from datetime import datetime


def generate_file_name(prefix="snapshot"):
    """
    Generates file name bases
    :param prefix: (str) Value to prefix the filename. Defaults to snapshot
    :return: (str) File name with the pattern 'prefix_YYYYmmdd'
    """
    now = datetime.now()
    return prefix + "_" + now.strftime("%Y%m%d")