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When we are working on a project, we often need to log some message for easier debugging. How do we configure logging once and use it across the project?

Set up logging for the project
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Suppose we have the following file in our project:

main.py
module1.py
module2.py

main.py is the entry point of our project and use functions or classes from module1 and module2.

The best practice is to set up logging in main.py like this:

import module1
import module2
import logging
# other imports

logging_level = logging.DEBUG
main_logger = logging.getLogger()
main_logger.setLevel(logging_level)

# Set up a stream handler to log to the console
stream_handler = logging.StreamHandler()
stream_handler.setLevel(logging_level)
formatter = logging.Formatter("%(name)s - %(levelname)s - %(message)s")
stream_handler.setFormatter(formatter)

# Add handler to logger
main_logger.addHandler(stream_handler)

# other codes

In the above code, we define a root logger main_logger via logging.getLogger() (notice that no name is provided to this method), and set up the handlers and logging level properly.

In module1 and module2, we only need to define a logger like this:

import logging

logger = logging.getLogger(__name__)

# then use logger.info() or logger.debug() in your code.

In other modules, if we define the logger using logging.getLogger(__name__), it will automatically use the settings from the root logger. This ensures that we only need to configure logging once in our project.

Changing logging level to disable some logging message
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There is also another benefit in using a unified config: we can easily control the logging message verbosity by changing the logging level. For example, in the above example, we set logging level in main.py to logging.DEBUG, so all logger.debug() and logger.info() message will be printed to the console. If we do not want to see debug messages anymore, we can set logging level to logging.INFO, which will disable any debug message across the whole project.

Disable logging message from other packages
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When we import 3rd party packages in our own project and set proper logging level for the project, if a logging message in 3rd party packages is above that level, the message will also be printed. Since we usually only care about logging messages in our project, we can disable logging message in 3rd party packages by setting their logger level higher, or by disabling logging entirely for 3rd party packages. Here is the code:

import logging

# suppose that the logger level of our module is logging.DEBUG, we set
# logger level of other module higher than logging.DEBUG
for m in ("urllib3", "foo", "bar"):
    logging.getLogger(m).setLevel(logging.CRITICAL)
    # To disable it entirely, use the following.
    # logging.getLogger(m).disabled = True