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!
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.
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.