[PyQt] Virtual methods and an extra reference

Kevin Watters kevinwatters at gmail.com
Wed Jun 18 19:03:49 BST 2008

There's a comment in siplib.c:5300ish in the sip_api_is_py_method
function that I'll reproduce here:

 * Note that the callable is never garbage collected.  The main
 * reason for this is that it's not possible to get hold of the
 * method cache without make incompatible changes to the SIP
 * API, particularly to support the cyclic garbage collector.
 * It would be a lot easier if the cache was held in the
 * Python object rather than the derived C++ class (and this
 * function would be passed a cache index instead of a pointer
 * to the cache entry).  Dropping the cache completely should
 * also be considered which would have the advantage of making
 * monkey patching predictable.  With cyclic garbage collector
 * support we could also just save a reference to a
 * reimplementation that was a method rather than save the
 * separate components, which would also allow a borrowed
 * reference to the reimplementation to be returned so that the
 * virtual handler wouldn't need to decrement its reference
 * count.
I'm tracking down a memory leak in my app--and I think it's boiling
down to a virtual method on one of my classes that has an extra
reference, one not coming from any Python object.
This virtual method is /bound/, though, so my understanding is that
it carries "self" in its im_self attribute and is keeping the rest of
my objects alive when they shouldn't be. Should I be worried about
that comment--and specifically that objects using virtual methods will
leak? Reading the code seems to say otherwise; that the comment is
applying to the case when a monkeypatched callable has been set on the
class--not when you override a method in a subclass like usual.

Phil, do you mind explaining the limitation this comment is describing
a bit more? I appreciate any help!

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