RFC: Proposal: Deterministic Object Destruction

[Chris Angelico]

On Fri, Mar 2, 2018 at 4:46 PM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Thu, Mar 1, 2018 at 10:35 PM, Chris Angelico <rosuav at gmail.com> wrote:
>> On Fri, Mar 2, 2018 at 4:16 PM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
>>> On Wed, Feb 28, 2018 at 8:00 PM, Chris Angelico <rosuav at gmail.com> wrote:
>>>> Not off hand, but I can provide an EXTREMELY real-world example of a
>>>> fairly tight loop: exceptions. An exception has a reference to the
>>>> local variables it came from, and those locals may well include the
>>>> exception itself:
>>>>
>>>> try:
>>>>     1/0
>>>> except Exception as e:
>>>>     print(e)
>>>>
>>>> The ZeroDivisionError has a reference to the locals, and 'e' in the
>>>> locals refers to that very exception object.
>>>
>>> The problem with this example of course is that the variable 'e' is
>>> scoped to the except block and automatically del'ed when it exits.
>>
>> Or, to be more accurate: The language design acknowledges that this
>> reference cycle is a fundamental problem, and the *solution* is that
>> there's an implicit "e = None; del e" at the end of the except block.
>>
>> You can easily defeat that protection with "except Exception as ee: e
>> = ee", if you want to demonstrate the cycle.
>
> Yes, but how often does this happen in practice? This situation was
> billed as "an EXTREMELY real-world example". In the real world, most
> of the time when you have an exception, you log it or handle it, and
> then you discard it.

The exception object itself is the cycle. The fact that the language
has a default way of handling this doesn't change that. What happens,
for instance, when you have another exception inside the except block,
and they chain? Once the top-level exception is freed, there's still a
definite reference cycle in the original. Or what if you have an
interactive debugger that lets you inspect objects at point of crash?
That's going to work by retaining a reference to the exception. So,
yep, this is stuff that happens every day. Not contrived, not
uncommon.

ChrisA