How can you ensure that a handler is added only once in Python logging?

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You can ensure that a handler is added only once in Python logging by checking if the handler already exists in the logger and only adding it if it doesn't exist. Here's an example:

import logging

logger = logging.getLogger()
handler = logging.StreamHandler()
formatter = logging.Formatter('%(asctime)s %(levelname)-8s %(message)s')

if not any(isinstance(h, logging.StreamHandler) for h in logger.handlers):
    handler.setFormatter(formatter)
    logger.addHandler(handler)

logger.info('Hello, world!')

In this example, we first create a logger object and a handler object. We then create a formatter object and set it on the handler. We check if there are any existing handlers of the same type (in this case, a StreamHandler) in the logger, and only add the handler if there aren't any. Finally, we log a message to the logger. This ensures that the handler is added only once, even if the script is run multiple times.