Advice needed on __del__

[Scott David Daniels]

André Roberge wrote:
> Scott David Daniels wrote:
> 
>> André Roberge wrote:
>>
>>> ...  Each time I refresh the screen, I could
>>> force that call, then check to see if Evil has been
>>> destroyed by Python, which would give me the information
>>> I need to destroy Evil_twin behind the scene myself -
>>> which is what I am really after.
>>
>> What might work is to keep a weakref to Evil in Evil_twin.
>> Upon refresh, you could check if the weakref has gone stale.
>> Is that good enough?
> 
> ...
Try something like this:
++ import weakref
>  class UsedRobot(object):
--     def __init__(self, ..., parent=Visible_world()):
++     def __init__(self, ..., jeckyll=None, parent=Visible_world()):
++         # need to know if we are good or evil.
>          # Visible_world is a Singleton - see Python cookbook
-->        true_robot = parent.addOneRobot(...)    # <---Evil_twin created
>          self.robot = parent.robot_dict[true_robot.name]
>          self.name = true_robot.name
>          self.program = rur_program() # Singleton
>          self.parent = parent
>          self.parent.object_dict[self.name] = True  # <--- new trick
>          self.program.wait_update_refresh(self.robot, self.name)
++         if jeckyll is None:
++             # Making a good twin.  Make an evil partner
++             twin = parent.addOneRobot(..., jeckyll=self)
++             self.twin = lambda: return twin # match weakref interface
++             # hold a ref to the evil twin, so it will be alive
++         else:
++             # Making an evil twin.  Make a weak ref to the good
++             twin = parent.addOneRobot(..., jeckyll=self, ...)
++             self.twin = weakref.ref(jeckyll)
-->    def __del__(self):
-->        self.parent.object_dict[self.name] = False
...

Somewhere else when deciding to draw a particular robot:
          ... robot = whatever ...
-->      <draw robot>
++       if robot.twin() is None:
++           # This must be an evil twin with a dead good twin
++           <avoid drawing robot>
++       else:
++           # this is either a good twin or
++           <draw robot>
...

--Scott David Daniels
Scott.Daniels at Acm.Org