ftp.retrbinary() help python

I have created a python script to connect to a remserver.

datfile = []
for dk in range(len(files)):
  dfnt=files[dk]
  dpst=dfnt.find('.dat')
  if dpst == 15:
    dlist = dfnt[:]
    datfile.append(dlist)

    assert datfile == ['a.dat','b.dat']
    # True

Which as you can see creates a list. Now I am passing this list to

ftp.retrbinary('datfile')

but this lines returns an error:

typeerror: retrbinary() takes at least 3 arguments (2 given)

not sure what is looking for?