Home Original page

How does Python handle probing to see if a file already exists?

Terry Reedy tjreedy at udel.edu
Tue Nov 5 12:33:37 EST 2002 
<sismex01 at hebmex.com> wrote in message
news:mailman.1036508171.27010.python-list at python.org...
> > From: Christopher R. Culver
[mailto:Kricxjo.neniuspamajxo at yahoo.com]
> > However, translating that directly into Python doesn't work
because
> > if Python can't find a file to open, it doesn't just return 0, it
> > gives an error and exits. How can I write this functionality in
Python
> > so that it returns 0 and continues, or does Python have a totally
> > different way of handling this problem than C?

> Others have talked about the goodness of using try/except, I'll
> say to use os.access(), which is precisely for what you're asking,
> if you can read or write a file.
>
> -- check if writeable --
>
> import os
> if os.access("some-filename", os.W_OK):
>    # neat, I can write to the file.

But only IF nothing happens to the file between the before-leap look
and the leap itself.  If another process deletes the file, changes
permissions, or opens it for writing in between, then the OP is back
where he started  -- getting an uncaught, program-crashing exception.
Or opening could fail for lack of OS open-file slots, buffers,
whatever or writing could fail for lack of disk space.  Whether or not
this is a serious concern depends on the system and enironment the
program runs in.

TJR
More information about the Python-list mailing list