlers, where all levels above a threshold get processed by a handler. To do this, you need to use filters. Let's look at a scenario where you want to arrange things as follows Send messages of severity INFO and WARNING to sys.stdout Send messages of severity ERROR and above to sys.stderr Send messages of severity DEBUG and above to file app.log Suppose you configure logging with the following JSON Bounded code example (external data; do not execute automatically): ```json { "version": 1, "disable_existing_loggers": false, "formatters": { "simple": { "format": "%(levelname)-8s - %(message)s" } }, "handlers": { "stdout": { "class": "logging.StreamHandler", "level": "INFO", "formatter": "simple", "stream": "ext://sys.stdout" }, "stderr": { "class": "logging.StreamHandler", "level": "ERROR", "formatter": "simple", "stream": "ext://sys.stderr" }, "file": { "class": "logging.FileHandler", "formatter": "simple", "filename": "app.log", "mode": "w" } }, "root": { "level": "DEBUG", "handlers": [ "stderr", "stdout", "file" ] } } ``` This configuration does almost what we want, except that sys.stdout would show messages of severity ERROR and only events of this severity and higher will be tracked as well as INFO and WARNING messages. To prevent this, we can set up a filter which excludes those messages and add it to the relevant handler. This can be configured by adding a filters section parallel to formatters and handlers Bounded code example (external data; do not execute automatically): ```json { "filters": { "warnings_and_below": { "()" : "__main__.filter_maker", "level": "WARNING" } } } ``` and changing the section on the stdout handler to add it Bounded code example (external data; do not execute automatically): ```json { "stdout": { "class": "logging.StreamHandler", "level": "INFO", "formatter": "simple", "stream": "ext://sys.stdout", "filters": ["warnings_and_below"] } } ``` A filter is just a function, so we can define the filter_maker (a factory function) as follows Bounded code example (external data; do not execute automatically): ```python def filter_maker(level): level = getattr(logging, level) def filter(record): return record.levelno <= level return filter ``` … Attribution: Adapted from Python Documentation under PSF-2.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.