Class destructor

[Jp Calderone]

On Thu, Feb 13, 2003 at 11:33:30AM -0500, Mongryong wrote:
> On Thu, 2003-02-13 at 09:46, Roberto Cavada wrote:
> > 
> > class A:
> >      def __init__(self):
> >          self.ref = self
> >          print "Built instance of A"
> >          return
> > 
> >      def __del__(self):
> >          print "Destroying instance of A"
> >          return
> > 
> >      pass # end of class
> > 
> > a1 = A()
> > del a1  # This does not call A.__del__
> 
> If you keep in mind that Python uses 'reference counting' for garbage
> collecting, you can see why the 'del a1' appears to be doing nothing. 

  Actually, reference counting and garbage collection are two separate
methods.  Straight reference counting is used for all non-cyclic structures. 
The garbage collector only comes into play when an actual cycle is found. 
It runs periodically and notices when a group of objects are only reachable
from themselves.  If there is no __del__ method, it destroys them, otherwise
it marks them as uncollectable garbage.

> From above, there are '2' references to the object created by 'a1=A()':
> a1 and a1.ref both point to that object.  When 'del a1' is executed 'a1'
> no longers points to anything (ie. a1=Null).

  This is a bad way to look at it.  a1 is no longer defined after you delete
it.

>  This reduces the 'reference count' to 1.  Hence, the object pointed to by
> 'a1' can not yet be deleted.  So, you now have something similar to a
> C/C++'s 'lost pointer'.

  "Lost pointer" (Which I believe is a less commonly used term for "dangling
pointer", though I may be mistaken) in C is when a pointer refers to an area
of memory that has been freed.  This isn't the case in Python, as it is
[almost] impossible to refer to freed memory locations.

  This really is just a cycle.

> 
> > a2 = A()
> > del a2.ref # Neither does this
> > del a2  # This calls it
> > 
> As explained above 'a2=A()' creates two references.  'del a2.ref'
> removes 1 reference. 'del a2' removes the other reference.  Hence, no
> more references, so the destructor is called.
> 
> So, if you wanted 'circular' references like the above example, you'll
> have to call delete the inner reference first.  
> 

  Jp

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