[PyQt] sip: Accessing members results in incorrect/missing data

[Phil Thompson]

On 22/01/2020 21:50, MatthijsBurgh wrote:
> Phil Thompson-5 wrote
>> On 22/01/2020 13:57, MatthijsBurgh wrote:
>>> The first changeset already fixed the issue.
>>> 
>>> The newer changeset creates the situation as desribed in
>>> http://python.6.x6.nabble.com/sip-Accessing-members-results-in-incorrect-missing-data-tp5266521p5266740.html,
>>> it looks like the release function isn't called anymore. I thought
>>> Python GC
>>> was able to break up circular references, but it looks like it isn't
>>> happening.
>>> 
>>> By running the following code:
>>> ```
>>> v = Vector(1,2,3)
>>> R = Rotation()
>>> F = Frame(R,v)
>>> v2 = F.p
>>> v3 = Frame(F).p
>>> v3 = Vector(2,3,4)
>>> ```
>>> The `release_Frame` is called(print shows up) after the second `v3`
>>> assignment when using your first changeset, but the print doesn't 
>>> show
>>> up
>>> with your second changeset, today's one.
>>> 
>>> So to me it looks like this will cause a memory leak.
>> 
>> In my own test cases the cycle is broken. Have you run gc.collect()? 
>> If
>> you print out the reference counts to they look correct?
>> 
>>> Side note: I would prefer the usage of a specific key for the member
>>> reference to the containing class.
>>> I don't think using the same key for both references will cause a
>>> problem
>>> with overwriting. As a class can't contain a member of the same type,
>>> only a
>>> member as a pointer to the same type. In which case, the reference
>>> isn't
>>> kept, if I understand the code correct.
>> 
>> There can be a problem (now fixed) if the type of the variable is
>> defined in a different module from the type of the container.
>> 
>> Phil
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> 
> In your last changeset (but also the second one), the new frame keeps 
> alive.
> Even after `v3` is assigned a new value. Therefore the unassigned 
> Frame,
> from which only `p` was accessed, stays alive till the end of the 
> Python
> session.
> 
> This happens because `Frame` has a ref to `p`, because of the caching 
> of the
> python object. And `p` has a reference to `Frame`, to keep the class 
> alive,
> to be able to access the member `p`.
> 
> I don't see an easy solution for this at the moment. Do you?

As I said, I don't seem to see the problem. The caching is needed as 
explained in the comments. Did you try gc.collect()?

Phil