A "scopeguard" for Python

Robert Kern robert.kern at gmail.com
Wed Mar 3 22:02:31 CET 2010

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 )
     # blah blah #2
     chdir( original_dir )

and not a try: else:

# blah blah #1
chdir( somewhere )
     # blah blah #2
     chdir( original_dir )

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.

> 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. You seem to 
have changed your mind about what you want to talk about. That's fine. We don't 
have to stick with the original topic, 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."

Do you acknowledge this?

> I'm not sure what that shows, except that you haven't grokked this yet.
>>>> 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.
>> Uh-huh.
> Yeah. Consider that you're now for the third time failing to grasp the
> concept of cleanup for a successful operation.

Oh, I do. But if I didn't want it to run on an exception, I'd just write the 
code without any try:s or with:s at all.

# blah blah #1
chdir( somewhere )
# blah blah #2
chdir( original_dir )

>>>> The with statement allows you to encapsulate repetitive boilerplate
>>>> into context managers, but a general purpose context manager like your
>>>> Cleanup class doesn't take advantage of this.
>>> I'm sorry but that's pretty meaningless. It's like: "A house allows you
>>> to encapsulate a lot of stinking garbage, but your house doesn't take
>>> advantage of that, it's disgustingly clean". Hello.
>> No, I'm saying that your Cleanup class is about as ugly as the try:
>> finally:. It just shifts the ugliness around. There is a way to use
>> the with statement to make things look better and more readable in
>> certain situations, namely where there is some boilerplate that you
>> would otherwise repeat in many places using try: finally:. You can
>> encapsulate that repetitive code into a class or a @contextmanager
>> generator and just call the contextmanager. A generic context manager
>> where you register callables doesn't replace any boilerplate. You
>> still repeat all of the cleanup code everywhere. What's more, because
>> you have to shove everything into a callable, you have significantly
>> less flexibility than the try: finally:.
> Sorry, but that's meaningless again. You're repeating that my house has
> no garbage in it.

No, I'm repeatedly saying that I think your solution stinks. I think it's ugly. 
I think it's restrictive. I think it does not improve on the available solutions.

> And you complain that it would be work to add garbage
> to it. Why do you want that garbage? I think it's nice without it!

And you are entitled to that opinion. I am giving you mine.

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

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