On Wed, 12 Jan 2005, Marilyn Davis wrote:
> I was looking at my use of file objects and file descriptors and I wrote
> this sample program and was very surprised by the result -- which makes
> me think there's something here that I don't understand. Where did my
> 'ooo' go?
>
> #! /usr/bin/env python
> import os
>
> fobj = open('/tmp/xxx','w')
> fobj.write('ooo\n')
> fp = fobj.fileno()
> os.write(fp,'x\n')
> os.close(fp)
Hi Marilyn,
Oh! Can you explain why you're mixing the low-level 'os.write()' and
'os.close()' stuff with the high-level file methods?
The 'os' functions work at a different level of abstraction than the file
object methods, so there's no guarantee that:
os.close(fp)
will do the proper flushing of the file object's internal character
buffers.
Try this instead:
###
fobj = open('/tmp/xxx','w')
fobj.write('ooo\n')
fobj.write('x\n')
fobj.close()
###
The documentation on os.write() says:
"""Note: This function is intended for low-level I/O and must be applied
to a file descriptor as returned by open() or pipe(). To write a ``file
object'' returned by the built-in function open() or by popen() or
fdopen(), or sys.stdout or sys.stderr, use its write() method."""
(http://www.python.org/doc/lib/os-fd-ops.html#l2h-1555)
I think the documentation is trying to say: "don't mix high-level and
low-level IO".
For most purposes, we can usually avoid using the low-level IO functions
os.open() and os.write(). If we're using the low-level file functions
because of pipes, then we can actually turn pipes into file-like objects
by using os.fdopen(). os.fdopen() is a bridge that transforms file
descriptors into file-like objects. See:
http://www.python.org/doc/lib/os-newstreams.html
for more information on os.fdopen().
I hope this helps!
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