How to change the format of logged messages temporarily, in Python?

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Here is a simple solution, that can be deduced from Vinay Sajip’s own HOWTO; it basically updates the logging formatter with setFormatter():

import logging

logger = logging.getLogger()  # Logger
logger_handler = logging.StreamHandler()  # Handler for the logger
logger.addHandler(logger_handler)

# First, generic formatter:
logger_handler.setFormatter(logging.Formatter('%(message)s'))
logger.error('error message')  # Test

# New formatter for the handler:
logger_handler.setFormatter(logging.Formatter('PROCESSING FILE xxx - %(message)s'))
logger.error('error message')  # Test

This correctly produces:

error message
PROCESSING FILE xxx - error message

(where xxx can be set dynamically to the file being processed, as asked for in the question).

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