Destructor

[Jeff Epler]

The destructor (__del__ method) of an object is never invoked while the
object still has non-weak references.  (AKA "the refcount is nonzero")

So merely having
    del f
or
    f = None
may not cause this necessary condition, as in
    f = g = Thingy()
    del f
(g still has a strong reference to the object that f referenced)

Furthermore, as I undersand the Language Reference, the destructor need
not be invoked as soon as the refcount drops to zero -- in fact,
destructors could be called in a different order than the refcounts
decrease, or be postponed forever.  So if you have
    class C:
	def __init__(self, name): self.name = name
	def __del__(self): print "dying", self.name

    c = C("c")
    d = C("d")

    del c
    print "blah"
    del d
you might get
    blah
    dying d
    dying c
or
    dying c
    blah
    dying d
or
    blah
but certainly not
    dying d
    blah
    dying c
(Probably the CPython implementation you have only prints the 'dying
c/blah/dying d' order, however)

The 1.5-era language manual only gives cycles and program termination
as reasons __del__ is not called.  I haven't checked a newer version of
the language manual..

Jeff