[pygtk] Garbage collection prematurely clears cyclical objects referenced from GTK callbacks

[Mark Seaborn]

On Wed, 2008-08-13 at 11:29 +0200, Hrvoje Nikšić wrote:

> Traceback (most recent call last):
>   File "bug.py", line 9, in <lambda>
>     w.connect('delete-event', lambda *args: self.die())
> NameError: free variable 'self' referenced before assignment in enclosing scope

This looks like the bug I reported last week:
http://bugzilla.gnome.org/show_bug.cgi?id=546802
- "Pygtk destroys cycle too early"

The assignment "self.window = w" creates a cycle between the Buggy
object, the gtk.Window() and the callback.  The cycle is wrongly freed
by Python's GC because Pygobject does not tell Python that the widget's
wrapper object is referenced by the underlying GObject.  As you have
found, in order to break the cycle Python zeros out the callback
closure's captured free variables, which is what causes the "referenced
before assignment" exception.

Regards,
Mark
-- 
Mark Seaborn
Software Engineer

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