when should I explicitly close a file?
in [Python]
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From: Lawrence D'Oliveiro on 17 Apr 2010 07:23 In message <d48b70da-5384-4dc6-9527-46c6b735ccc0(a)r1g2000yqb.googlegroups.com>, gelonida wrote: > I've been told, that following code snippet is not good. > > open("myfile","w").write(astring) ... I do that for reads, but never for writes. For writes, you want to give a chance for write errors to raise an exception and alert the user, instead of failing silently, to avoid inadvertent data loss. Hence the explicit close.
From: Lie Ryan on 17 Apr 2010 08:33 On 04/17/10 21:23, Lawrence D'Oliveiro wrote: > In message > <d48b70da-5384-4dc6-9527-46c6b735ccc0(a)r1g2000yqb.googlegroups.com>, gelonida > wrote: > >> I've been told, that following code snippet is not good. >> >> open("myfile","w").write(astring) ... > > I do that for reads, but never for writes. > > For writes, you want to give a chance for write errors to raise an exception > and alert the user, instead of failing silently, to avoid inadvertent data > loss. Hence the explicit close. In short, in case of doubt, just be explicit. Since in python nothing is guaranteed about implicit file close, you must always explicitly close it.
From: Adam Tauno Williams on 23 Apr 2010 06:14 On Fri, 2010-04-23 at 16:29 +1200, Lawrence D'Oliveiro wrote: > In message <mailman.2119.1271898215.23598.python-list(a)python.org>, Chris > Rebert wrote: > > On Wed, Apr 21, 2010 at 5:53 PM, Lawrence D'Oliveiro wrote: > >> In message <4bc9aadb$1(a)dnews.tpgi.com.au>, Lie Ryan wrote: > >>> Since in python nothing is guaranteed about implicit file close ... > >> It is guaranteed that objects with a reference count of zero will be > >> disposed. > >> In my experiments, this happens immediately. > > Experiment with an implementation other than CPython and prepare to be > > surprised. > Any implementation that doesn't do reference-counting is brain-damaged. Why? There are much better ways to do memory management / garbage collection; especially when dealing with large applications.
From: Alf P. Steinbach on 23 Apr 2010 07:19 * Adam Tauno Williams: > On Fri, 2010-04-23 at 16:29 +1200, Lawrence D'Oliveiro wrote: >> In message <mailman.2119.1271898215.23598.python-list(a)python.org>, Chris >> Rebert wrote: >>> On Wed, Apr 21, 2010 at 5:53 PM, Lawrence D'Oliveiro wrote: >>>> In message <4bc9aadb$1(a)dnews.tpgi.com.au>, Lie Ryan wrote: >>>>> Since in python nothing is guaranteed about implicit file close ... >>>> It is guaranteed that objects with a reference count of zero will be >>>> disposed. >>>> In my experiments, this happens immediately. >>> Experiment with an implementation other than CPython and prepare to be >>> surprised. >> Any implementation that doesn't do reference-counting is brain-damaged. > > Why? Depends on what the statement was meant to mean. But for a literal context-free interpretation e.g. the 'sys.getrefcount' function is not documented as CPython only and thus an implementation that didn't do reference counting would not be a conforming Python implementation. Whether it uses reference counting to destroy objects at earliest opportunity is another matter. > There are much better ways to do memory management / garbage > collection; especially when dealing with large applications. Depends on whether you're talking about Python implementations or as a matter of general principle, and depends on how you define "better", "large" and so on. On its own it's a pretty meaningless statement. But although a small flame war erupted the last time I mentioned this, I think a case can be made that Python is not designed for programming-in-the-large. And that the current CPython scheme is eminently suitable for small scripts. But it has its drawbacks, especially considering the various ways that stack frames can be retained, and considering the documentation of 'gc.garbage', ... "Objects that have __del__() methods and are part of a reference cycle cause the entire reference cycle to be uncollectable, including objects not necessarily in the cycle but reachable only from it." .... which means that a programming style assuming current CPython semantics and employing RAII can be detrimental in a sufficiently large system. Cheers & hth., - Alf
From: Steven D'Aprano on 22 Apr 2010 00:03
On Thu, 22 Apr 2010 12:53:51 +1200, Lawrence D'Oliveiro wrote: > In message <4bc9aadb$1(a)dnews.tpgi.com.au>, Lie Ryan wrote: > >> Since in python nothing is guaranteed about implicit file close ... > > It is guaranteed that objects with a reference count of zero will be > disposed. Not all Python implementations have reference counts at all, e.g. Jython and IronPython. Neither of those close files immediately. > In my experiments, this happens immediately. Are your experiments done under PyPy, CLPython, or Pynie? -- Steven |