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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def rotation_filename(self, default_name: str) -> str: """ Modify the filename of a log file when rotating. This is provided so that a custom filename can be provided. :param default_name: The default name for the log file. """

        if self.namer is None:
            return default_name

        return self.namer(default_name)