Python logging filter being applied to all handlers when only added to one

I am adding logging to a Python project, and I wish to log to both terminal and file. To do this I am using a streamHandler and a fileHandler.

To make it easier to quickly differentiate between log levels, I wish to add colour to the levelname for the logs that are output to the terminal, and to achieve this I have added a filter.

However, despite the filter only being applied to the streamHandler, messages in the log file are seeing the colour code applied. How can this be, given the filter is not added to the fileHandler?

Example Code

Note that the filter in this example only includes a format for logging.ERROR, as this will log to both terminal and file. But in reality I have a format for all log levels.

import loggingclass ColourLevelNameFilter(logging.Filter):    FORMATS = {        logging.ERROR: '\x1b[031m'+'{0}'+'\x1b[0m',    }    def filter(self, record):        levelname_format = self.FORMATS.get(record.levelno)        record.levelname = levelname_format.format(record.levelname)        return Truedef main():    time_format = '%Y-%m-%d %H:%M:%S'    text_format = '%(asctime)s %(levelname)s => %(message)s'    formatter = logging.Formatter(text_format, time_format)    stream_handler = logging.StreamHandler()    stream_handler.setFormatter(formatter)    stream_handler.setLevel(logging.INFO)    stream_handler.addFilter(ColourLevelNameFilter())    log_file = 'test-error.log'    file_handler = logging.FileHandler(log_file, 'w')    file_handler.setFormatter(formatter)    file_handler.setLevel(logging.WARNING)    logger = logging.getLogger('my-app')    logger.setLevel(logging.INFO)    logger.addHandler(stream_handler)    logger.addHandler(file_handler)    logger.error('Testing logging of error.')if __name__ == '__main__':    main()