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POwhy are my weakrefs dead in the water when they point to a method?
 

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  1. POwhy are my weakrefs dead in the water when they point to a method?
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    <blockquote> <p><strong>Possible Duplicate:</strong><br> <a href="https://stackoverflow.com/questions/599430/why-doesnt-the-weakref-work-on-this-bound-method">Why doesn&#39;t the weakref work on this bound method?</a> </p> </blockquote> <p>I'm using weakrefs in an observer-pattern and noticed an interesting phenomenon. If I create an object and add one of it's methods as an observer of an Observable, the reference is dead almost instantly. Can anyone explain what is happening?</p> <p>I'm also interested in thoughts for why this might be a bad idea. I've decided not to use the weakrefs and just make sure to clean up after myself properly with Observable.removeobserver, but my curiosity is killing me here.</p> <p>Here's the code:</p> <pre><code>from weakref import ref class Observable: __observers = None def addobserver(self, observer): if not self.__observers: self.__observers = [] self.__observers.append(ref(observer)) print 'ADDING observer', ref(observer) def removeobserver(self, observer): self.__observers.remove(ref(observer)) def notify(self, event): for o in self.__observers: if o() is None: print 'observer was deleted (removing)', o self.__observers.remove(o) else: o()(event) class C(Observable): def set(self, val): self.notify(val) class bar(object): def __init__(self): self.c = C() self.c.addobserver(self.foo) print self.c._Observable__observers def foo(self, x): print 'callback', x #never reached b = bar() b.c.set(3) </code></pre> <p>and here's the output:</p> <pre><code>ADDING observer &lt;weakref at 0xaf1570; to 'instancemethod' at 0xa106c0 (foo)&gt; [&lt;weakref at 0xaf1570; dead&gt;] observer was deleted (removing) &lt;weakref at 0xaf1570; dead&gt; </code></pre> <p>the main thing to note is that the print statement after the call to addobserver shows that the weakref is already dead.</p>
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    1. USAdam Fraser
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    1. CODo you realize that `__observers` is a class variable and thus shared across all instances of `Observable` (this includes subclasses)? Also, if you're using name mangling (imho you shouldn't, but that's a different topic), you shouldn't access the mangled names from outside. Specifically, you hardcode a class name and access something which you presumably try to keep private.
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    2. COAh, so I should get rid of the class var declaration and use `if "__observers" in self.__dict__` instead of the `if not self.__observers`. Right-o. Almost missed that. Also, I know the private var should be kept private, I was just printing it for demonstration purposes. This whole name-mangling thing is new to me though, so thanks for putting a name to it (no pun intended) so I could google it... I wasn't sure why __observers was getting renamed to _Observable__observers. I'll probably call it _observers instead.
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      1. USAdam Fraser
    3. COdelnan: it appears your comment about `__observers` being a class variable isn't entirely true. Initially it is, but when `addobserver` is called, `self.__observers = []` initializes the instance variable. The class variable is still accessible (and still `None`), but only reachable by `Observable._Observable__observers`.
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      1. USAdam Fraser
 

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