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

[Greg Ewing]

Gerhard Häring wrote:

> It's also possible to check if a file exists before trying to open it:
> 
> if os.path.exists("/usr/bin/foo"):
>     # more code here
> 
> But the existence of the file does not guarantee you have the necessary
> permissions to actually open it. It could also be deleted in the millisecond or
> so between your check and trying to open it.


The problem in Christopher's case is actually the other
way around -- a file could be *created* in between probing
and opening a new file.

This presents a slight problem. What we really need to do
is attempt to open the new file with a mode which says
"create the file, or fail if it already exists" -- but
there doesn't seem to be any such mode.

On Unix, it can be done using lower-level calls:

    from os import open, fdopen, O_CREAT, O_EXCL
    try:
      fd = open(new_file_name, O_CREAT | O_EXCL, 0666)
      f = fdopen(fd, "w")
    except IOError:
      ...

-- 
Greg Ewing, Computer Science Dept,
University of Canterbury,	
Christchurch, New Zealand
http://www.cosc.canterbury.ac.nz/~greg