def sen2chars(sen, is_latin=False):
    """
    Convert sentence to char sequence.
    :param sen: str, like '今天天气很好'
    :param is_latin: whether to transform whitespace to underline.
    :return: char list, like ['今', '天', '天', '气', '很', '好']
    """
    if is_latin:
        sen = sen.replace(' ', '_')
    return [char for char in sen if not char.isspace()]