metaclasses

[Alex Martelli]

Dmitri Mouromtsev wrote:

> Hello all!
> 
> I've tried to override __del__ method for class object, but nothing to
> happen. Why? Does the __del__ method called when class object deleted or
> not?

Yes, but del doesn't delete an object, just a reference; the object
gets deleted only when all references to it disappear -- and classes
are referred to by other places.  Watch...:

class MyMetaclass(type):

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

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

class MyClass(object):
    __metaclass__ = MyMetaclass

class Sub(MyClass):
    pass

del Sub

import gc,pprint

def isActuallySub(x):
    return type(x) is MyMetaclass and x is not MyClass
a = [x for x in gc.get_objects() if isActuallySub(x)]
del x
referrers = [x for x in gc.get_referrers(a[0]) if x is not a]

print len(referrers),'still holding on to Sub...:'
pprint.pprint(referrers)

[alex at lancelot mydata]$ python ap.py
initialized MyClass
initialized Sub
1 containers still hold on to Sub...:
[(<class '__main__.Sub'>, <class '__main__.MyClass'>, <type 'object'>)]
[alex at lancelot mydata]$


So, object <class '__main__.Sub'> cannot be deleted because of
this reference to it.  Exactly where this tuple comes from is
an interesting puzzle (I think it's object keeping track of all
of its subclasses, but that's just a guess), but in practice I
do think it means no class object is ever truly deleted...


Alex