keras/src/callbacks/csv_logger.py/CSVLogger/__init__
class CSVLogger:    def __init__(self, filename, separator=",", append=False):
        super().__init__()
        self.sep = separator
        self.filename = file_utils.path_to_string(filename)
        self.append = append
        self.writer = None
        self.keys = None
        self.append_header = True