convert ftp.retrbinary to file object? - Python language lacks expression?

[Martin Franklin]

Robert wrote:
> I just tried to convert a (hugh size) ftp.retrbinary run into a
> pseudo-file object with .read(bytes) method in order to not consume
> 500MB on a copy operation.
> 
> First I thought, its easy as usual with python using something like
> 'yield' or so.
> 
> Yet I didn't manage to do (without using threads or rewriting
> 'retrbinary')? Any ideas?
> 
> #### I tried a pattern like:
>     .... 
>     def open(self,ftppath,mode='rb'):
>         class FTPFile: #TODO
>             ...
>             def iter_retr()
>                 ...
>                 def callback(blk):
>                     how-to-yield-from-here-to-iter_retr blk???
>                 ftp.retrbinary("RETR %s" % relpath,callback)
>             def read(self, bytes=-1):
>                 ... 
>                 self.buf+=self.iter.next()
>             ...
>         ....


Hmmmm this is nearly there I think...:

import ftplib

class TransferAbort(Exception): pass

class FTPFile:
     def __init__(self, server, filename):
         self.server = server
         self.filename = filename
         self.offset = 0

     def callback(self, data):
         self.offset = self.offset + len(data)
         self.data = data
         ## now quit the RETR command?
         raise TransferAbort("stop right now")

     def read(self, amount):
         self.ftp = ftplib.FTP(self.server)
         self.ftp.login()
         try:
             self.ftp.retrbinary("RETR %s" %self.filename, self.callback,
                 blocksize=amount,
                 rest=self.offset)
         except TransferAbort:
             return self.data


f = FTPFile("HOSTNAME", "FILENAME")

print f.read(24)
print f.read(24)


I open the ftp connection inside the read method as it caused an error 
(on the second call to read) when I opened it in  __init__ ???

HTH
Martin