python+twisted+gtk: KeyboardInterrupt causes free variable?

Question

I'm using twisted with GTK, and the following code runs when a connection could not be established:

def connectionFailed(self, reason):
    #show a "connect failed" dialog
    dlg = gtk.MessageDialog(
        type=gtk.MESSAGE_ERROR,
        buttons=gtk.BUTTONS_CLOSE,
        message_format="Could not connect to server:\n%s" % (
            reason.getErrorMessage()))
    responseDF = defer.Deferred()
    dlg.set_title("Connection Error")
    def response(dialog, rid):
        dlg.hide_all()
        responseDF.callback(rid)
    dlg.connect("response", response)
    dlg.show_all()

    self.shutdownDeferreds.append(responseDF)

self.shutdownDeferreds is a list of deferreds that is set up so that the reactor does not stop until they are all called.

Now, I happened to press CTRL+C at the same time as the connection failed. The dialog did pop up, but when I press Close, I get:

Traceback (most recent call last):
  File "C:\Users\DrClaud\bumhunter\gui\controller.py", line 82, in response
    dlg.hide_all()
NameError: free variable 'dlg' referenced before assignment in enclosing scope
Traceback (most recent call last):
  File "C:\Users\DrClaud\bumhunter\gui\controller.py", line 82, in response
    dlg.hide_all()
NameError: free variable 'dlg' referenced before assignment in enclosing scope

Any ideas why that might happen?

Answer 1

Shouldn't that be:

def response(dialog, rid):
    dialog.hide_all()
    responseDF.callback(rid)

or really, for clarity,

def response(self, rid):
    self.hide_all()
    responseDF.callback(rid)

(I might be wrong about this, I've done barely any GTK.) If so, the problem is that you are referencing dlg in the function, which makes it a closure (it captures dlg from its surrounding scope). The KeyboardInterrupt will cause weird and wonderful behaviour, because it could destroy that scope.