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

Ivan Pozdeev vano at mail.mipt.ru
Mon Jun 4 16:52:38 EDT 2018

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
> 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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