Save to a file, but avoid overwriting an existing file

Cameron Simpson cs at zip.com.au
Wed Mar 12 23:19:25 CET 2014


On 12Mar2014 13:29, zoom <zoom at yahoo.com> wrote:
> I would like to assure that when writing to a file I do not
> overwrite an existing file, but I'm unsure which is the best way to
> approach to this problem. As I can see, there are at least two
> possibilities:
> 
> 1. I could use fd = os.open("x", os.O_WRONLY | os.O_CREAT | os.O_EXCL)
> which will fail - if the file exists. However, I would prefer if the
> program would try to save under different name in this case, instead
> of discarding all the calculation done until now - but I' not too
> well with catching exceptions.

Others have menthions tempfile, though of course you have the same collision
issue when you come to rename the temp file if you are keeping it.

I would run with option 1 for your task.

Just iterate until os.open succeeds.

However, you need to distinuish _why_ an open fails. For example,
if you were trying to make files in a directory to which you do not
have write permission, or just a directory that did not exist,
os.open would fail not matter what name you used, so your loop would
run forever.

Therefore you need to continue _only_ if you get EEXIST. Otherwise abort.

So you'd have some code like this (totally untested):

  # at top of script
  import errno

  # where you make the file
  def open_new(primary_name):
    try:
      fd = os.open(primary_name, os.O_WRONLY | os.O_CREAT | os.O_EXCL)
    except OSError as e:
      if e.errno != errno.EEXIST:
        raise
    else:
      return primary_name, fd
    n = 1
    while True:
      secondary_name = "%s.%d" % (primary_name, n)
      try:
        fd = os.open(secondary_name, os.O_WRONLY | os.O_CREAT | os.O_EXCL)
      except OSError as e:
        if e.errno != errno.EEXIST:
          raise
      else:
        return secondary_name, fd
      n += 1

  # where you need the file
  path, fd = open_new("x")

That gets you a function your can reuse which returns the file's
name and the file descriptor.

Cheers,
-- 
Cameron Simpson <cs at zip.com.au>

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