Python method reference kills garbage collection?

[Mark Hammond]

Troels Walsted Hansen wrote:
> Consider the following program. What is so special about references to the
> object's own methods that cause the object to never be garbage collected? 

Just answered that.

 > Is this a cyclic reference problem,

Yes.  self.f is a bound method.  A bound method has a reference to a 
class and an instance.  The instance in this case is 'self', so you have 
a cycle.

If you really want to do this, store a reference to the unbound method.  Eg:

 >>> class A:
... 	def __init__(self):
... 		self.f = self.__class__.func
... 	def __del__(self):
... 		print "dieing"
... 	def func(self):
... 		print "func"
... 		
 >>> a=A()
 >>> a.f
<unbound method A.func>
 >>> a.func
<bound method A.func of <__main__.A instance at 0x011C9E08>>

To create a bound method from the unbound method (generally as a temp, 
just before calling it), use the new module:

 >>> import new
 >>> new.instancemethod(a.f, a, A)
<bound method A.func of <__main__.A instance at 0x011C9E08>>

And it will self-destruct OK:
 >>> a=None
 >>>

Ahh - but it didn't die!  Oh yeah - the bound method we just created 
will be stored in "_" (as this was entered interactively.  So enter any 
new expression to replace "_"

 >>> 1
dieing
1
 >>>

Yay :)

Mark.