mimicking a file in memory

[Peter Otten]

mgierdal at gmail.com wrote:

> On Nov 20, 4:37 pm, Jarek Zgoda <jzg... at o2.usun.pl> wrote:
> 
>> Try with StringIO/cStringIO, these modules are supposed to give you
>> in-memoryobjects compatible with file object interface.
> 
> I found this solution not working.
> I had similar problem: I wanted to write some string into the in-
> memory file, then transfer it via ftp to some file and forget in-
> memory  content.
> 
> from ftplib import FTP
> ftp = FTP('ftp.server.org')
> ftp.login('ID','pswd')
> import StringIO
> filename = 'some_file.txt'
> command = 'STOR ' + filename
> outfile = StringIO.StringIO()
> outfile.write(some_string + '\n')

Try it again with 

outfile.seek(0) 

before the storlines() call.

> ftp.storlines(command, outfile)
> ftp.quit()
> outfile.close()
> 
> The file shows up on the FTP server, but with ZERO length. 

I believe you would see the same problem if outfile were a real file.

> I think the
> problem is that ftp.storelines attempts to use outfile's read()
> function, which is not present in StringIO objects (they use
> getvalue() instead). Quite an annoying inconsistency.

>>> "read" in dir(StringIO.StringIO())
True

A quick look into the source code can put an end to the rest of that
speculation:

    def storlines(self, cmd, fp):
        '''Store a file in line mode.'''
        self.voidcmd('TYPE A')
        conn = self.transfercmd(cmd)
        while 1:
            buf = fp.readline()
            if not buf: break
            if buf[-2:] != CRLF:
                if buf[-1] in CRLF: buf = buf[:-1]
                buf = buf + CRLF
            conn.sendall(buf)
        conn.close()
        return self.voidresp()

storlines() will happily accept every object fp featuring a readline()
method.

> Any thoughts, please?

Keep the library docs under your pillow and the library source on your
screen :)

Peter