pymysql 写入null

a='NULL'

def updata():
    conn = pymysql.connect(host=host, port=port, user=user, passwd=password, db=db)
    cursor = conn.cursor()
    sql="insert into T_SENSOR_TRANSDUCER_ABBR(ABBR_CODE,VAR_TYPE_ID,VAR_TYPE_TRANSDUCER_LIST,QUALITYCONTROL_VERSION) VALUES('{}',{},'{}',{})".format("test",1,"test1",a)
    print(sql)
    cursor.execute(sql)
    conn.commit()
    print("haha")
updata()```