ftplib with unknown file names

[rbt]

How can I use ftplib to retrieve files when I do not know their names? I 
can do this to get a listing of the directory's contents:

ftp_server.retrlines('LIST')


The output from this goes to the console and I can't figure out how to 
turn that into something I can use to actually get the files (like a 
list of file names). I read a bit about the callback function that can 
be passed to retrlines but I couldn't figure out how to use it.

Any help is appreciated.

Thanks!