[PyQt] Re: Python frame never dies on application errors

[Sundance]

Fabio Menegazzo wrote:

> Consider a widget in the display. When an exception happens the Python
> frame never dies. Am I doing something wrong?

Hi Fabio,

In your example, your exception occurs while the frame for 
SlotRaiseError() is still on the stack. The traceback thus keeps a 
pointer to that frame, which is why the stub variable's refcount doesn't 
reach 0. You are trying to use sys.exc_clear(), but that function makes 
strictly no promise as to what variables are going to get cleared, or 
when (see http://effbot.org/pyref/sys.exc_clear.htm). In particular, it 
doesn't clear the reference to the last traceback that is available in 
the sys module.

Contrast the example:

---[ Example ]---------------------------------------------------------
import sys
from PyQt4 import Qt

class Stub:
  
  def __init__( s ):
    print "I'm a stub instance and I was just created!"
    
  def __del__( s ):
    print "Whoops, my refcount is zero and now I die!"


class TestClass:
  
  def causeHavoc( s ):
    
    stub = Stub()
    raise Exception( "Whoops, an error occurred!" )
    
  def clearTraceback( s ):
    
    print "Deleting last traceback..."
    sys.last_traceback = None  ## HERE!
    
  def start( s ):
    
    app = Qt.QApplication( sys.argv )
    Qt.QTimer.singleShot( 1000, s.causeHavoc )
    Qt.QTimer.singleShot( 2000, s.clearTraceback )
    Qt.QTimer.singleShot( 3000, app.quit )
    app.exec_()

t = TestClass()
t.start()
-----------------------------------------------------------------------

In this example, the stub variable is correctly collected when the 
reference to the traceback is deleted.

In short: business as usual. Python works fine, it just keeps a 
reference to the last traceback that you didn't know about. :)

Hope this helps,

-- S.