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?
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?
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. 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. 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.
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.
--
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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