[Python-Dev] Why not using "except: (...) raise" to cleanup on error?

Ivan Pozdeev vano at mail.mipt.ru
Mon Jun 4 18:03:03 EDT 2018

On 05.06.2018 0:54, Ivan Pozdeev wrote:
> On 04.06.2018 23:52, Ivan Pozdeev wrote:
>> On 04.06.2018 20:11, Chris Angelico wrote:
>>> On Tue, Jun 5, 2018 at 2:57 AM, Yury Selivanov 
>>> <yselivanov.ml at gmail.com> wrote:
>>>> On Mon, Jun 4, 2018 at 12:50 PM Chris Angelico <rosuav at gmail.com> 
>>>> wrote:
>>>>> On Tue, Jun 5, 2018 at 2:11 AM, Victor Stinner 
>>>>> <vstinner at redhat.com> wrote:
>>>> [..]
>>>>>> For me, it's fine to catch any exception using "except:" if the 
>>>>>> block
>>>>>> contains "raise", typical pattern to cleanup a resource in case of
>>>>>> error. Otherwise, there is a risk of leaking open file or not 
>>>>>> flushing
>>>>>> data on disk, for example.
>>>>> Pardon the dumb question, but why is try/finally unsuitable?
>>>> Because try..finally isn't equivalent to try..except? Perhaps you
>>>> should look at the actual code:
>>>> https://github.com/python/cpython/blob/b609e687a076d77bdd687f5e4def85e29a044bfc/Lib/asyncio/base_events.py#L1117-L1123 
> In this particular code, it looks like just KeyboardInterrupt needs to 
> be handled in addition to Exception -- and even that's not certain 
> 'cuz KeyboardInterrupt is an abnormal termination and specifically 
> designed to not be messed with by the code ("The exception inherits 
> from |BaseException| 
> <https://docs.python.org/3/library/exceptions.html?highlight=generatorexit#BaseException> 
> so as to not be accidentally caught by code that catches |Exception| 
> <https://docs.python.org/3/library/exceptions.html?highlight=generatorexit#Exception> 
> and thus prevent the interpreter from exiting.").

> It only makes sense to catch it in REPL interfaces where the user 
> clearly wants to terminale the current command rather than the entire 
> program.
Remembered `pip`, too -- there, it's justified by it working in 
> If e.g. a warning is upgraded to exception, this means that some code 
> is broken from user's POV, but not from Python team's POV, so we can't 
> really be sure if we can handle this situation gracefully: our cleanup 
> code can fail just as well!
>>> Oh. Duh. Yep, it was a dumb question. Sorry! The transport should ONLY
>>> be closed on error.
>> I smell a big, big design violation here.
>> The whole point of Exception vs BaseException is that anything not 
>> Exception is "not an error", has a completely different effect on the 
>> program than an error, and thus is to be dealt with completely 
>> differently. For example, warnings do not disrupt the control flow, 
>> and GeneratorExit is normally handled by the `for` loop machinery.
>> That's the whole point why except: is strongly discouraged.
>> Be _very_ careful because when a system has matured, the risk of 
>> making bad to disastrous design decisions skyrockets (because "the 
>> big picture" grows ever larger, and it's ever more difficult to 
>> account for all of it).
>> The best solution I know of is an independent sanity-check against 
>> the project's core design principles: focus solely on them and say if 
>> the suggestion is in harmony with the existing big picture. This 
>> prevents the project from falling victim to 
>> https://en.wikipedia.org/wiki/Design_by_committee in the long run. 
>> This is easier to do for someone not intimately involved with the 
>> change and the affected area 'cuz they are less biased in favor of 
>> the change and less distracted by minute details.
>> Someone may take up this role to "provide a unified vision" (to 
>> reduce the load on a single 
>> http://meatballwiki.org/wiki/BenevolentDictator , different projects 
>> have tried delegates (this can run afoul of 
>> https://en.wikipedia.org/wiki/Conway%27s_law though) and a 
>> round-robin approach (Apache)).
>> The best way, however, would probably be for anyone dealing with a 
>> design change to remember to make this check.
>> This is even easier in Python, 'cuz the core values are officially 
>> formulated as Python Zen, and any module has one or two governing 
>> principles at its core, tops, that can be extracted by skimming 
>> through its docs.
>>> ChrisA
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> -- 
> Regards,
> Ivan


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