列被标记为表的主键的成员，但没有 Python 端或服务器端默认生成器

Question

这是我正在制作的表格：

 URL = 'postgresql://{}:{}@{}:{}/{}'
 DB_URL = URL.format(
     DB_USER_NAME,
     DB_PASSWORD,
     DB_HOST,
     str(DB_PORT),
     DB_NAME)

 print("creating db engine...")

 # Connect to database
 CONN = create_engine(DB_URL, client_encoding="UTF-8")
 META_DATA = MetaData(bind=CONN, reflect=True)

 print("creating users table")
 TABLE_EXISTS = CONN.dialect.has_table(CONN, "users")
 USERS_TABLE = None

 # Create tables
 if not TABLE_EXISTS:
     USERS_TABLE = \
         Table("test_user", META_DATA,
               Column("id", Integer, Sequence("user_id_seq"), primary_key=True),
               Column("first_name", String(255)),
               Column("last_name", String(255))
              )
     META_DATA.create_all(CONN)
 else:
     USERS_TABLE = Table("test_user", META_DATA, autoload=True)


 if USERS_TABLE != None:
     print("done")
 else:
     raise Exception("Halp")

 # Insertions
 INS_EXPRESSION = USERS_TABLE.insert(None).values(first_name="Foo", last_name="Bar")

 CONN.execute(INS_EXPRESSION)

 # Selections
 SELECTION = select([USERS_TABLE])
 RESULT = CONN.execute(SELECTION)
 for row in RESULT:
     print(row)

当我运行此代码并进行插入时，它第一次工作。但是如果我第二次运行它，我会收到这个错误：

SAWarning: Column 'test_user.id' is marked as a member of the primary key for table 'test_user', but has no Python-side or server-side default generator indicated, nor does it indicate 'autoincrement=True' or 'nullable=True', and no explicit value is passed. Primary key columns typically may not store NULL.

失败的行记录为：

DETAIL:  Failing row contains (null, Foo, Bar).

我究竟做错了什么？？