Python/PyQt upgrade triggers strange bug

Percy: Look, look, I just can't take the pressure of all these omens anymore!

Edmund: Percy...

Percy: No, no, really, I'm serious! Only this morning in the courtyard I saw a horse with two heads and two bodies!

Edmund: Two horses standing next to each other?

Percy: Yes, I suppose it could have been.

Blackadder, "Witchsmeller Pursuivant"

Today I saw a bug with one head and two bodies. Upgrading from Ubuntu to 9.10 to 10.4 broke a tool bar button in Leo, the world's best code editor / project manager / note sorter. The upgrade involved transitions from Python 2.6.4 -> 2.6.5 and PyQt 4.6 -> 4.7.2. The forward and back browsing buttons supplied by Leo's nav_qt plugin stopped working.

After Brain had been debugging, testing, googling, comparing etc. for over two hours, Intuition wanders past and says, "oh, ha, why not try

def __init__ (self,c):
         self.c = c
+        c._prev_next = self
         self.makeButtons()

Sometimes, Brain doesn't like Intuition very much.

Fortunately Brain was able to save some face, as

-        act_l = QtGui.QAction(icon_l, 'prev', ib_w)           
-        act_r = QtGui.QAction(icon_r, 'next', ib_w)           
+        act_l = QtGui.QAction(icon_l, 'prev', ib_w, triggered=self.clickPrev)   
+        act_r = QtGui.QAction(icon_r, 'next', ib_w, triggered=self.clickNext)  

was also required.

So it seems like the upgrade caused two changes which both had the same symptom, making debugging a challenge. It seems like the plugin class instance or the actions it was creating are now being garbage collected where they weren't before. The c._prev_next = self would prevent the instance being collected, although it's unclear that it should also prevent the actions being collected. You would think the GUI's links to the actions would be enough to protect them, so perhaps that bug body wasn't an old glitch going away, but a new one being introduced. OTOH the gui must have a link to the actions, as it's able to trigger them.

The triggered=self.clickPrev addition presumably covers a change in the emission of 'clicked()' by QToolButton, or a change in default actions, or something. Passing the parameter that way is a PyQt alternative to act_r.connect(act_r, QtCore.SIGNAL("triggered()"),self.clickNext), which would probably also have worked.