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>
)
<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__*"
def __del__(cls):
print 'deleted', cls.__name__
class MyClass(object):
if 'meta' in sys.argv:
print "*meta*"
__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 ...
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