On 12/03/2014 22:19, Cameron Simpson wrote:
On 12Mar2014 13:29, zoom <z...@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,
I haven't looked but would things be easier if the new exception
hierarchy were used
http://docs.python.org/3.3/whatsnew/3.3.html#pep-3151-reworking-the-os-and-io-exception-hierarchy
?
--
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.
Mark Lawrence
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