Fix Python – Using python Logging with AWS Lambda

Question 1

As the AWS documentation suggests:

import logging
logger = logging.getLogger()
logger.setLevel(logging.INFO)
def my_logging_handler(event, context):
    logger.info('got event{}'.format(event))
    logger.error('something went wrong')

Now I made:

import logging
logging.basicConfig(level = logging.INFO)
logging.info("Hello World!")

The first snippet of code prints in the Cloud Watch console, but the second one no.

I didn’t see any difference as the two snippets are using the root logger.

Question 2

The reason that logging does not seem to work is because the AWS Lambda Python runtime pre-configures a logging handler that, depending on the version of the runtime selected, might modify the format of the message logged, and might also add some metadata to the record if available. What is not preconfigured though is the log-level. This means that no matter the type of log-message you try to send, it will not actually print.

As AWS documents themselves, to correctly use the logging library in the AWS Lambda context, you only need to set the log-level for the root-logger:

import logging
logging.getLogger().setLevel(logging.INFO)

If you want your Python-script to be both executable on AWS Lambda, but also with your local Python interpreter, you can check whether a handler is configured or not, and fall back to basicConfig (which creates the default stderr-handler) otherwise:

if len(logging.getLogger().handlers) > 0:
    # The Lambda environment pre-configures a handler logging to stderr. If a handler is already configured,
    # `.basicConfig` does not execute. Thus we set the level directly.
    logging.getLogger().setLevel(logging.INFO)
else:
    logging.basicConfig(level=logging.INFO)

Question

Answer