NameError: name '__callback' is not defined

I am trying to use gupnp (GNU library for UPnP and DLNA operations), using GObjectIntrospection:

import functools
import gi
gi.require_version( 'GSSDP', '1.0' )
from gi.repository import GSSDP
from gi.repository import Gtk

__callback = None
__network = None
__msgType = None
__client = None
__browser = None

def start( cb, network = '0.0.0.0', msgtype = 'ssdp:all' ):
  global __client, __browser, __callback
  # Si ya hay un descubrimiento en marcha, no hacemos nada.
  if __client != None:
    if cb == __callback:
      return
    else:
      raise( 'Ya hay un descubrimiento en marcha' )

  __callback = cb
  __client = GSSDP.Client.new( )
  __client.set_network( network )

  __browser = GSSDP.ResourceBrowser.new( __client, msgtype )
  __browser.connect( 'resource-available', __available )
  __browser.connect( 'resource-unavailable', __unavailable )
  __browser.set_active( True )

def stop( ):
  __browser.set_active( False )

def __available( notUsed, msn, locations ):
  __callback( msn, locations )

def __unavailable( notUsed, msn ):
  __callback( msn, None )

def debug( msn, locations ):
  if locations == None:
    print( 'PERDIDO', msn )
  else:
    print( 'ENCONTRADO', msn )
    for v in locations:
      print( v )

start( debug )
Gtk.main( )

To work, it works, at least to discover UPnP devices:

  

FOUND uuid: 20809696-105a-3721-e8b8-c4a366da3c13 :: upnp: rootdevice    link
  FOUND uuid: 20809696-105a-3721-e8b8-c4a366da3c13    link
  ...

The funny thing happens when I try to stop it, by pressing Ctrl + C :

  

Traceback (most recent call last):
    File "gssdp.py", line 38, in __unavailable
  NameError: name '__callback' is not defined

How? O_o But I'm using __callback everywhere!

The complete message is:

  

^ CTraceback (most recent call last):
    File "gssdp.py", line 49, in < module & gtk.main ()
    File "/usr/lib/python3/dist-packages/gi/overrides/Gtk.py", line 1629, in main
      return _Gtk_main (* args, ** kwargs)
    File "/usr/lib/python3.6/contextlib.py", line 88, in exit
  next (self.gen)
    File "/usr/lib/python3/dist-packages/gi/_ossighelper.py", line 251, in register_sigint_fallback
      signal.default_int_handler (signal.SIGINT, None)
  KeyboardInterrupt   Traceback (most recent call last):
    File "gssdp.py", line 38, in __unavailable
  NameError: name '__callback' is not defined
  Error in sys.excepthook:
  Traceback (most recent call last):
    File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 53, in apport_excepthook
    File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 24, in enabled
  ImportError: sys.meta_path is None, Python is likely shutting down

The code is not intended to be used independently, but is part of a Gtk application. When I use it from the real application, with a window on screen, the same curious exception occurs when closing it, doing click in the corresponding icon of its title bar.

Note : I'm sure it's not a typo ; I have copied / pasted __callback , and it continues to be produced.

EDITO

I made some small changes:

start( debug )
GObject.timeout_add( 2000, stop )
GObject.timeout_add( 2500, Gtk.main_quit )
Gtk.main( )

This way, I stop the discovery at 2 seconds, and I finish the application at 2.5 seconds.

In doing so, the exception does not occur .