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

Ivan Pozdeev vano at mail.mipt.ru
Mon Jun 4 17:54:34 EDT 2018

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 
so as to not be accidentally caught by code that catches |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.

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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