我正在尝试在Apache Airflow中使用FTPHook类。以下是以ftp.gnu.org为例的测试代码：

from airflow.contrib.hooks.ftp_hook import FTPHook
hook = FTPHook('gnu')
print(hook.list_directory(''))

此代码返回错误：

{base_hook.py:84} INFO - Using connection to: id: gnu. Host: ftp.gnu.org,Port: None,Schema: None,Login: None,Password: None,extra: {}
Traceback (most recent call last):
  File "test.py",line 5,in <module>
    print(hook.list_directory(''))
  File "/home/ubuntu/.anaconda3/envs/airflow/lib/python3.7/site-packages/airflow/contrib/hooks/ftp_hook.py",line 127,in list_directory
    conn.cwd(path)
  File "/home/ubuntu/.anaconda3/envs/airflow/lib/python3.7/ftplib.py",line 631,in cwd
    return self.voidcmd(cmd)
  File "/home/ubuntu/.anaconda3/envs/airflow/lib/python3.7/ftplib.py",line 278,in voidcmd
    return self.voidresp()
  File "/home/ubuntu/.anaconda3/envs/airflow/lib/python3.7/ftplib.py",line 251,in voidresp
    resp = self.getresp()
  File "/home/ubuntu/.anaconda3/envs/airflow/lib/python3.7/ftplib.py",line 246,in getresp
    raise error_perm(resp)
ftplib.error_perm: 530 Please login with USER and PASS.

但是，如果我们将这行代码hook.get_conn().login()放在print函数之前，则一切正常。这是错误还是我做错了什么？