WeakValueDictionary

[Andrew Dalke]

Bryan wrote:
> i have another question about weak references.   how do you know that b 
> is dead?
> 
> 
>  >>> a = A()
>  >>> b = weakref.ref(a)
>  >>> b
> <weakref at 011739F0; to 'instance' at 01135CB0>
>  >>> del a
>  >>> b
> <weakref at 011739F0; dead>

Quoting from the weakref PEP at
   http://www.python.org/peps/pep-0205.html

     A weak reference object will allow access to the referenced object
     if it hasn't been collected and to determine if the object still
     exists in memory.  Retrieving the referent is done by calling the
     reference object.  If the referent is no longer alive, this will
     return None instead.

 >>> class Spam:
...   def hello(self): print "Hello!"
...
 >>> a = Spam()
 >>> b = weakref.ref(a)
 >>> b().hello()
Hello!
 >>> del a
 >>> b().hello()
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
AttributeError: 'NoneType' object has no attribute 'hello'
 >>> b()
 >>> b() is None
True
 >>>

				Andrew
				dalke at dalkescientific.com