ftplib - 226 message not received

[Brendan]

I am trying to download a file within a very large zipfile. I need two
partial downloads of the zipfile. The first to get the file byte
offset, the second to get the file itself which I then inflate.

I am implementing the partial downloads as follows:

con = ftp.transfercmd('RETR ' + filename, rest_offset)  # the data
socket
while True:
    block = con.recv(blocksize)
# stop transfer while it isn't finished yet
    if bytes_recv >= buf_length:
        break
    elif not block:
        break
    buf = ''.join([buf, block])
    bytes_recv += len(block)
con.close()

My problem is that even though the socket is closed, I have no way to
receive the 226 response from server so I can proceed with the next
download.  Of course I could close the ftp connection entirely and
open a new one, but I am hoping to avoid doing so.