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In my project, I use the Python logging package for logging service.

My setup looks like this:

import logging

logger = logging.getLogger('demo project')
formatter = logging.Formatter(
    "[%(asctime)s][%(name)s][%(levelname)s] %(message)s", "%Y-%m-%d %H:%M:%S"
)

stream_handler = logging.StreamHandler()
stream_handler.setFormatter(formatter)
stream_handler.setLevel(logging.INFO)
# stream_handler.setLevel(logging.DEBUG)
logger.addHandler(stream_handler)

logger.setLevel(logging.INFO)

logger.info("info message")

Missing log message
#

This works well when I show info messages. Later, I want to show some debug messages via my logger. So I set the logger’s logging level to DEBUG, and use logger.debug() to print the debug message:

logger.setLevel(logging.DEBUG)

logger.debug("some debug info")

Unfortunately, this does not work as intended. When I run my code, no debugging message is shown. Why?

Separate logging levels: logger vs handler
#

It has something to do how the logging messages are actually logged. Remember that in our first code snippet, we set the logging level both for the logger and the handler (in this case, a stream handler).

The levels we use for logger and the handler together decide which message can be logged and which should be rejected. The logger’s level is like the first gate/filter, and the handler’s level is like the second filter.

When a logging message comes, it first passes the logger, and then the handler. If the logging message’s level is no less than the logger’s level, it then goes to the handler. If its level is also no less than handler’s level, the message gets logged.

Take the snippet in the second code snippet as an example, the logger’s level is DEBUG, and the handler’s level is INFO. When we want to log some debug messages, it has the same level as logger’s level. However, its level is below the handler’s level (DEBUG vs INFO), so it gets rejected by the handler (i.e., the handler will not log it, we see nothing on the terminal output). On the other hand, if we use a high level for logger, for example ERROR, and use a low level for handler, e.g., DEBUG, only error loggings will be logged, since all other logging messages will be rejected by the logger.

The Python doc also has a nice diagram showing the workflow of logging processing, which is much more detailed.

The usefulness of separate logging levels
#

What is the benefit of separate logging level of logger and handler? With this, we can achieve flexible logging plans. For example, we may want to log all messages on the stream handler, but only want to log error messages on the file handler to save storage space.

stream_handler = logging.StreamHandler()
stream_handler.setFormatter(formatter)
stream_handler.setLevel(logging.DEBUG)
logger.addHandler(stream_handler)

file_handler = logging.FileHandler('demo-project.log')
file_handler.setFormatter(formatter)
file_handler.setLevel(logging.ERROR)
logger.addHandler(file_handler)

logger.setLevel(logging.DEBUG)

logger.debug("debug message")
logger.info("info message")
logger.warning("warning message")
logger.error("error message")