new-style classes keep track of their subclasses through weak-refs so they remain in general collectible.

consider this code: (inspired by this post comp.lang.python post id: <b3khka$2g6$1@news.wplus.spb.ru>

I cannot find that article by id alone; groups.google.com doesn't have it.

<test.py> import sys

class MyMetaclass(type):

def __init__(cls, name, bases, dict): super(MyMetaclass, cls).__init__(name, bases, dict) print 'initialized', cls.__name__

if 'meta__del__' in sys.argv: print "*meta__del__*"

I'm guessing this should be indented the same as the following line.

def __del__(cls): print 'deleted', cls.__name__

class MyClass(object): if 'meta' in sys.argv: print "*meta*"

Ditto.

__metaclass__ = MyMetaclass pass

class Sub(MyClass): pass

del Sub

import gc

gc.collect() # force involved weak-refs to be cleared

print "MyClass subclasses",MyClass.__subclasses__() print "garbage",gc.garbage

</test.py>

Now:

C:\exp\py-subclasses-gc>\transit\Py23\python test.py MyClass subclasses [] garbage []

C:\exp\py-subclasses-gc>\transit\Py23\python test.py meta *meta* initialized MyClass initialized Sub MyClass subclasses [] garbage []

also Sub with both metaclass type or MyMetaclass without __del__ is collectible and collected but:

C:\exp\py-subclasses-gc>\transit\Py23\python test.py meta meta__del__ *meta__del__* *meta* initialized MyClass initialized Sub MyClass subclasses [<class '__main__.Sub'>] garbage [<class '__main__.Sub'>]

if MyMetaclass grows a __del__ method Sub is no longer collectible ...

In general objects whose class has a __del__ method are not collectible. The GC implementation has a good reason for this; someone else may be able to explain it. --Guido van Rossum (home page: http://www.python.org/~guido/)