code: def bind(self, callback, sender=None):
          '''Bind a ``callback`` for a given ``sender``.'''
          key = (_make_id(callback), _make_id(sender))
          self.callbacks.append((key, callback)f bind(self, callback, sender=None):
          '''Bind a ``callback`` for a given ``sender``.'''
          key = (_make_id(callback), _make_id(sender))
          self.callbacks.append((key, callback))
docs: Bind a ``callback`` for a given ``sender``.