weakref example

output

mithrandi@lorien /tmp> pypy weakrefs.py 
before disable: w() = <__main__.C object at 0x00007f5e52d41c58>
after del: w() = <__main__.C object at 0x00007f5e52d41c58>
after collect: w() = None
hi mom
after enable: w() = None

weakrefs.py

from weakref import ref
from gc import collect, enable, disable

class C(object): pass

def cb(x):
    print 'hi mom'

def test():
    o = C()
    w = ref(o, cb)
    print 'before disable: w() =', w()
    disable()
    del o
    print 'after del: w() =', w()
    collect()
    print 'after collect: w() =', w()
    enable()
    print 'after enable: w() =', w()

test()