[PyQt] Lambdas, Closures and Garbage Collection

Phil Thompson phil at riverbankcomputing.com
Fri Sep 5 09:37:40 BST 2008

On Wed, 03 Sep 2008 15:27:19 +0200, charles matthew chen
<charles.chen at temboo.com> wrote:
> Hi Phil,
>    Thanks for getting back to us.
> On Tue, Sep 2, 2008 at 11:09 PM, Phil Thompson 
> <phil at riverbankcomputing.com> wrote:
>  > You want to force programmers to keep an explicit reference to a
> lambda?
>  > That rather misses the point doesn't it?
>   Yes, however, this behavior is a serious source of memory leaks and 
> worse for our project. 
>    Let's say the lambda calls a bound method of an instance - that 
> instance and everything it has a strong reference to will never be 
> garbage collected.  We could explicitly disconnect the lambda/closure 
> from the signal, but that too would require us to maintain an explicit 
> reference to whatever callable we're using as a slot. 
>    That is, my understanding is this: if you connect a lambda A as a 
> slot on a signal of QObject B, and lambda A references instance C, C 
> will not be garbage collected until QObject B is deleted/gc'd. 
>    Actually, I'm not 100% sure yet, but I'm working on isolating a bug 
> wherein even in the case that B is deleted and gc'd, lambda A (and hence 
> instance C) are not garbage collected.  If I can confirm, I'll post a 
> short snippet that reproduces this issue to the list.
> On Tue, Sep 2, 2008 at 11:09 PM, Phil Thompson 
> <phil at riverbankcomputing.com> wrote:
>  > Probably.
>    I've looked at the source of SIP and not sure how to go about making 
> this patch.  sipConnectRx() and sipDisconnectRx() are a bit complex.  
> Can you give us any guidance on the best way to go about this?

Not easily - partly because I don't think the idea is a good one. In fact,
at some point, connect will take a strong reference to any slot (not just
non-methods). The reason for the current distinction is historical.

When the code was first written (without support for lambdas and partials)
the cyclic garbage collector didn't exist. I will eventually get around to
making sure the data structures supporting method slots work with the
collector - at which point strong references can be taken.


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