Question

Lets assume we have a class in python:

class A(object):
    def __del__(self):
        print "Del!"

__del__ is called upon deleting/garbage collection of any A instance.

Is is possible to do the same for a class? I would like to have some method called when the class itself is garbage collected, which I assume is being done at the script exit.

Thanks in advance for any pointers!

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Edit: Just as I have expected, everyone is trying to drive me away from using this technique (I would probably make such a comment myself:)), though the question still stands: is it possible?

I want to the the following: I have a class with a static member that needs to be cleaned.

class A(object):
    class Meta(type):
        def __new__(mcs, name, bases, attrs):
            attrs['conn'] = sqlite3.connect( DB_FILE )
            return type.__new__(mcs, name, bases, attrs)

    __metaclass__ = Meta

I would like A.conn.close() to be called, but just before the program closes – i.e. when I know that no more instances of A will be ever created. I know I can do this with atexit, but this just seems very ugly.

Answer 1

The problem is that classes contain circular references back to themselves – so when they are deleted they are not easily collected – thus the __del__ method odf the metaclass is not called.

I could trigger it being called using Pypy’s Python implementation, but not using cpython – either 2.6 or 3.2. And even to trigger that, I had to manually invoke the garbage collector –
The Python environment at program exit is known to be full of inconsitencies, and the chances of the __del__ method being called while enough internal information on the class would exist to allow a sae shut down would be very slim.

Here is my Pypy session where I did trigger the call to the class’ __del__

 ~]$ pypy                                                                                       
Python 2.5.2 (78826, Nov 29 2010, 00:18:05)                                                                       
[PyPy 1.4.0] on linux2                                                                                            
Type "help", "copyright", "credits" or "license" for more information.                                            
And now for something completely different: ``sorry, I'll teach the cactus how                                    
to swim later''                                                                                                   
>>>> import gc
>>>> class Meta(type):         
....    def __del__(cls):                          
....       print ("Arghh!!")                                              
....                                                                      
>>>> class A(object):                                                                     
....   __metaclass__ = Meta                                                                                     
....                                                                                                                                                                                                                
>>>> del A                                                                                                        
>>>> gc.collect()                                                                                                 
Arghh!!                                                                                                           
0                                                                                                                 
>>>>

2022 – as of Python 3.11 a06, the __del__ method in the metaclass works in cPython, if one calls gc.collect() after deleting all references to the class, just like it happens with pypy on this example.