Steven D'Aprano wrote:
On Sat, 02 Jan 2010 09:40:44 -0800, Aahz wrote:
OTOH, if you want to do something different depending on whether the
file exists, you need to use both approaches:
if os.path.exists(fname):
try:
f = open(fname, 'rb')
data = f.read()
f.close()
return data
except IOError:
logger.error("Can't read: %s", fname) return ''
else:
try:
f = open(fname, 'wb')
f.write(data)
f.close()
except IOError:
logger.error("Can't write: %s", fname)
return None
Unfortunately, this is still vulnerable to the same sort of race
condition I spoke about.
Even more unfortunately, I don't know that there is any fool-proof way of
avoiding such race conditions in general. Particularly the problem of
"open this file for writing only if it doesn't already exist".
<snip>
In Windows, there is a way to do it. It's just not exposed to the
Python built-in function open(). You use the CreateFile() function,
with /dwCreationDisposition/ of CREATE_NEW.
It's atomic, and fails politely if the file already exists.
No idea if Unix has a similar functionality.
DaveA
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