WeakValueDictionary retaining reference to object with no more strong references

Question

>>> from weakref import WeakValueDictionary
>>> class Foo(object):
...     pass
>>> foo = Foo()
>>> db = WeakValueDictionary()
>>> db['foo-id'] = foo
>>> del foo
>>> dict(db)
{'foo-id': <__main__.Foo object at 0x4dd946c>}

Why does it show this instead of an empty dictionary? Note that this code produces the result I'd expect:

>>> db2 = WeakValueDictionary()
>>> db2['disposable-id'] = Foo()
>>> dict(db2)
{}

It also behaves as expected when executing a script (instead of the interactive interpreter):

from weakref import WeakValueDictionary
class Foo(object):
    pass
foo = Foo()
db = WeakValueDictionary()
db['foo-id'] = foo
del foo
print str(dict(foo))
# prints {}

Answer 1

WeakValueDictionary does not guarantee that entries will be removed when there are no normal references. What it guarantees is that it will not prevent garbage collection in due course - your object is garbage collectable, not garbage collected. The entry will disappear when garbage collection happens.

Answer 2

If you have only been trying this in an interactive shell, I believe it has to do with the way the garbage collection is working under that interface and the global scope operations.

Try this from a script:

foo.py

from weakref import WeakValueDictionary

class Foo(object):
    pass

f = Foo()
d = WeakValueDictionary()
d['f'] = f 

print dict(d)
del f 
print dict(d)

And then...

$ python foo.py

{'f': <__main__.Foo object at 0x101f496d0>}
{}

Now, try this from an interactive python shell, moving the operation under a functions scope:

from weakref import WeakValueDictionary

class Foo(object):
    pass

f = Foo()
d = WeakValueDictionary()
d['f'] = f 

def main():
    global f
    print dict(d)
    del f 
    print dict(d)

main()

#{'f': <__main__.Foo object at 0x100479f10>}
#{}