RFC: Proposal: Deterministic Object Destruction

[Chris Angelico]

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:
>> On Thu, Mar 1, 2018 at 1:46 PM, Rick Johnson
>> <rantingrickjohnson at gmail.com> wrote:
>>> On Wednesday, February 28, 2018 at 5:02:17 PM UTC-6, Chris Angelico wrote:
>>>
>>>> Here's one example: reference cycles. When do they get detected?
>>>> Taking a really simple situation:
>>>>
>>>> class Foo:
>>>>     def __init__(self):
>>>>         self.self = self
>>>
>>> *shudders*
>>>
>>> Can you provide a real world example in which you need an
>>> object which circularly references _itself_? This looks like
>>> a highly contrived example used to (1) merely win the
>>> argument, and (2) Bump fib() up one position from it's
>>> current position as "the worst introductory example of how
>>> to write a function in the history of programming tutorials"
>>
>> 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.

ChrisA