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@gmail.com> wrote:
On Mon, Jun 4, 2018 at 12:50 PM Chris Angelico <rosuav@gmail.com> wrote:
On Tue, Jun 5, 2018 at 2:11 AM, Victor Stinner <vstinner@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 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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-- 
Regards,
Ivan