A "scopeguard" for Python
in [Python]
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From: Alf P. Steinbach on 4 Mar 2010 10:48 * Jean-Michel Pichavant: > Alf P. Steinbach wrote: >>> From your post, the scope guard technique is used "to ensure some >>> desired cleanup at the end of a scope, even when the scope is exited >>> via an exception." This is precisely what the try: finally: syntax is >>> for. >> >> You'd have to nest it. That's ugly. And more importantly, now two >> people in this thread (namely you and Mike) have demonstrated that >> they do not grok the try functionality and manage to write incorrect >> code, even arguing that it's correct when informed that it's not, so >> it's a pretty fragile construct, like goto. > > You want to execute some cleanup when things go wrong, use try except. > You want to do it when things go right, use try else. You want to > cleanup no matter what happen, use try finally. > > There is no need of any Cleanup class, except for some technical > alternative concern. Have you considered that your argument applies to the "with" construct? You have probably not realized that. But let me force it on you: when would you use "with"? Check if that case is covered by your argument above. Now that you've been told about the "with" angle, don't you think it's a kind of weakness in your argument that it calls for removing "with" from the language? I recommend that you think about why your argument is invalid. Or, as I like to say, why your argument is completely bogus. Cheers & hth., - Alf
From: Robert Kern on 4 Mar 2010 10:48 On 2010-03-03 18:49 PM, Alf P. Steinbach wrote: > * Robert Kern: >> On 2010-03-03 15:35 PM, Alf P. Steinbach wrote: >>> * Robert Kern: >>>> On 2010-03-03 13:32 PM, Alf P. Steinbach wrote: >>>>> * Robert Kern: >>>>>> On 2010-03-03 11:18 AM, Alf P. Steinbach wrote: >>>>>>> * Robert Kern: >>>>>>>> On 2010-03-03 09:56 AM, Alf P. Steinbach wrote: >>>>>>>>> * Mike Kent: >>>>>>>>>> What's the compelling use case for this vs. a simple try/finally? >>>>>>>>> >>>>>>>>> if you thought about it you would mean a simple "try/else". >>>>>>>>> "finally" is >>>>>>>>> always executed. which is incorrect for cleanup >>>>>>>> >>>>>>>> Eh? Failed execution doesn't require cleanup? The example you >>>>>>>> gave is >>>>>>>> definitely equivalent to the try: finally: that Mike posted. >>>>>>> >>>>>>> Sorry, that's incorrect: it's not. >>>>>>> >>>>>>> With correct code (mine) cleanup for action A is only performed when >>>>>>> action A succeeds. >>>>>>> >>>>>>> With incorrect code cleanup for action A is performed when A fails. >>>>>> >>>>>> Oh? >>>>>> >>>>>> $ cat cleanup.py >>>>>> >>>>>> class Cleanup: >>>>>> def __init__( self ): >>>>>> self._actions = [] >>>>>> >>>>>> def call( self, action ): >>>>>> assert( callable( action ) ) >>>>>> self._actions.append( action ) >>>>>> >>>>>> def __enter__( self ): >>>>>> return self >>>>>> >>>>>> def __exit__( self, x_type, x_value, x_traceback ): >>>>>> while( len( self._actions ) != 0 ): >>>>>> try: >>>>>> self._actions.pop()() >>>>>> except BaseException as x: >>>>>> raise AssertionError( "Cleanup: exception during cleanup" ) >>>>>> >>>>>> def print_(x): >>>>>> print x >>>>>> >>>>>> with Cleanup() as at_cleanup: >>>>>> at_cleanup.call(lambda: print_("Cleanup executed without an >>>>>> exception.")) >>>>>> >>>>>> with Cleanup() as at_cleanup: >>>>> >>>>> *Here* is where you should >>>>> >>>>> 1) Perform the action for which cleanup is needed. >>>>> >>>>> 2) Let it fail by raising an exception. >>>>> >>>>> >>>>>> at_cleanup.call(lambda: print_("Cleanup execute with an exception.")) >>>>>> raise RuntimeError() >>>>> >>>>> With an exception raised here cleanup should of course be performed. >>>>> >>>>> And just in case you didn't notice: the above is not a test of the >>>>> example I gave. >>>>> >>>>> >>>>>> $ python cleanup.py >>>>>> Cleanup executed without an exception. >>>>>> Cleanup execute with an exception. >>>>>> Traceback (most recent call last): >>>>>> File "cleanup.py", line 28, in <module> >>>>>> raise RuntimeError() >>>>>> RuntimeError >>>>>> >>>>>>>> The actions are always executed in your example, >>>>>>> >>>>>>> Sorry, that's incorrect. >>>>>> >>>>>> Looks like it to me. >>>>> >>>>> I'm sorry, but you're >>>>> >>>>> 1) not testing my example which you're claiming that you're >>>>> testing, and >>>> >>>> Then I would appreciate your writing a complete, runnable example that >>>> demonstrates the feature you are claiming. Because it's apparently not >>>> "ensur[ing] some desired cleanup at the end of a scope, even when the >>>> scope is exited via an exception" that you talked about in your >>>> original post. >>>> >>>> Your sketch of an example looks like mine: >>>> >>>> with Cleanup as at_cleanup: >>>> # blah blah >>>> chdir( somewhere ) >>>> at_cleanup.call( lambda: chdir( original_dir ) ) >>>> # blah blah >>>> >>>> The cleanup function gets registered immediately after the first >>>> chdir() and before the second "blah blah". Even if an exception is >>>> raised in the second "blah blah", then the cleanup function will still >>>> run. This would be equivalent to a try: finally: >>>> >>>> # blah blah #1 >>>> chdir( somewhere ) >>>> try: >>>> # blah blah #2 >>>> finally: >>>> chdir( original_dir ) >>> >>> Yes, this is equivalent code. >>> >>> The try-finally that you earlier claimed was equivalent, was not. >> >> Okay, but just because of the position of the chdir(), right? > > Yes, since it yields different results. > >>>> and not a try: else: >>>> >>>> # blah blah #1 >>>> chdir( somewhere ) >>>> try: >>>> # blah blah #2 >>>> else: >>>> chdir( original_dir ) >>> >>> This example is however meaningless except as misdirection. There are >>> infinitely many constructs that include try-finally and try-else, that >>> the with-Cleanup code is not equivalent to. It's dumb to show one such. >>> >>> Exactly what are you trying to prove here? >> >> I'm just showing you what I thought you meant when you told Mike that >> he should have used a try/else instead of try/finally. >> >>> Your earlier claims are still incorrect. >>> >>>> Now, I assumed that the behavior with respect to exceptions occurring >>>> in the first "blah blah" weren't what you were talking about because >>>> until the chdir(), there is nothing to clean up. >>>> >>>> There is no way that the example you gave translates to a try: else: >>>> as you claimed in your response to Mike Kent. >>> >>> Of course there is. >>> >>> Note that Mike wrapped the action A within the 'try': >>> >>> >>> <code author="Mike" correct="False"> >>> original_dir = os.getcwd() >>> try: >>> os.chdir(somewhere) >>> # Do other stuff >>> finally: >>> os.chdir(original_dir) >>> # Do other cleanup >>> </code> >>> >>> >>> The 'finally' he used, shown above, yields incorrect behavior. >>> >>> Namely cleanup always, while 'else', in that code, can yield correct >>> behavior /provided/ that it's coded correctly: >>> >>> >>> <code author="Alf" correct="ProbablyTrue" disclaimer="off the cuff"> >>> original_dir = os.getcwd() >>> try: >>> os.chdir(somewhere) >>> except Whatever: >>> # whatever, e.g. logging >>> raise >>> else: >>> try: >>> # Do other stuff >>> finally: >>> os.chdir(original_dir) >>> # Do other cleanup >>> </code> >> >> Ah, okay. Now we're getting somewhere. Now, please note that you did >> not have any except: handling in your original example. So Mike made a >> try: finally: example to attempt to match the semantics of your code. >> When you tell him that he should 'mean a simple "try/else". "finally" >> is always executed. which is incorrect for cleanup', can you >> understand why we might think that you were saying that try: finally: >> was wrong and that you were proposing that your code was equivalent to >> some try: except: else: suite? > > No, not really. His code didn't match the semantics. Changing 'finally' > to 'else' could make it equivalent. Okay, please show me what you mean by "changing 'finally' to 'else'." I think you are being hinty again. It's not helpful. The most straightforward interpretation of those words means that you literally just want to remove the word 'finally' and replace it with 'else' in Mike's example. Obviously you don't mean that because it is a syntax error. try: else: is not a construct in Python. There is a try: except: else:, but there is no point to doing that if you don't have anything in the except: clause. Neither Mike's example nor your original one have any except: clause. Why do you think that we would interpret those words to mean that you wanted the example you give just above? >>>>> 2) not even showing anything about your earlier statements, which were >>>>> just incorrect. >>>>> >>>>> You're instead showing that my code works as it should for the case >>>>> that >>>>> you're testing, which is a bit unnecessary since I knew that, but >>>>> thanks >>>>> anyway. >>>> >>>> It's the case you seem to be talking about in your original post. >>> >>> What's this "seems"? Are you unable to read that very short post? >> >> I say "seems" because my understandings of what you meant in your >> original post and your response to Mike disagreed with one another. >> Now I see that your later posts were talking about minor discrepancy >> about which errors you wanted caught by the finally: and which you >> didn't. > > It's absolutely not a minor discrepancy whether some code is executed or > not. It can have arbitrarily large effect. And from my point of view the > discussion of that snippet has not been about what errors I "want" > caught by the 'finally'; it's been about whether two snippets of code > yield the same effect or not: Mike's code was incorrect not because it > did something else, but because as code that did something else it was > not an equivalent to the code that I posted. > > >> I was confused because it seemed that you were saying that try: >> finally: was completely wrong and that "try/else" was right. It >> confused me and at least one other person. >> >>>> , but I do ask you to acknowledge that you originally were talking >>>> about a feature that "ensure[s] some desired cleanup at the end of a >>>> scope, even when the scope is exited via an exception." >>> >>> Yes, that's what it does. >>> >>> Which is I why I wrote that. >>> >>> This should not be hard to grok. >>> >>> >>>> Do you acknowledge this? >>> >>> This seems like pure noise, to cover up that you were sputing a lot of >>> incorrect statements earlier. >> >> No, I'm just trying to clarify what you are trying to say. The above >> statement did not appear to accord with your later statement: 'if you >> thought about it you would mean a simple "try/else". "finally" is >> always executed. which is incorrect for cleanup.' It turns out that >> what you really meant was that it would be incorrect for cleanup to be >> executed when an error occurred in the chdir() itself. >> >> Now, I happen to disagree with that. > > Well, I was pretty unclear, almost hint-like, sorry about that, mea > culpa, but you have it slightly wrong. You wrote then "The example you > gave is definitely equivalent to the try: finally: that Mike posted." > And it isn't. I agree that it does behave differently with respect to when an exception is raised in chdir(). I was wrong on that point. I thought you were claiming that it behaved differently when there was an exception in the "# Do other stuff" block because you were being (and are still being) unclear. >> There are a couple of ways to do this kind of cleanup depending on the >> situation. Basically, you have several different code blocks: >> >> # 1. Record original state. >> # 2. Modify state. >> # 3. Do stuff requiring the modified state. >> # 4. Revert to the original state. >> >> Depending on where errors are expected to occur, and how the state >> needs to get modified and restored, there are different ways of >> arranging these blocks. The one Mike showed: >> >> # 1. Record original state. >> try: >> # 2. Modify state. >> # 3. Do stuff requiring the modified state. >> finally: >> # 4. Revert to the original state. >> >> And the one you prefer: >> >> # 1. Record original state. >> # 2. Modify state. >> try: >> # 3. Do stuff requiring the modified state. >> finally: >> # 4. Revert to the original state. >> >> These differ in what happens when an error occurs in block #2, the >> modification of the state. In Mike's, the cleanup code runs; in yours, >> it doesn't. For chdir(), it really doesn't matter. Reverting to the >> original state is harmless whether the original chdir() succeeds or >> fails, and chdir() is essentially atomic so if it raises an exception, >> the state did not change and nothing needs to be cleaned up. >> >> However, not all block #2s are atomic. Some are going to fail partway >> through and need to be cleaned up even though they raised an >> exception. Fortunately, cleanup can frequently be written to not care >> whether the whole thing finished or not. > > Yeah, and there are some systematic ways to handle these things. You > might look up Dave Abraham's levels of exception safety. Mostly his > approach boils down to making operations effectively atomic so as to > reduce the complexity: ideally, if an operation raises an exception, > then it has undone any side effects. > > Of course it can't undo the launching of an ICBM, for example... > > But ideally, if it could, then it should. I agree. Atomic operations like chdir() help a lot. But this is Python, and exceptions can happen in many different places. If you're not just calling an extension module function that makes a known-atomic system call, you run the risk of not having an atomic operation. > If you call the possibly failing operation "A", then that systematic > approach goes like this: if A fails, then it has cleaned up its own > mess, but if A succeeds, then it's the responsibility of the calling > code to clean up if the higher level (multiple statements) operation > that A is embedded in, fails. > > And that's what Marginean's original C++ ScopeGuard was designed for, > and what the corresponding Python Cleanup class is designed for. And try: finally:, for that matter. >> Both formulations can be correct (and both work perfectly fine with >> the chdir() example being used). Sometimes one is better than the >> other, and sometimes not. You can achieve both ways with either your >> Cleanup class or with try: finally:. >> >> I am still of the opinion that Cleanup is not an improvement over try: >> finally: and has the significant ugliness of forcing cleanup code into >> callables. This significantly limits what you can do in your cleanup >> code. > > Uhm, not really. :-) As I see it. Well, not being able to affect the namespace is a significant limitation. Sometimes you need to delete objects from the namespace in order to ensure that their refcounts go to zero and their cleanup code gets executed. Tracebacks will keep the namespace alive and all objects in it. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
From: Robert Kern on 4 Mar 2010 11:04 On 2010-03-04 09:48 AM, Alf P. Steinbach wrote: > * Jean-Michel Pichavant: >> Alf P. Steinbach wrote: >>>> From your post, the scope guard technique is used "to ensure some >>>> desired cleanup at the end of a scope, even when the scope is exited >>>> via an exception." This is precisely what the try: finally: syntax >>>> is for. >>> >>> You'd have to nest it. That's ugly. And more importantly, now two >>> people in this thread (namely you and Mike) have demonstrated that >>> they do not grok the try functionality and manage to write incorrect >>> code, even arguing that it's correct when informed that it's not, so >>> it's a pretty fragile construct, like goto. >> >> You want to execute some cleanup when things go wrong, use try except. >> You want to do it when things go right, use try else. You want to >> cleanup no matter what happen, use try finally. >> >> There is no need of any Cleanup class, except for some technical >> alternative concern. > > Have you considered that your argument applies to the "with" construct? > > You have probably not realized that. > > But let me force it on you: when would you use "with"? When there is a specific context manager that removes the need for boilerplate. > Check if that case is covered by your argument above. > > Now that you've been told about the "with" angle, don't you think it's a > kind of weakness in your argument that it calls for removing "with" from > the language? No, it only argues that "with Cleanup():" is supernumerary. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
From: Jean-Michel Pichavant on 4 Mar 2010 11:32 Alf P. Steinbach wrote: > * Jean-Michel Pichavant: >> Alf P. Steinbach wrote: >>>> From your post, the scope guard technique is used "to ensure some >>>> desired cleanup at the end of a scope, even when the scope is >>>> exited via an exception." This is precisely what the try: finally: >>>> syntax is for. >>> >>> You'd have to nest it. That's ugly. And more importantly, now two >>> people in this thread (namely you and Mike) have demonstrated that >>> they do not grok the try functionality and manage to write incorrect >>> code, even arguing that it's correct when informed that it's not, so >>> it's a pretty fragile construct, like goto. >> >> You want to execute some cleanup when things go wrong, use try >> except. You want to do it when things go right, use try else. You >> want to cleanup no matter what happen, use try finally. >> >> There is no need of any Cleanup class, except for some technical >> alternative concern. > > Have you considered that your argument applies to the "with" construct? > > You have probably not realized that. > > But let me force it on you: when would you use "with"? > > Check if that case is covered by your argument above. > > Now that you've been told about the "with" angle, don't you think it's > a kind of weakness in your argument that it calls for removing "with" > from the language? > > I recommend that you think about why your argument is invalid. > > Or, as I like to say, why your argument is completely bogus. > > > Cheers & hth., > > - Alf I am using python 2.5, so I know nothing about the with statement, and it may possible my arguments apply to it, you could remove it from the language, it wouldn't bother me at all. I just don't see in what you've written (adding a class, with some __entry__, __exit__ protocol, using a with statement) what cannot be achieved with a try statement in its simpliest form. Try except may be lame and noobish, but it works, is easy to read and understood at first glance. It looks like to me that 'with' statements are like decorators: overrated. Sometimes people could write simple readable code, but yet they're tempted by the geek side of programming: using complex constructs when there's no need to. I myself cannot resist sometimes ;-) JM
From: Alf P. Steinbach on 4 Mar 2010 11:56
* Robert Kern: > On 2010-03-03 18:49 PM, Alf P. Steinbach wrote: >> * Robert Kern: [snip] >>> can you >>> understand why we might think that you were saying that try: finally: >>> was wrong and that you were proposing that your code was equivalent to >>> some try: except: else: suite? >> >> No, not really. His code didn't match the semantics. Changing 'finally' >> to 'else' could make it equivalent. > > Okay, please show me what you mean by "changing 'finally' to 'else'." I > think you are being hinty again. It's not helpful. [snip middle of this paragraph] > Why do you think that we would interpret those words > to mean that you wanted the example you give just above? There's an apparent discrepancy between your call for an example and your subsequent (in the same paragraph) reference to the example given. But as to why I assumed that that example, or a similar correct one, would be implied, it's the only meaningful interpretation. Adopting a meaningless interpretation when a meaningful exists is generally just adversarial, but in this case I was, as you pointed out, extremely unclear, and I'm sorry: I should have given such example up front. Will try to do so. [snip] > >>> There are a couple of ways to do this kind of cleanup depending on the >>> situation. Basically, you have several different code blocks: >>> >>> # 1. Record original state. >>> # 2. Modify state. >>> # 3. Do stuff requiring the modified state. >>> # 4. Revert to the original state. >>> >>> Depending on where errors are expected to occur, and how the state >>> needs to get modified and restored, there are different ways of >>> arranging these blocks. The one Mike showed: >>> >>> # 1. Record original state. >>> try: >>> # 2. Modify state. >>> # 3. Do stuff requiring the modified state. >>> finally: >>> # 4. Revert to the original state. >>> >>> And the one you prefer: >>> >>> # 1. Record original state. >>> # 2. Modify state. >>> try: >>> # 3. Do stuff requiring the modified state. >>> finally: >>> # 4. Revert to the original state. >>> >>> These differ in what happens when an error occurs in block #2, the >>> modification of the state. In Mike's, the cleanup code runs; in yours, >>> it doesn't. For chdir(), it really doesn't matter. Reverting to the >>> original state is harmless whether the original chdir() succeeds or >>> fails, and chdir() is essentially atomic so if it raises an exception, >>> the state did not change and nothing needs to be cleaned up. >>> >>> However, not all block #2s are atomic. Some are going to fail partway >>> through and need to be cleaned up even though they raised an >>> exception. Fortunately, cleanup can frequently be written to not care >>> whether the whole thing finished or not. >> >> Yeah, and there are some systematic ways to handle these things. You >> might look up Dave Abraham's levels of exception safety. Mostly his >> approach boils down to making operations effectively atomic so as to >> reduce the complexity: ideally, if an operation raises an exception, >> then it has undone any side effects. >> >> Of course it can't undo the launching of an ICBM, for example... >> >> But ideally, if it could, then it should. > > I agree. Atomic operations like chdir() help a lot. But this is Python, > and exceptions can happen in many different places. If you're not just > calling an extension module function that makes a known-atomic system > call, you run the risk of not having an atomic operation. > >> If you call the possibly failing operation "A", then that systematic >> approach goes like this: if A fails, then it has cleaned up its own >> mess, but if A succeeds, then it's the responsibility of the calling >> code to clean up if the higher level (multiple statements) operation >> that A is embedded in, fails. >> >> And that's what Marginean's original C++ ScopeGuard was designed for, >> and what the corresponding Python Cleanup class is designed for. > > And try: finally:, for that matter. Not to mention "with". Some other poster made the same error recently in this thread; it is a common fallacy in discussions about programming, to assume that since the same can be expressed using lower level constructs, those are all that are required. If adopted as true it ultimately means the removal of all control structures above the level of "if" and "goto" (except Python doesn't have "goto"). >>> Both formulations can be correct (and both work perfectly fine with >>> the chdir() example being used). Sometimes one is better than the >>> other, and sometimes not. You can achieve both ways with either your >>> Cleanup class or with try: finally:. >>> >>> I am still of the opinion that Cleanup is not an improvement over try: >>> finally: and has the significant ugliness of forcing cleanup code into >>> callables. This significantly limits what you can do in your cleanup >>> code. >> >> Uhm, not really. :-) As I see it. > > Well, not being able to affect the namespace is a significant > limitation. Sometimes you need to delete objects from the namespace in > order to ensure that their refcounts go to zero and their cleanup code > gets executed. Just a nit (I agree that a lambda can't do this, but as to what's required): assigning None is sufficient for that[1]. However, note that the current language doesn't guarantee such cleanup, at least as far as I know. So while it's good practice to support it, to do everything to let it happen, it's presumably bad practice to rely on it happening. > Tracebacks will keep the namespace alive and all objects > in it. Thanks!, I hadn't thought of connecting that to general cleanup actions. It limits the use of general "with" in the same way. Cheers, - Alf Notes: [1] An 'except' clause deletes variables, but since it has no knowledge of the code it's placed in the only alternatives would be a presumably costly check of prior existence, or letting it pollute the namespace. |