Re: Q on explicitly calling file.close
> except: > return 0 So wrong on so many levels... -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
En Thu, 10 Sep 2009 08:26:16 -0300, David C. Ullrich escribió: On Wed, 9 Sep 2009 15:13:49 -0700 (PDT), r wrote: On Sep 9, 4:19 pm, Charles Yeomans wrote: I removed the except block because I prefer exceptions to error codes. how will the caller know an exception has occurred? What if logic depends on the validation that a file *had* or *had not* been written too, huh? Oh I see! But what happens if the filename does not exist? What then? "open" will blow chucks thats what! Here is a version for our paranoid- schizophrenic-sadomasochist out there... Well first, we agree that putting the open() in the try part of a try-finally is wrong. try-finally is supposed to ensure that _allocated_ resources are cleaned up. What you do below may work. But it's essentially throwing out exception handling and using error codes instead. There are plenty of reasons why exceptions are preferred. The standard thing is this: def UseResource(rname): r = get(rname) try: r.use() finally r.cleanup() And it is so widely used that it got its own syntax (the with statement) and library support (contextlib, for creating custom context managers). In any decent version of Python this idiom becomes: with get(rname) as r: r.use assuming get(rname) returns a suitable object (that defines __enter__ and __exit__) def egor_read_file(fname, mode='rb'): print 'yes, master' try: f = open(fname, mode=mode) except IOError: return (0, 'But, the file no open master!') try: s = f.read() except NameError: return (0, 'the file still no open master!') try: f.close() except: print 'That file sure is tricky master! return (s, 'Whew! here is the file contents, master') What's above seems simpler. More important, if you do it this way then you _always_ have to check the return value of egor_read_file and take appropriate action - complicates the code everywhere the function is called as well as making the function more complicated. Doing it as in UseResource() above you don't need to worry about whether UseResource() failed _except_ in situations where you're certain that that's the right level to catch the error. That's the standard argument showing why structured exceptions are a Good Thing. I'd say everyone should be aware of that, giving the ample usage of exceptions in Python, but looks like people requires a reminder from time to time. -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Wed, 9 Sep 2009 15:13:49 -0700 (PDT), r wrote: >On Sep 9, 4:19 pm, Charles Yeomans wrote: >(snip:) >> Unfortunately, both of these simple templates have the following >> problem -- if open fails, a NameError will be raised from the finally >> block. > >(snip) >> I removed the except block because I prefer exceptions to error codes. > >how will the caller know an exception has occurred? What if logic >depends on the validation that a file *had* or *had not* been written >too, huh? > >> In addition to fixing the latent bug in the second simple template, I >> took the opportunity to correct your heinous violation of command- >> query separation. >> >> Charles Yeomans > >Oh I see! But what happens if the filename does not exist? What then? >"open" will blow chucks thats what! Here is a version for our paranoid- >schizophrenic-sadomasochist out there... Well first, we agree that putting the open() in the try part of a try-finally is wrong. try-finally is supposed to ensure that _allocated_ resources are cleaned up. What you do below may work. But it's essentially throwing out exception handling and using error codes instead. There are plenty of reasons why exceptions are preferred. The standard thing is this: def UseResource(rname): r = get(rname) try: r.use() finally r.cleanup() Now if the get() fails, UseResource will indeed blow chunks. That's what it _should_ do. Generally speaking, you should use try-except very rarely, making certain you're catching the exception at the appropriate level. Here for example in _some_ uses of UseResource() you _want_ the application to terminate if the get() fails. In that case you'd just call UseResource() without any try-excpt. In some other situation (maybe the user provided a filename that may or may not exist) you want to trap the exception when you _call_ UseResource(): try: UseResource(rname) except WhateverError: print error message >def egor_read_file(fname, mode='rb'): >print 'yes, master' >try: > f = open(fname, mode=mode) >except IOError: >return (0, 'But, the file no open master!') > >try: >s = f.read() >except NameError: >return (0, 'the file still no open master!') > >try: >f.close() >except: >print 'That file sure is tricky master! > >return (s, 'Whew! here is the file contents, master') What's above seems simpler. More important, if you do it this way then you _always_ have to check the return value of egor_read_file and take appropriate action - complicates the code everywhere the function is called as well as making the function more complicated. Doing it as in UseResource() above you don't need to worry about whether UseResource() failed _except_ in situations where you're certain that that's the right level to catch the error. > >> MRAB wrote: >> You should've used raw strings. :-) > >rats!, you got me on that one :-) David C. Ullrich "Understanding Godel isn't about following his formal proof. That would make a mockery of everything Godel was up to." (John Jones, "My talk about Godel to the post-grads." in sci.logic.) -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sep 9, 4:19 pm, Charles Yeomans wrote: (snip:) > Unfortunately, both of these simple templates have the following > problem -- if open fails, a NameError will be raised from the finally > block. (snip) > I removed the except block because I prefer exceptions to error codes. how will the caller know an exception has occurred? What if logic depends on the validation that a file *had* or *had not* been written too, huh? > In addition to fixing the latent bug in the second simple template, I > took the opportunity to correct your heinous violation of command- > query separation. > > Charles Yeomans Oh I see! But what happens if the filename does not exist? What then? "open" will blow chucks thats what! Here is a version for our paranoid- schizophrenic-sadomasochist out there... def egor_read_file(fname, mode='rb'): print 'yes, master' try: f = open(fname, mode=mode) except IOError: return (0, 'But, the file no open master!') try: s = f.read() except NameError: return (0, 'the file still no open master!') try: f.close() except: print 'That file sure is tricky master! return (s, 'Whew! here is the file contents, master') > MRAB wrote: > You should've used raw strings. :-) rats!, you got me on that one :-) -- free beer tomorrow @ http://jjsenlightenments.blogspot.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sep 9, 2009, at 4:50 PM, r wrote: On Sep 9, 3:18 pm, David C Ullrich wrote: (snip) These days I've actually got the syntax and spelling memorized - I can type "close()" without needing to look it up! +1 You are so right David! I think some people around here need to look up "code reuse". Here are a couple of simple templates for our friends to study... def read_file(fname, mode='rb'): '''open file and return contents''' try: f = open(fname, mode) s = f.read() return s except: return 0 finally: f.close() def write_file(fname, s, mode='wb'): '''open file, truncate, and write string''' try: f = open(fname, mode) f.write(s) return 1 except: return 0 finally: f.close() Unfortunately, both of these simple templates have the following problem -- if open fails, a NameError will be raised from the finally block. def read_file(fname, mode='rb'): '''open file and return contents''' f = open(fname, mode) try: return f.read() finally: f = f.close() I removed the except block because I prefer exceptions to error codes. def write_file(fname, s, mode='wb'): '''open file, truncate, and write string''' f = open(fname, mode) try: f.write(s) finally: f = f.close() In addition to fixing the latent bug in the second simple template, I took the opportunity to correct your heinous violation of command- query separation. Charles Yeomans -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
r wrote: [snip] #-- Double Extra Creidit --# Create a backup_file() function that takes as arg and creates a copy of the file with the extension ".bak"... backup_file('C:\test.txt') -> 'C:\test.bak' You should've used raw strings. :-) -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sep 9, 3:18 pm, David C Ullrich wrote: (snip) > These days I've actually got the syntax and spelling memorized - > I can type "close()" without needing to look it up! +1 You are so right David! I think some people around here need to look up "code reuse". Here are a couple of simple templates for our friends to study... def read_file(fname, mode='rb'): '''open file and return contents''' try: f = open(fname, mode) s = f.read() return s except: return 0 finally: f.close() def write_file(fname, s, mode='wb'): '''open file, truncate, and write string''' try: f = open(fname, mode) f.write(s) return 1 except: return 0 finally: f.close() >>> s = read_file(fname) >>> if s: ... s += 'morecrap' >>> write_file(s) #-- Extra Credit --# Create an append_file() function that takes a and as args and appends to the file returning 1 on success, and 0 on failure. #-- Double Extra Creidit --# Create a backup_file() function that takes as arg and creates a copy of the file with the extension ".bak"... >>> backup_file('C:\test.txt') -> 'C:\test.bak' -- def get_enlightened(): import webbrowser url = 'http://jjsenlightenments.blogspot.com/' webbrowser.open(url) -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sat, 05 Sep 2009 23:41:08 +, Steven D'Aprano wrote: > On Sat, 05 Sep 2009 16:14:02 +, kj wrote: > >> Finally, I was under the impression that Python closed filehandles >> automatically when they were garbage-collected. [...] > > (3) For quick and dirty scripts, or programs that only use one or two > files, relying on the VM to close the file is sufficient (although lazy > in my opinion *wink*) Many years ago I read some big Python guy say something to the effect that you didn't "need" to close files, but if you don't it will bite you eventually. So I learned to type "close()". Of course I wouldn't worry about that in quick and dirty scripts. Then there was that quick and dirty script that gradually mutated into a not-so-quick and filthy monstrosity that a certain project depended on. Stopped working one day. Me being dim, it took almost a day to find the problem... (I don't recall how exactly an open file caused a problem, it was still just three or four files altogether.) These days I've actually got the syntax and spelling memorized - I can type "close()" without needing to look it up! > but for long-running applications using many > files, or for debugging, you may want more control over what happens > when. -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
En Sun, 06 Sep 2009 21:20:52 -0300, Stephen Hansen escribió: On Sun, Sep 6, 2009 at 4:31 PM, r wrote: On Sep 6, 1:14 pm, "Jan Kaliszewski" wrote: > 05-09-2009 r wrote: > > i find the with statement (while quite useful in general > > practice) is not a "cure all" for situations that need and exception > > caught. But what does that even mean? What's the -problem- that it isn't 'curing'? That 'with' doesn't completely replace all uses of 'try'? It was never meant to-- that seems like a completely different problem area. But, if you do want it to handle exception catching, that's just a question of making a context manager that does so. import contextlib @contextlib.contextmanager def file_with_errorhandling(filename, mode, exceptions=(IOError,OSError)): fp = file(filename, mode) try: yield fp except exceptions: print "Handle file-operation errors gracefully here." fp.close() with file_with_errorhandling(filename, 'r') as fp: fp.read() # or whatever True, the context manager provided by file objects just lets exceptions propagate up but that's usually the desired behavior. You can make context managers for particular problems you run across that handle exceptions if you want. Note that to correctly emulate the file built-in context manager, fp.close() should be inside a finally clause. The with statement isn't about never having to type try again, I don't think. The with statement is intended as a replacement for common try/finally blocks, not try/except blocks as the OP seems to imply. -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sun, Sep 6, 2009 at 4:31 PM, r wrote: > On Sep 6, 1:14 pm, "Jan Kaliszewski" wrote: > > 05-09-2009 r wrote: > > > i find the with statement (while quite useful in general > > > practice) is not a "cure all" for situations that need and exception > > > caught. > > > > In what sense? > > *ahem*! in the sense that the with statement (while quite useful in > general practice) is not a "cure all" for situations that need and > exception caught ;-) But what does that even mean? What's the -problem- that it isn't 'curing'? That 'with' doesn't completely replace all uses of 'try'? It was never meant to-- that seems like a completely different problem area. But, if you do want it to handle exception catching, that's just a question of making a context manager that does so. import contextlib @contextlib.contextmanager def file_with_errorhandling(filename, mode, exceptions=(IOError,OSError)): fp = file(filename, mode) try: yield fp except exceptions: print "Handle file-operation errors gracefully here." fp.close() with file_with_errorhandling(filename, 'r') as fp: fp.read() # or whatever True, the context manager provided by file objects just lets exceptions propagate up but that's usually the desired behavior. You can make context managers for particular problems you run across that handle exceptions if you want. The with statement isn't about never having to type try again, I don't think. --S -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sep 6, 1:14 pm, "Jan Kaliszewski" wrote: > 05-09-2009 r wrote: > > i find the with statement (while quite useful in general > > practice) is not a "cure all" for situations that need and exception > > caught. > > In what sense? *ahem*! in the sense that the with statement (while quite useful in general practice) is not a "cure all" for situations that need and exception caught ;-) > I think that: > > with open(...) as f: > foo... > > is equivalent to: > > f = open(...) > try: > foo... > finally: > f.close() > > Obviously it doesn't substitute catching with 'except' My sentiments exactly...? Get Enlightened -> http://jjsenlightenments.blogspot.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
05-09-2009 r wrote: i find the with statement (while quite useful in general practice) is not a "cure all" for situations that need and exception caught. In what sense? I think that: with open(...) as f: foo... is equivalent to: f = open(...) try: foo... finally: f.close() Obviously it doesn't substitute catching with 'except', but I don't see how it could disturb that. Cheers, *j -- Jan Kaliszewski (zuo) -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
> > It's just too bad that 'with' doesn't support multiple separate "x as y" >> clauses. >> > > The developers already agreed with you ;-). > > "With more than one item, the context managers are processed as if multiple > with statements were nested: > > with A() as a, B() as b: >suite > is equivalent to > > with A() as a: >with B() as b: >suite > Changed in version 3.1: Support for multiple context expressions. > " > Doh! I hadn't noticed that. Alas, I'm /just barely now/ moving to Python 2.5, I won't be able to even consider getting near Python 3 for -- a year or so, I'm expecting. If ever! (I suspect this will also be in 2.7) > Ah, that's a beacon of hope. --S -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
Stephen Hansen wrote: This is precisely why the with statement exists; to provide a cleaner way to wrap a block in setup and teardown functions. Closing is one. Yeah, you get some extra indentation-- but you sorta have to live with it if you're worried about correct code. I think it's a good compromise between your examples of nasty and nice :) def compromise(from_, to_): with file(to_) as to_h: with file(from_) as from_h: for line in from_h: print >> to_h, munge(line) It's just too bad that 'with' doesn't support multiple separate "x as y" clauses. The developers already agreed with you ;-). "With more than one item, the context managers are processed as if multiple with statements were nested: with A() as a, B() as b: suite is equivalent to with A() as a: with B() as b: suite Changed in version 3.1: Support for multiple context expressions. " (I suspect this will also be in 2.7) Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sat, Sep 5, 2009 at 6:51 PM, kj wrote: > In <02b2e6ca$0$17565$c3e8...@news.astraweb.com> Steven D'Aprano < > st...@remove-this-cybersource.com.au> writes: > > >(3) For quick and dirty scripts, or programs that only use one or two > >files, relying on the VM to close the file is sufficient (although lazy > >in my opinion *wink*) > > It's not a matter of laziness or industriousness, but rather of > code readability. The real problem here is not the close() per > se, but rather all the additional machinery required to ensure that > the close happens. When the code is working with multiple file > handles simultaneously, one ends up with a thicket of try/finally's > that makes the code just *nasty* to look at. E.g., even with only > two files, namely an input and an output file, compare: > This is precisely why the with statement exists; to provide a cleaner way to wrap a block in setup and teardown functions. Closing is one. Yeah, you get some extra indentation-- but you sorta have to live with it if you're worried about correct code. I think it's a good compromise between your examples of nasty and nice :) def compromise(from_, to_): with file(to_) as to_h: with file(from_) as from_h: for line in from_h: print >> to_h, munge(line) It's just too bad that 'with' doesn't support multiple separate "x as y" clauses. As for why you didn't see much of that style code in the wild-- that's because the with statement is just pretty new. 2.5 had it with a future statement, and although there's lots of code out there that /supports/ Python 2.5, there's not /that/ much which /requires/ it of yet. As time goes on and people stop wanting to support earlier versions of Python in various libraries, you'll see more of it in publicly available code. I'm personally rewriting a huge chunk of code in the office to convert everything from try/finally to with statements as we move our codebase to 2.5 finally, and am quite gleeful. It's a lot cleaner and clearer. --S -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sun, 06 Sep 2009 01:51:50 +, kj wrote: > In <02b2e6ca$0$17565$c3e8...@news.astraweb.com> Steven D'Aprano > writes: > >>(3) For quick and dirty scripts, or programs that only use one or two >>files, relying on the VM to close the file is sufficient (although lazy >>in my opinion *wink*) > > It's not a matter of laziness or industriousness, but rather of code > readability. The real problem here is not the close() per se, but > rather all the additional machinery required to ensure that the close > happens. When the code is working with multiple file handles > simultaneously, one ends up with a thicket of try/finally's that makes > the code just *nasty* to look at. Yep, that's because dealing with the myriad of things that *might* (but probably won't) go wrong when dealing with files is *horrible*. Real world code is almost always much nastier than the nice elegant algorithms we hope for. Most people know they have to deal with errors when opening files. The best programmers deal with errors when writing to files. But only a few of the most pedantic coders even attempt to deal with errors when *closing* the file. Yes, closing the file can fail. What are you going to do about it? At the least, you should notify the user, then continue. Dying with an uncaught exception in the middle of processing millions of records is Not Cool. But close failures are so rare that we just hope we'll never experience one. It really boils down to this... do you want to write correct code, or elegant code? -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
In <02b2e6ca$0$17565$c3e8...@news.astraweb.com> Steven D'Aprano writes: >(3) For quick and dirty scripts, or programs that only use one or two >files, relying on the VM to close the file is sufficient (although lazy >in my opinion *wink*) It's not a matter of laziness or industriousness, but rather of code readability. The real problem here is not the close() per se, but rather all the additional machinery required to ensure that the close happens. When the code is working with multiple file handles simultaneously, one ends up with a thicket of try/finally's that makes the code just *nasty* to look at. E.g., even with only two files, namely an input and an output file, compare: def nice(from_, to_): to_h = file(to_, "w") for line in file(from_): print >> to_h, munge(line) def nasty(from_, to_): to_h = file(to_, "w") try: from_h = file(from_) try: for line in from_h: print >> to_h, munge(line) finally: from_h.close() finally: to_h.close() I leave to your imagination the joys of reading the code for hairy(from_, to_, log_), where log_ is a third file to collect warning messages. kynn -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sat, 05 Sep 2009 16:14:02 +, kj wrote: > Finally, I was under the impression that Python closed filehandles > automatically when they were garbage-collected. (In fact (3) suggests > as much, since it does not include an implicit call to fh.close.) If so, > the difference between (1) and (3) does not seem very big. What am I > missing here? (1) Python the language will close file handles, but doesn't guarantee when. Some implementations (e.g. CPython) will close them immediately the file object goes out of scope. Others (e.g. Jython) will close them "eventually", which may be when the program exists. (2) If the file object never goes out of scope, say because you've stored a reference to it somewhere, the file will never be closed and you will leak file handles. Since the OS only provides a finite number of them, any program which uses large number of files is at risk of running out. (3) For quick and dirty scripts, or programs that only use one or two files, relying on the VM to close the file is sufficient (although lazy in my opinion *wink*) but for long-running applications using many files, or for debugging, you may want more control over what happens when. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sep 5, 2:47 pm, Dennis Lee Bieber wrote: (snip) > > Finally, I was under the impression that Python closed filehandles > > automatically when they were garbage-collected. (In fact (3) > > suggests as much, since it does not include an implicit call to > > fh.close.) If so, the difference between (1) and (3) does not seem > > very big. What am I missing here? True, but i find the with statement (while quite useful in general practice) is not a "cure all" for situations that need and exception caught. In that case the laborious finger wrecking syntax of "f.close ()" must be painstaking typed letter by painful letter. f-.-c-l-o-s-e-(-) It's just not fair ;-( -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
CPython uses reference counting, so an object is garbage collected as soon as there are no references to it, but that's just an implementation detail. Other implementations, such as Jython and IronPython, don't use reference counting, so you don't know when an object will be garbage collected, which means that the file might remain open for an unknown time afterwards in case 1 above. Most people use CPython, so it's not surprising that case 1 is so common. Additionally, many scripts just use a small number of files (say, 1-5 files) so having a file-handle open for the duration of the run it minimal overhead. On the other hand, when processing thousands of files, I always explicitly close each file to make sure I don't exhaust some file-handle limit the OS or interpreter may enforce. -tkc -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
On Sep 5, 1:17 pm, Dave Angel wrote: > kj wrote: > > There's something wonderfully clear about code like this: > > > # (1) > > def spam(filename): > > for line in file(filename): > > do_something_with(line) > > > It is indeed pseudo-codely beautiful. But I gather that it is not > > correct to do this, and that instead one should do something like > > > # (2) > > def spam(filename): > > fh = file(filename) > > try: > > for line in fh: > > do_something_with(line) > > finally: > > fh.close() > > > ...or alternatively, if the with-statement is available: > > > # (3) > > def spam(filename): > > with file(filename) as fh: > > for line in fh: > > do_something_with(line) > > > Mind you, (3) is almost as simple as (1) (only one additional line), > > but somehow it lacks (1)'s direct simplicity. (And it adds one > > more indentation level, which I find annoying.) Furthermore, I > > don't recall ever coming across either (2) or (3) "in the wild", > > even after reading a lot of high-quality Python code (e.g. standard > > library modules). > > > Finally, I was under the impression that Python closed filehandles > > automatically when they were garbage-collected. (In fact (3) > > suggests as much, since it does not include an implicit call to > > fh.close.) If so, the difference between (1) and (3) does not seem > > very big. What am I missing here? > > > kynn > > We have to distinguish between reference counted and garbage collected. > As MRAB says, when the reference count goes to zero, the file is > immediately closed, in CPython implementation. So all three are > equivalent on that platform. > > But if you're not sure the code will run on CPython, then you have to > have something that explicitly catches the out-of-scopeness of the file > object. Both your (2) and (3) do that, with different syntaxes. > > DaveA Stop being lazy and close the file. You don't want open file objects just floating around in memory. Even the docs says something like "yes, python will free the memory associated with a file object but you can never *really* be sure *when* this will happen, so just explicitly close the damn thing!". Besides, you can't guarantee that any data has been written without calling f.flush() or f.close() first. What if your program crashes and no data is written? huh? I guess i could put my pants on by jumping into both legs at the same time thereby saving one step, but i my fall down and break my arm. I would much rather just use the one leg at a time approach... -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
kj wrote: There's something wonderfully clear about code like this: # (1) def spam(filename): for line in file(filename): do_something_with(line) It is indeed pseudo-codely beautiful. But I gather that it is not correct to do this, and that instead one should do something like # (2) def spam(filename): fh = file(filename) try: for line in fh: do_something_with(line) finally: fh.close() ...or alternatively, if the with-statement is available: # (3) def spam(filename): with file(filename) as fh: for line in fh: do_something_with(line) Mind you, (3) is almost as simple as (1) (only one additional line), but somehow it lacks (1)'s direct simplicity. (And it adds one more indentation level, which I find annoying.) Furthermore, I don't recall ever coming across either (2) or (3) "in the wild", even after reading a lot of high-quality Python code (e.g. standard library modules). Finally, I was under the impression that Python closed filehandles automatically when they were garbage-collected. (In fact (3) suggests as much, since it does not include an implicit call to fh.close.) If so, the difference between (1) and (3) does not seem very big. What am I missing here? kynn We have to distinguish between reference counted and garbage collected. As MRAB says, when the reference count goes to zero, the file is immediately closed, in CPython implementation. So all three are equivalent on that platform. But if you're not sure the code will run on CPython, then you have to have something that explicitly catches the out-of-scopeness of the file object. Both your (2) and (3) do that, with different syntaxes. DaveA -- http://mail.python.org/mailman/listinfo/python-list
Re: Q on explicitly calling file.close
kj wrote: There's something wonderfully clear about code like this: # (1) def spam(filename): for line in file(filename): do_something_with(line) It is indeed pseudo-codely beautiful. But I gather that it is not correct to do this, and that instead one should do something like # (2) def spam(filename): fh = file(filename) try: for line in fh: do_something_with(line) finally: fh.close() ...or alternatively, if the with-statement is available: # (3) def spam(filename): with file(filename) as fh: for line in fh: do_something_with(line) Mind you, (3) is almost as simple as (1) (only one additional line), but somehow it lacks (1)'s direct simplicity. (And it adds one more indentation level, which I find annoying.) Furthermore, I don't recall ever coming across either (2) or (3) "in the wild", even after reading a lot of high-quality Python code (e.g. standard library modules). Finally, I was under the impression that Python closed filehandles automatically when they were garbage-collected. (In fact (3) suggests as much, since it does not include an implicit call to fh.close.) If so, the difference between (1) and (3) does not seem very big. What am I missing here? CPython uses reference counting, so an object is garbage collected as soon as there are no references to it, but that's just an implementation detail. Other implementations, such as Jython and IronPython, don't use reference counting, so you don't know when an object will be garbage collected, which means that the file might remain open for an unknown time afterwards in case 1 above. Most people use CPython, so it's not surprising that case 1 is so common. -- http://mail.python.org/mailman/listinfo/python-list