Re-reading the patch, I notice it’s in the test module, not on the FTP
class, so I guess it’s perfectly fine. Is the string returned by a real
FTP server “331 username ok”? (My point is that I think it would be
better to have different strings for cmd_noop and cmd_user.)

I meant code outside the standard library. I thought this was in ftplib,
but since it’s in test_ftplib we don’t have to fear breaking other
people’s code.

In summary, the diff for ftplib seems good to me, with a minor remark
about the docstrings, and I don’t feel confident enough to give a review
about the tests.